A Dictionary of Units
(created by Frank Tapson)

This provides a summary of most of the units of measurement to be found in use around the world today (and a few of historical interest), together with the appropriate conversion factors needed to change them into a 'standard' unit of the S I.

The units may be found either by looking under the category in which they are used [such as length, mass, density, energy etc.], or else by picking one unit from an alpabetically ordered list of units. There are NO units of currency.

There is an outline of the S I; a list of its basic defining standards and also some of its derived units; then another list of all the S I prefixes and some notes on conventions of usage.

There is a short historical note on measures generally; descriptions of the Metric system, the U K (Imperial) system with a statement on the implementation of 'metrication' in the U K, and the U S system.

Finally there is a list of other sources concerned with the topic of measures and units (with a few references to currencies) and also some notes about this document.


The Systeme International [S I]

Le Systeme international d'Unites officially came into being in October 1960 and has been adopted by nearly all countries, though the amount of actual usage varies considerably.

It is based upon 7 principal units, 1 in each of 7 different categories -

			Category		Name 		Abbreviation

			Length			metre 			m
			Mass			kilogram		kg
			Time			second			s
			Electric current	ampere			A
			Temperature		kelvin			K
			Amount of substance	mole			mol
			Luminous intensity	candela			cd

Definitions of these basic units are given.

Each of these units may take a prefix.

From these basic units many other units are derived and named.

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Definitions of the Seven Basic S I Units

metre [m]
The metre is the basic unit of length. It is the distance light travels, in a vacuum, in 1/299792458th of a second.
kilogram [kg]
The kilogram is the basic unit of mass. It is the mass of an international prototype in the form of a platinum-iridium cylinder kept at Sevres in France. It is now the only basic unit still defined in terms of a material object, and also the only one with a prefix[kilo] already in place.
second [s]
The second is the basic unit of time. It is the length of time taken for 9192631770 periods of vibration of the caesium-133 atom to occur.
ampere [A]
The ampere is the basic unit of electric current. It is that current which produces a specified force between two parallel wires which are 1 metre apart in a vacuum.It is named after the French physicist Andre Ampere (1775-1836).
kelvin [K]
The kelvin is the basic unit of temperature. It is 1/273.16th of the thermodynamic temperature of the triple point of water. It is named after the Scottish mathematician and physicist William Thomson 1st Lord Kelvin (1824-1907).
mole [mol]
The mole is the basic unit of substance. It is the amount of substance that contains as many elementary units as there are atoms in 0.012 kg of carbon-12.
candela [cd]
The candela is the basic unit of luminous intensity. It is the intensity of a source of light of a specified frequency, which gives a specified amount of power in a given direction.

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Derived Units of the S I

From the 7 basic units of the SI many other units are derived for a variety of purposes. Only some of them are explained here. The units printed in bold are either basic units or else, in some cases, are themselves derived.
farad [F]
The farad is the SI unit of the capacitance of an electrical system, that is, its capacity to store electricity. It is a rather large unit as defined and is more often used as a microfarad. It is named after the English chemist and physicist Michael Faraday (1791-1867).
hertz [Hz]
The hertz is the SI unit of the frequency of a periodic phenomenon. One hertz indicates that 1 cycle of the phenomenon occurs every second. For most work much higher frequencies are needed such as the kiloherz [kHz] and megaherz [MHz]. It is named after the German physicist Heinrich Rudolph Herz (1857-94).
joule [J]
The joule is the SI unit of work or energy. One joule is the amount of work done when an applied force of 1 newton moves through a distance of 1 metre in the direction of the force.It is named after the English physicist James Prescott Joule (1818-89).
newton [N]
The newton is the SI unit of force. One newton is the force required to give a mass of 1 kilogram an acceleration of 1 metre per second per second. It is named after the English mathematician and physicist Sir Isaac Newton (1642-1727).
ohm [*]
The ohm is the SI unit of resistance of an electrical conductor. Its symbol, shown here as [*] is the Greek letter known as 'omega'. It is named after the German physicist Georg Simon Ohm (1789-1854).
pascal [Pa]
The pascal is the SI unit of pressure. One pascal is the pressure generated by a force of 1 newton acting on an area of 1 square metre. It is a rather small unit as defined and is more often used as a kilopascal [kP]. It is named after the French mathematician, physicist and philosopher Blaise Pascal (1623-62).
volt [V]
The volt is the SI unit of electric potential. One volt is the difference of potential between two points of an electical conductor when a current of 1 ampere flowing between those points dissipates a power of 1 watt. It is named after the Italian physicist Count Alessandro Giuseppe Anastasio Volta (1745-1827).
watt [W]
The watt is used to measure power or the rate of doing work. One watt is a power of 1 joule per second. It is named after the Scottish engineer James Watt (1736-1819).

Note that prefixes may be used in conjunction with any of the above units.

