The imperial system of units defines the historic unit system of the UK and Commonwealth, as laid out in the Weights and Measures Act 1824. Since 1985, UK government redefined the imperial system of units in terms of SI units (Weights and Measures Act, 1985). This completed the movement from the imperial system of units, to the SI system of units in the UK. Consequently all modern technical and scientific work should be undertaken in SI units, and the data in this section should be of historic interest only.
However you will find many of these units in common use within the UK, especially in relation to civil engineering and building standards. Also note that despite sharing common names, and history, the units defined in the customary system of the United States may be different to the ‘same’ British imperial units.
All data in this section derives from the The Units of Measurement Regulations (1995), as the latest statutory instrument defining the relationship between SI and British imperial units. Note that later statutory instruments update the definition of the SI units metre and kilogram, but do not change the fundamental values.
Length
Imperial unit
Corresponding Metric Unit
Metric Equivalent
inch
centimetre
2.54 centimetres
hand
metre
0.1016 metre
foot
metre
0.3048 metre
yard
metre
0.9144 metre
fathom
metre
1.8288 metres
chain
metre,
20.1168 metres
furlong
kilometre
0.201168 kilometre
mile
kilometre
1.609344 kilometres
nautical mile (UK)
metre
1853 metres
Area
Imperial unit
Corresponding Metric Unit
Metric Equivalent
square inch
square centimetre
6.4516 square centimetres
square foot
square metre
0.09290304 square metre
square yard
square metre
0.83612736 square metre
rood
square metre
1011.7141056 square metres
acre
square metre
4046.8564224 square metres
square mile
square kilometre
2.589988110336 square kilometres
Capacity
Imperial unit
Corresponding Metric Unit
Metric Equivalent
fluid ounce
millilitre
28.4130625 millilitres
gill
litre
0.1420653125 litre
pint
litre
0.56826125 litre
quart
litre
1.1365225 litres
gallon
litre
4.54609 litres
Pressure
Imperial unit
Corresponding Metric Unit
Metric Equivalent
inch of water
pascal
249.08891 pascals
Mass
Imperial unit
Corresponding Metric Unit
Metric Equivalent
grain
gram
0.06479891 gram
dram
gram
1.7718451953125 grams
ounce (avoirdupois)
gram
28.349523125 grams
troy ounce
gram
31.1034768 grams
pound
kilogram
0.45359237 kilogram
stone
kilogram
6.35029318 kilograms
quarter
kilogram
12.70058636 kilograms
cental
kilogram
45.359237 kilograms
hundredweight
kilogram
50.80234544 kilograms
ton
tonne
1.0160469088 tonnes
Force
Imperial unit
Corresponding Metric Unit
Metric Equivalent
pound-force
newton
4.4482216152605 newtons
ton-force
kilonewton
9.96401641818352 kilonewtons
Volume
Imperial unit
Corresponding Metric Unit
Metric Equivalent
cubic inch
cubic centimetre
16.387064 cubic centimetres
cran
cubic decimetre
170.478375 cubic decimetres
cubic foot
cubic metre
0.028316846592 cubic metre
bushel
cubic metre
0.03636872 cubic metre
cubic yard
cubic metre
0.764554857984 cubic metre
Power
Imperial unit
Corresponding Metric Unit
Metric Equivalent
horsepower
kilowatt
0.74569987158227022 kilowatt
Temperature
Imperial unit
Corresponding Metric Unit
Metric Equivalent
degree Fahrenheit
degree Celsius
See Below
To convert degree Fahrenheit, \(°F\), into degree Celsius, \(°C\)
\[°C = \frac{\left( °F - 32 \right)}{1.8}\]
To convert degree Celsius, \(°C\), into degree Fahrenheit, \(°F\)