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The Prefixes of the S I

The S I allows the sizes of units to be made bigger or smaller by the use of appropriate prefixes. For example, the electrical unit of a watt is not a big unit even in terms of ordinary household use, so it is generally used in terms of 1000 watts at a time. The prefix for 1000 is kilo so we use kilowatts[kW] as our unit of measurement. For makers of electricity, or bigger users such as industry, it is common to use megawatts[MW] or even gigawatts[GW]. The full range of prefixes with their [symbols or abbreviations] and their multiplying factors which are also given in other forms is

	yotta [Y] 1 000 000 000 000 000 000 000 000	= 10^24
	zetta [Z] 1 000 000 000 000 000 000 000		= 10^21
	exa   [E] 1 000 000 000 000 000 000		= 10^18
	peta  [P] 1 000 000 000 000 000			= 10^15
	tera  [T] 1 000 000 000 000 		   	= 10^12	
	giga  [G] 1 000 000 000 		   (a thousand millions = a billion)
	mega  [M] 1 000 000 			   (a million)
	kilo  [k] 1 000 			   (a thousand)
		  1
	milli [m] 0.001 			   (a thousandth)
	micro [*] 0.000 001 			   (a millionth)
	nano  [n] 0.000 000 001 		   (a thousand millionth)
	pico  [p] 0.000 000 000 001			= 10^-12	
	femto [f] 0.000 000 000 000 001			= 10^-15
	atto  [a] 0.000 000 000 000 000 001		= 10^-18
	zepto [z] 0.000 000 000 000 000 000 001		= 10^-21
	yocto [y] 0.000 000 000 000 000 000 000 001	= 10^-24
[*] the symbol used here is the Greek letter known as 'mu'
All of the S I prefixes are multiples or sub-multiples of 1000. However, these are inconvenient for many purposes and so hecta (x 100), deca (x 10), deci (x 0.1), and centi (x 0.01) are also used.

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Conventions of Usage in the S I

There are various rules laid down for the use of the SI and its units as well as some observations to be made that will help in its correct use.

Any unit may take only ONE prefix. For example 'millimillimetre' is incorrect and should be written as 'micrometre'.
Prefixes which make a unit bigger are written in capital letters (M G T etc.), but when they make a unit smaller then lower case (m n p etc.) is used. The one exception to this is kilo [k] to avoid any possible confusion with kelvin [K].
A unit which is named after a person is written all in lower case (newton, volt, pascal etc.) when named in full, but using a capital letter (N V P etc.) when abbreviated. An exception to this rule is the litre which, if written as a lower case 'l' could be mistaken for a '1' (one) and so a capital 'L' is allowed as an alternative.
Units written in abbreviated form are never pluralised. So 'm' could always be either 'metre' or 'metres'. 'ms' would represent 'metre second'.
To make numbers easier to read they may be divided into groups of 3 separated by spaces (or half-spaces) but NOT commas.
The SI preferred way of showing a decimal fraction is to use a comma (123,456) to separate the whole number from its fractional part. The practice of using a point, as is common in English-speaking countries, is acceptable providing only that the point is placed ON the line of the bottom edge of the numbers (123.456).
It will be noted that many units are eponymous, that is they are named after persons. This is always someone who was prominent in the early work done within the field in which the unit is used.

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A Brief History of Measurement

One of the earliest types of measurement concerned that of length. These measurements were usually based on parts of the body. A well documented example (the first) is the Egyptian cubit which was derived from the length of the arm from the elbow to the outstretched finger tips. By 2500 BC this had been standardised in a royal master cubit made of black marble (about 52 cm). This cubit was divided into 28 digits (roughly a finger width) which could be further divided into fractional parts, the smallest of these being only just over a millimetre.

In England units of measurement were not properly standardised until the 13th century, though variations (and abuses) continued until long after that. For example, there were three different gallons (ale, wine and corn) up until 1824 when the gallon was standardised.

In the U S A the system of weights and measured first adopted was that of the English, though a few differences came in when decisions were made at the time of standardisation in 1836. For instance, the wine-gallon of 231 cubic inches was used instead of the English one (as defined in 1824) of about 277 cubic inches. The U S A also defined a separate dry gallon of about 269 cubic inches.

Even as late as the middle of the 20th century there were some differences in UK and US measures which were nominally the same. The UK inch measured 2.53998 cm while the US inch was 2.540005 cm. Both were standardised at 2.54 cm in July 1959.

In France the metric system officially started in June 1799 with the declared intent of being 'For all people, for all time'. The unit of length was the metre which was defined as being one ten-millionth part of a quarter of the earth's circumference. The production of this standard required a very careful survey to be done which took several years. However, as more accurate instruments became available so the 'exactness' of the standard was called into question. Later efforts were directed at finding some absolute standard based on an observable physical phenomenon. Over two centuries this developed into the S I. So maybe their original slogan was more correct than anyone could have foreseen then.

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Metric System of Measurements

	     	Length			                      Area
	  10 millimetres = 1 centimetre			100 sq. mm     = 1 sq. cm
	  10 centimetres = 1 decimeter		     10 000 sq. cm     = 1 sq. metre
	  10 decimetres  = 1 metre			100 sq. metres = 1 are
	  10 metres      = 1 dekametre		 	100 ares       = 1 hectare
	  10 dekametres  = 1 hectometre		     10 000 sq. metres = 1 hectare
	  10 hectometres = 1 kilometre	  		100 hectares   = 1 sq. kilometre
	1000 metres      = 1 kilometre		  1 000 000 sq. metres = 1 sq. kilometre
	  		  			  
	
								Capacity
			 	 			 10 millilitres = 1 centilitre
		Volume	 	 		         10 centilitree = 1 decilitre
   	1000 cu. mm = 1 cu. cm				 10 decilitres  = 1 litre
   1 000 000 cu. cm = 1 cu. metre	       	       1000 litres      = 1 cu. metre
	
					
					Mass					
				1000 grams     = 1 kilogram			
				1000 kilograms = 1 tonne
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The U K (Imperial) System of Measurements

	         Length			      	            Area			
	  12 inches   = 1 foot			 144 sq. inches = 1 square foot
	   3 feet     = 1 yard			   9 sq. feet   = 1 square yard
	  22 yards    = 1 chain			4840 sq. yards  = 1 acre
	  10 chains   = 1 furlong		 640 acres      = 1 square mile
	   8 furlongs = 1 mile
	5280 feet     = 1 mile				
	1760 yards    = 1 mile				Capacity
						20 fluid ounces = 1 pint 
		Volume			         4 gills        = 1 pint	
	1728 cu. inches = 1 cubic foot		 2 pints        = 1 quart		
	  27 cu. feet   = 1 cubic yard		 4 quarts       = 1 gallon (8 pints)		 
						 
		Mass						 
	437.5 grains = 1 ounce				Troy Weights
	 16 ounces   = 1 pound (7000 grains)	24 grains        = 1 pennyweight
	 14 pounds   = 1 stone			20 pennyweights  = 1 ounce (480 grains)
	  8 stones   = 1 hundredweight [cwt]	12 ounces        = 1 pound (5760 grains)
	 20 cwt      = 1 ton (2240 pounds)

	   Apothecaries' Measures	          Apothecaries' Weights
	 20 minims      = 1 fl.scruple		20 grains   = 1 scruple
	  3 fl.scruples = 1 fl.drachm		 3 scruples = 1 drachm
	  8 fl.drachms  = 1 fl.ounce		 8 drachms  = 1 ounce (480 grains)
	 20 fl.ounces   = 1 pint		12 ounces   = 1 pound (5760 grains)	 
The old Imperial (now UK) system was originally defined by three standard measures - the yard, the pound and the gallon which were held in London. They are now defined by reference to the S I measures of the metre, the kilogram and the litre. These equivalent measures are exact.

1 yard = 0.9144 metres - same as US
1 pound = 0.453 592 37 kilograms - same as US
1 gallon = 4.546 09 litres
Note particularly that the UK gallon is a different size to the US gallon so that NO liquid measures of the same name are the same size in the UK and US systems.
Also that the ton(UK) is 2240 pounds while a ton(US) is 2000 pounds. These are also referred to as a long ton and short ton respectively.

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Metrication in the U K

There have been three major Weights and Measures Acts in recent times (1963, 1976 and 1985) all gradually abolishing various units, as well re-defining the standards. All the Apothecaries' measures are gone, and of the Troy measures, only the ounce remains. Currently legislation has decreed that -

From the 1st October 1995, for economic, public health, public safety and administrative purposes, only metric units are allowed EXCEPT that -

  • pounds and ounces for weighing of goods sold from bulk
  • pints and fluid ounces for beer, cider, waters, lemonades and fruit juices in RETURNABLE containers
  • therms for gas supply
  • fathoms for marine navigation
may be used until 31st December 1999.

The following may continue to be used WITHOUT time limit -

  • miles, yards, feet and inches for road traffic signs and related measurements of speed and distance
  • pints for dispensing draught beer and cider, and for milk in RETURNABLE containers
  • acres for land registration purposes
  • troy ounces for transactions in precious metals.
Sports are exempt from all of this, but most of them have (voluntarily) changed their relevant regulations into statements of equivalent metric measures.

The above only covers matters of trade.
Consenting adults (in private) may use any units they please.

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The U S System of Measurements

Most of the US system of measurements is the same as that for the UK. The biggest differences to be noted are in Capacity which has both liquid and dry measures as well as being based on a different standard - the US liquid gallon is smaller than the UK gallon. There is also a measurement known at the US survey foot. It is gradually being phased out as the maps and land plans are re-drawn under metrication. (The changeover is being made by putting 39.37 US survey feet = 12 metres)
	        Length			      	        Area			
	  12 inches   = 1 foot			 144 sq. inches = 1 square foot
	   3 feet     = 1 yard			   9 sq. feet   = 1 square yard
	 220 yards    = 1 furlong		4840 sq. yards  = 1 acre
	   8 furlongs = 1 mile	 		 640 acres      = 1 square mile 
	5280 feet     = 1 mile			   1 sq.mile    = 1 section	
	1760 yards    = 1 mile			  36 sections   = 1 township	
			
	         Volume
	1728 cu. inches = 1 cubic foot
	  27 cu. feet   = 1 cubic yard

	      Capacity (Dry)		              Capacity (Liquid)
					       	  16 fluid ounces = 1 pint 
	   2 pints   = 1 quart			   4 gills        = 1 pint	
	   8 quarts  = 1 peck	 		   2 pints        = 1 quart		
	   4 pecks   = 1 bushel	 		   4 quarts       = 1 gallon (8 pints)

	         Mass						 
	437.5 grains = 1 ounce				Troy Weights
	 16 ounces   = 1 pound (7000 grains)	24 grains        = 1 pennyweight
	 14 pounds   = 1 stone			20 pennyweights  = 1 ounce (480 grains)
	100 pounds   = 1 hundredweight [cwt]	12 ounces        = 1 pound (5760 grains)
	 20 cwt      = 1 ton (2000 pounds)

	  Apothecaries' Measures	   	Apothecaries' Weights
	 60 minims    = 1 fl.dram		20 grains   = 1 scruple
	  8 fl.drams  = 1 fl.ounce		 3 scruples = 1 dram
	 16 fl.ounces = 1 pint		 	 8 drams    = 1 ounce (480 grains)
						12 ounces   = 1 pound (5760 grains)	 
As with the UK system these measures were originally defined by physical standard measures - the yard, the pound, the gallon and the bushel.They are now all defined by reference to the S I measures of the metre, the kilogram and the litre. These equivalent measures are exact.

1 yard = 0.9144 metres - same as UK
1 pound = 0.453 592 37 kilograms - same as UK
1 gallon (liquid) = 3.785 411 784 litres
1 bushel (dry) = 35.239 070 166 88 litres
Note particularly that the US gallon is a different size to the UK gallon so that NO liquid measures of the same name are the same size in the US and UK systems.
Also that the ton(US) is 2000 pounds while a ton(UK) is 2240 pounds. These are also referred to as a short ton and long ton respectively.

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Categories of Units

length
area
volume or capacity
mass

density, area
density, line
density, volume
energy
force
fuel consumption
line density
mass per unit length
mass per unit area
mass per unit volume
power
pressure
speed
spread rate (by mass)
spread rate (by volume)
stress
torque
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List of Units

The units are listed in alphabetical order but scanning can be speeded up by selecting the initial letter of the unit from these individual letters or groups

A - B - C - D - E - F - G - H - IJ - K - L - M

N - O - PQ - R - S - T - UVW - XYZ


A

acres
angstroms
ares
astronomical units
atmospheres

B

barleycorns
barrels (oil)
bars
British thermal units
Btu/hour etc.
bushels

C

calories
calories per hour etc.
carats, metric
centigrade heat units
centilitres
centimetres
centimetres of mercury or water
centimetres per minute etc.
chains (surveyors')
circular inches
cubic (+ any units)
cubic measures per area
cubits

D

decilitres
denier
drex
dynes

E

ells (UK)
ems (pica)
ergs (energy)
ergs (torque)

F

fathoms
feet
feet of water
feet per hour etc.
fluid ounces
foot pounds-force
foot pounds-force per minute etc.
foot poundals
furlongs

G

gallons
gallons per area
gigajoules
gigawatts
grains
grains per gallon
grams
gram-force centimetres
grams per area
grams per cm
grams per (any volume)

H

hands
hectares
hides
horsepower
horsepower hours
hundredweights

IJ

inches
inches of mercury or water
inches of rain (by mass)
inches of rain (by volume)
inches per minute etc.
joules
joules per hour etc.

K

kilocalories
kilocalories per hour etc.
kilograms-force
kilogram-force metres (energy)
kilogram-force metres (torque)
kilogram-force metres per hour etc.
kilogram-force per area
kilograms
kilograms per area
kilograms per metre
kilograms per volume
kilojoules
kilojoules per hour etc.
kilometres
kilometres per hour etc.
kilometres per litre
kilonewton per square metre
kilonewtons
kilopascals
kilowatts
kilowatt hours
kips (weight)
kips (force)
kips per square inch
knots

L

leagues
light years
links (surveyors')
litres
litres per area

M

Mach number
megajoules
meganewtons
meganewtons per square metre
megawatts
metres
metres of water
metres per second etc.
microns (=micrometres)
miles
miles per gallon
miles per hour etc.
millibars
milligrams per cm
milligrams per (any volume)
millilitres
millimetres of mercury or water
millimetres of rain (by mass)
millimetres of rain (by volume)

N

newton metres (energy)
newton metres (torque)
newtons (per area)
newtons (force)
newtons (weight)

O

ounces
ounces per inch
ounces per area
ounces per volume

PQ

parsecs
pascals
perch (=rods or poles)
picas
pints
points (printers')
poundals
poundals per square foot
pounds
pounds per area
pounds per foot
pounds per volume
pounds-force
pound-force inches
pounds-force per area
quarts

R

roods

S

slugs (or g-pounds)
stones
square (+ any units)
squares (of timber)
sthenes

T

tex
therms
tonnes
ton-force metres
tonnes-force
tonnes-force per area
tonnes per hectare
tonnes per km
tonnes per volume
ton-force feet
tons
tons-force
tons-force per area
tons per acre
tons per mile
tons per volume
townships
troy ounce

UVW

watt second
watt hours
watts

XYZ

yards  
yards per hour etc.

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Length

The S I unit of length is the metre. To change any of these other units of length into their equivalent values in metres use the operation and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree of accuracy. Where some uncertainty is indicated it means that a good idea of the size of the unit can be given but that a better value would depend upon knowing the period and/or culture in which the unit was being used.

			angstroms		divide by 10 000 000 000 #
			astronomical units	x 149 598 550 000 
			barleycorns		x 0.008 467	
			centimetres		x 0.01 #
			chains (surveyors')	x 20.1168 #
			cubits			x (0.45 to 0.5)
			ells (UK)		x 0.875 (but many variations)
			ems (pica)		x 0.004 233 3
			fathoms			x 1.8288 #

			feet (UK and US)	x 0.3048 #
			feet (US survey)	x 0.3048 006 096
			furlongs		x 201.168 #
			hands			x 0.106 #
			inches			x 0.0254 #
			kilometres		x 1000 #
			leagues			x (4000 to 5000)
			light years		x 9 460 500 000 000 000
			links (surveyors')	x 0.201 168 #
			
			metres [m]		1
			
			microns (=micrometres)	x 0.000 001 #
			miles (UK and US)	x 1609.344 #
			miles (nautical)	x 1852 #
			parsecs			x 30 856 770 000 000 000
			perch (=rods or poles)	x 5.0292 #
			picas			x 0.004 233 333
			points (printers')	x 0.000 352 777
			yards			x 0.9144 #
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Area

The S I unit of area is the square metre. To change any of these other units of area into their equivalent values in square metres use the operation and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree of accuracy. Where some uncertainty is indicated it means that a good idea of the size of the unit can be given but that a better value would depend upon knowing the period and/or culture in which the unit was being used.

			acres			x 4046.856 422 4 #
			ares			x 100 #
			circular inches		x 0.000 506 707 479		
			hectares		x 10 000 #
			hides			x 485 000 (with wide variations)
			roods			x 1011.714 105 6 #
			square centimetres	x 0.000 1 #
			square feet (UK and US)	x 0.092 903 04 #
			square feet (US survey)	x 0.092 903 411 613
			square inches		x 0.000 645 16 #
			square kilometres	x 1 000 000 #
			
			square metres		1
			
			square miles		x 2 589 988.110 336 #
			square millimetres	x 0.000 001 #
			squares (of timber)	x 9.290 304 #
			square rods (or poles)	x 25.292 852 64 #	
			square yards		x 0.836 127 36 #
			townships		x 93 239 571.972
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Volume or Capacity

The S I unit of volume is the cubic metre. However, this seems to be much less used than the litre (1000 litres = 1 cubic metre).To change any of these other units of volume into their equivalent values in litres use the operation and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree of accuracy.

			barrels (oil)		x 158.987 294 928 #
			bushels (UK)		x 36.368 72 #
			bushels, dry (US)	x 35.239 070 166 88 #
			centilitres		x 0.01 #
			cubic centimetres	x 0.001 #
			cubic decimetres	1
			cubic dekametres	x 1 000 000 #
			cubic feet	 	x 28.316 846 592 #
			cubic inches	 	x 0.016 387 064 #

			cubic metres	 	x 1000 #
			cubic millimetres	x 0.000 001 #
			cubic yards	 	x 764.554 857 984 #
			decilitres	 	x 0.1 #
			fluid ounces (UK)	x 0.028 413 062 5 #
			fluid ounces (US)	x 0.029 573 534 562 5 #
			gallons (UK)	 	x 4.546 09 #
			gallons, dry (US)	x 4.404 883 770 86 #
			gallons, liquid (US)	x 3.785 411 784 #
			
			litres [l or L]		1
			
			millitres		x 0.001 #
			pints (UK)		x 0.568 261 25 #
			pints, dry (US)		x 0.550 610 471 357 5 #
			pints, liquid (US)	x 0.473 176 473 #
			quarts (UK)		x 1.136 522 5 #
			quarts, dry (US)	x 1.101 220 942 715 #
			quarts, liquid (US)	x 0.946 352 946 #
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Mass (or Weight)

The S I unit of mass is the kilogram. To change any of these other units of mass into their equivalent values in kilograms use the operation and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree of accuracy.

 			carats, metric		x 0.000 2 #
			grains			x 0.000 064 798 91 #
			grams			x 0.001 #
			hundredweights, long	x 50.802 345 44 #
			hundredweights, short	x 45.359 237 #
			
			kilograms [kg]		1
			
			kips			x 453.592 37 #
			newtons			divide by 9.806 65 #
			ounces, avoirdupois	x 0.028 349 523 125 #
			ounces, troy		x 0.031 103 476 8 #
			pounds			x 0.453 592 37 #
			slugs (or g-pounds)	x 0.068 521
			stones			x 6.350 293 18 #
			tons (UK or long)	x 1016.046 908 8 #
			tons (US or short)	x 907.184 74 #
			tonnes			x 1000 #
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Line density

Line density is a measure of mass per unit length. The S I compatible unit of line density is kilograms/metre. A major use of line density is in the textile industry to indicate the coarseness of a yarn or fibre. For that purpose the SI unit is rather large so the preferred unit there is the tex. (1 tex = 1 gram/kilometre) To change any of these other units of line density into their equivalent values in kilograms/metre use the operation and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree of accuracy.

 			denier			divide by  9 000 000 #
			drex			divide by 10 000 000 #
			grams/centimetre	divide by 10 #
			grams/kilometre (tex)	divide by  1 000 000 #
			grams/metre		divide by 1000 #
			grams/millimetre	1
			kilograms/kilometre	divide by 1000 #

			kilograms/metre 	1

			milligrams/centimetre	divide by 10 000 #
			milligrams/millimetre	divide by   1000 #
			ounces/inch		x 1.116 125
			ounces/foot		x 0.093 01
			pounds/inch		x 17.858
			pounds/foot		x 1.488 164
			pounds/yard		x 0.496 055
			pounds/mile		x 0.000 281 849
			tex			divide by 1 000 000 #
			tons(UK)/mile		x 0.631 342
			tons(US)/mile		x 0.563 698
			tonnes/kilometre	1
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Density

Density is the shortened term generally used in place of the more accurate description volumetric density.It is a measure of mass per unit volume. The S I compatible unit of density is kilograms/cubic metre. However, this a rather large unit for most purposes (iron is over 7000, wood is about 600 and even cork is over 200). A much more useful size of unit is kilograms/litre (for which the previous values then become 7, 0.6 and 0.2 respectively). This unit also has the great advantage of being numerically unchanged for grams/cubic centimetre and tonnes/cubic metre. To change any of these other units of density into their equivalent values in kilograms/litre use the operation and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree of accuracy.

 			grains/gallon(UK)	divide by  70 156
			grains/gallon(US)	divide by  58 418
			grams/cubic centimetre	1
			grams/litre		divide by 1000 #
			grams/millilitre	1
			kilograms/cubic metre	divide by 1000 #
			milligrams/millitre	divide by 1000 #
			milligrams/litre	divide by 1 000 000 #
			
			kilograms/litre 	1

			ounces/cubic inch	x 1.729 994 044
			ounces/gallon(UK)	x 0.006 236 023
			ounces/gallon(US)	x 
			pounds/cubic inch	x 27.679 904
			pounds/cubic foot	x 0.016 018 463
			pounds/gallon(UK)	x 0.099 776 373
			pounds/gallon(US)	x 0.119 826 427
			tonnes/cubic metre	1
			tons(UK)/cubic yard	x 1.328 939 184
			tons(US)/cubic yard	x 1.186 552 843
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Energy or work

The S I unit of energy or work is the joule. To change any of these other units of energy or work into their equivalent values in joules use the operation and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree of accuracy.

			British thermal units	divide by 1055.056 #
			calories		divide by 4.1868 #
			centigrade heat units	x 1900.4
			ergs			divide by 10 000 000 #
			foot pounds-force	x 1.355 817
			foot poundals		x 0.042 140
			gigajoules [GJ]		x 1000 000 000 #
			horsepower hours	x 2 684 520

			joules [J]		1

			kilocalories		divide by 4186.8 #
			kilogram-force metres	x 9.806 65 #
			kilojoules [kJ]		x 1000 #
			kilowatt hours [kWh]	x 3 600 000 #
			megajoules [MJ]		x 1 000 000 #
			newton metres [Nm]	x 1 #
			therms			x 105 575 000 #
			watt seconds [Ws]	1 
			watt hours [Wh]		x 3600 #
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Force

The S I unit of force is the newton. To change any of these other units of force into their equivalent values in newtons use the operation and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree of accuracy.

 			dynes			divide by 100 000 #
			kilograms force		x 9.806 65 #
			kilonewtons [kN]	x 1000 #
			kips			x 4448.222
			meganewtons [MN]	x 1 000 000 #

			newtons [N]		1

			pounds force		x 4.448 222
			poundals		x 0.138 255
			sthenes (=kN)		x 1000
			tonnes force		x 9806.65 #
			tons(UK) force		x 9964.016
			tons(US) force		x 8896.443
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Fuel Consumption

Fuel consumption of any means of transport (car, aeroplane, ship etc.) that uses fuel is a measure giving the relationship between the distance travelled for an amount of fuel used. The most common example is the car where it is usually expressed (in English-speaking countries) in miles per gallon.
It could also be expressed in gallons per mile. However, for a car the latter method gives a rather small figure: 35 miles per gallon is about 0.0286 gallons per mile. In that case it would be better to give a figure for 100 miles, so it would be 2.86 gallons per 100 miles. That is the metric way of expressing fuel consumption - as litres per 100 kilometres.
The rules below give a way of converting some of these different forms of measurement of fuel consumption. All the values are given to an accuracy of 4 significant figures.


To change 			into
	miles per gallon (UK)		miles per gallon (US) multiply by 0.833
	miles per gallon (UK)		miles per litre multiply by 0.22
	miles per litre			miles per gallon (UK) multiply by 4.456
	miles per gallon (UK)		kilometres per litre multiply by 0.354

	miles per gallon (US)		miles per gallon (UK) multiply by 1.2
	miles per gallon (US)		miles per litre multiply by 0.2642
	miles per litre			miles per gallon (US) multiply by 3.785
	miles per gallon (US)		kilometres per litre multiply by 0.4251

	X miles per gallon	        gallons per 100 miles, divide 100 by X
					(both gallons must of the same type)

	X miles per gallon (UK)	        litres per 100 miles, divide 282.5 by X
	X miles per gallon (US)		litres per 100 miles, divide 235.2 by X	
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Power

The S I unit of power is the watt. To change any of these other units of energy or work into their equivalent values in watts use the operation and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree of accuracy.

			Btu/hour		x 0.293 071
			Btu/minute		x 17.584 263
			Btu/second		x 1055.055
			calories/hour		x 0.001 639
			calories/minute	 	x 0.069 78
			calories/second		x 4.1868
			ft lb-force/minute	x 0.022 597
			ft lb-force/second	x 1.355 82
			gigawatts [GW]		x 1 000 000 000
			horsepower		x 745.7

			watts [W]		1

			joules/hour		divide by 3600 #
			joules/minute		divide by 60 #
			joules/second		1
			kilocalories/hour	x 1.163
			kilocalories/minute	x 69.78
			kg-force metres/hour	x 0.002 724
			kg-force metres/minute	x 0.163 444
			kiowatts [kW]		x 1000 #
			megawatts [MW]		x 1 000 000 #
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Pressure or Stress

The S I unit of pressure is the pascal. The units of pressure are defined in the same way as those for stress - force/unit area. To change any of these other units of pressure (or stress) into their equivalent values in pascals use the operation and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree of accuracy.

 			atmospheres		x 101 325
			bars			x 100 000 #
			centimetres of mercury	x 1333.22
			centimetres of water	x 98.066 5 #
			feet of water		x 2989.066 92 #
			inches of water		x 249.088 91 #
			inches of mercury	x 3386.388
			kg-force/sq.centimetre	x 98 066 5 #
			kg-force/sq.metre	x 9.806 65 #
			kilonewton/sq.metre	x 1000 #
			kilopascal [kPa]	x 1000 #
			kips/sq.inch		x 6 894 760
			meganewtons/sq.metre	x 1 000 000 #
			metres of water		x 9806.65 #
			millibars		x 100 #

			pascals [Pa]		1

			millimetres of mercury	x 133.322
			millimetres of water	x 9.806 65 #
			newtons/sq.centimetre	x 10 000
			newtons/sq.metre	1
			newtons/sq.millimetre	x 1 000 000 #
			pounds-force/sq.foot	x 47.880
			pounds-force/sq.inch	x 6894.757
			poundals/sq.foot	x 1.448 16
			tons(UK)-force/sq.foot	x 107 251
			tons(UK)-force/sq.inch	x 15 444 256
			tons(US)-force/sq.foot	x 95 760
			tons(US)-force/sq.inch	x 13 789 500
			tonnes-force/sq.cm	x 98 066 500 #
			tonnes-force/sq.metre	x 9806.65 #	
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Speed

The S I compatible unit of speed is metres/second. To change any of these other units of speed into their equivalent values in metres/second use the operation and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree of accuracy.

 			centimetres/minute	divide by 6000 #
			centimetres/second	divide by 100 #
			feet/hour		divide by 11 811
			feet/minute		x 0.0005 08 #
			feet/second		x 0.3048 #
			inches/minute		divide by 2362.2
			inches/second		x 0.0254 #
			kilometres/hour		x 3.6 #
			kilometres/second	x 1000 #
			knots			x 0.514 444
			Mach number		x 331.5
			metres/hour		divide by 3600 #
			metres/minute		divide by 60 #
			
			metres/second [m/s]	1

			miles/hour		x 0.447 04 #
			miles/minute		x 26.8224 #
			miles/second		x 1609.344 #
			yards/hour		divide by 3937
			yards/minute		x 0.015 24
			yards/second		x 0.9144 #
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Spread Rate (by mass)

The spread rate of a substance is a measure of how much of it there is covering a unit area. The 'how much' can be measured by volume or by mass. The S I compatible unit of spread rate by mass is kilograms/square metre. It is also a measure of area density (mass/unit area) and is similar to - but not the same as - pressure, which is force/unit area. For the rainfall conversions a density of 1 kg/litre has been assumed. To change any of these other units of spread rate into their equivalent values in kilograms/square metre use the operation and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree of accuracy.

 			grams/sq.centimetre	x 10 #
			grams/sq.metre		divide by 1000 #
			inches of rainfall	x 2.54
			kilograms/hectare	divide by 10 000 #
			kilograms/sq.centimetre	x 10 000 #
			milligrams/sq.metre	divide by 1000 #
			millimetres of rainfall	1

			kilograms/sq.metre 	1

			ounces/sq.foot		x 0.305 152
			ounces/sq.inch		x 43.942
			ounces/sq.yard		divide by 49.494
			pounds/acre		divide by 8921.791
			pounds/sq.foot		x 4.882 428
			pounds/sq.inch		x 703.07
			pounds/sq.yard		x 0.542 492
			tonnes/hectare		divide by 10 #
			tons(UK)/acre		divide by 3.982 942
			tons(US)/acre		divide by 4.460 896
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Spread Rate (by volume)

The spread rate of a substance is a measure of how much of it there is covering a unit area. The 'how much' can be measured by volume or by mass. The S I compatible unit of spread rate by volume is cubic metres/square metre. However, this is a rather large unit for most purposes and so litres/square metre is often preferred. To change any of these other units of spread rate into their equivalent values in litres/square metre use the operation and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree of accuracy.

 			cubic feet/acre		divide by 142.913
			cubic inches/sq.yard	divide by 51.024
			cubic yards/sq.mile	divide by 3387.577
			cubic metres/hectare	divide by 10 #
			cubic metres/sq.km	divide by 1000 #
			cubic metres/sq.metre	x 1000 #
			fl. ounces(UK)/sq.yard	divide by 29.428

			litres/square metre 	1

			gallons(UK)/acre	divide by 890.184
			gallons(US)/acre	divide by 1069.066
			gallons(UK)/hectare	divide by 2199.692
			gallons(US)/hectare	divide by 2641.721
			inches of rainfall	x 25.4 #
			litres/hectare		divide by 10 000 #
			millitres/sq.metre	divide by 1000 #
			millimetres of rainfall	1
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Torque

The S I compatible unit of torque is the newton metre. To change any of these other units of torque into their equivalent values in newton metres use the operation and conversion factor given. Those marked with # are exact. Other values are given to an appropriate degree of accuracy.

 			ergs (= dyne cm)	divide by 10 000 000 #
			gram-force centimetres	x 0.000 098 066 5 #
			kg-force centimetres	x 0.098 066 5 #
			kg-force metres		x 9.806 5 #
			newton centimetres	divide by 100 #
			
			newton metres [Nm]	1

			ounce-force inches	divide by 141.612
			pound-force inches	x 0.112 984
			pound-force feet	x 1.355 818
			poundal feet		x 0.042 140
			ton(UK)-force feet	x 3 037.032
			ton(US)-force feet	x 2 711.636
			tonne-force metres	x 9 806.65 #
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Other Sources

Conversion Tables of Units for Science and Engineering by Ari L Horvath
Macmillan Reference Books, London, 1986 (147 pages)
ISBN 0 333 40857 8
Probably the most comprehensive set of conversion factors in print, covering both old and modern units. There are 77 tables covering categories from Length to Radiation dosage. The Length table alone lists 107 units together with the conversion factors needed to change each one into metres.

The Dent Dictionary of Measurement by Darton and Clark
J M Dent, London, 1994 (538 pages)
ISBN 0 460 861379
Very comprehensive coverage of all kinds of units (including currencies), ordered in conventional dictionary form, and giving several conversion factors.

The Economist Desk Companion
Random Century, London, 1992 (272 pages)
ISBN 0 7126 9816 7
A handy compendium of units used in Science, Medicine, Engineering, Industry, Commerce, Finance and many other places, together with all the necessary conversion factors. There is also much other incidental (but related) information.

The Encyclopaedia Britannica
The modern E B has many references to units, but extensive use needs to be made of the index to find them all. It gives a wide selection of weights and measures from countries around the world and the appropriate conversion factors.

World Weights and Measures
Statistical Office of the United Nations, New York 1955 (225 pages)
A very comprehensive survey of each country in the world (as it was then) from Aden to Zanzibar, giving the units used in each for Length, Area and Capacity with their British and Metric equivalents. There is an appendix on the measures used for selected commodities. Currencies are also given. The indexes are very thorough.

The Weights and Measures of England by R D Connor
H M S O, London, 1987 (422 pages)
ISBN 0 460 86137 9
A scholarly and detailed account of the history of the development of the British (Imperial) system of weights and measures from the earliest times.

British Weights and Measures by R E Zupko
A history from Antiquity to the Seventeenth Century
The University of Wisconsin Press, 1977 [248 pages]
ISBN 0 299 07340 8
The actual history occupies only 100 pages. There is then an extensive list of the various units used in commerce, tables of many pre-Imperial units, a long list of pre-metric measures used in Europe together with their British and metric equivalents, and nearly 40 pages giving other sources.

The World of Measurements by H Arthur Klein
Allen and Unwin, London, 1975 (736 pages)
ISBN 0 04 500024 7
A very readable and comprehensive account of the history of units used in measuring, from the earliest known beginnings and around the world.

International Standards Organisation [I S O]

www.iso.ch

This organisation (based in switzerland) is responsible for the world-wide publication of standards for just about anything for which standards can be set.


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Last Updated: January
26, 2001