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Comparison of the Imperial and US customary systems facts for kids

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Both the imperial and United States customary systems of measurement derive from earlier English systems used in the Middle Ages, that were the result of a combination of the local Anglo-Saxon units inherited from German tribes and Roman units brought by William the Conqueror after the Norman Conquest of England in 1066.

Having this shared heritage, the two systems are quite similar, but there are differences. The US customary system is based on English systems of the 18th century, while the Imperial system was defined in 1824, after American independence.


The main difference is in units of volume. The American system has two gallons: a wet and a dry one. The imperial gallon is bigger than each of these. However, the imperial fluid ounce is slightly smaller than the American one.

Comparison of Imperial and U.S. volume measures
1 U.S. fluid ounce = 29.573 529 562 5 mililitres ≈ 1.041 Imperial fluid ounces
1 Imperial fluid ounce = 28.413 062 5 mililitres ≈ 0.961 U.S. fluid ounce
1 liquid U.S. gallon = 3.785 411 784 litres ≈ 0.833 Imperial gallon
1 Imperial gallon = 4.546 09 litres ≈ 1.201 liquid U.S. gallons
1 dry U.S. gallon = 4.404 842 803 2 litres ≈ 0.968 Imperial gallon
1 Imperial gallon = 4.546 09 litres ≈ 1.032 dry U.S. gallons


A baby bottle with measurements in metric, imperial and US customary

The international yard is exactly 0.9144 metres. This definition was agreed upon by the United States, Canada, the United Kingdom, South Africa, Australia and New Zealand in 1959. However, the United States continued to use its old length units for surveying purposes. The US survey foot is exactly 36/39.37 metre.


A one-US-gallon petrol can purchased near the US–Canada border showing equivalents in imperial gallons and litres

Another difference is in units of mass. In Britain people often use the stone. One stone is 14 pounds. Eight stones are one hundredweight (112 lb). Twenty hundredweights are one ton. So one British ton is 2240 pounds. It is very close to the tonne of the metric system.

In the United States people do not use the stone. They usually use a hundredweight of 100 pounds. This hundredweight is also called a short hundredweight. Twenty short hundredweights are one short ton. The British hundredweight is also called the long hundredweight and the Britsh ton is also called the long ton.

Comparison of current imperial, US and metric volume measures
  • Approximate values are denoted with ≈
  • Exact values are denoted with ≡
  • Definitions are marked in bold
Unit name Imperial measures US fluid measures US dry measures Metric measures

fluid ounces
Imperial fluid ounce
(fl oz)

1 fl oz

0.96075994040 fl oz
0.94710208333 fl oz (food)

28.4130625 mL
0.0284130625 L

US fluid ounce
(fl oz)

1.04084273079 fl oz

1 fl oz
0.98578431875 fl oz (food)

29.5735295625 mL
0.0295735295625 L

US fluid ounce
(food nutrition labelling)
(fl oz) (food)

1.05585239184 fl oz

1.01442068106 fl oz
1 fl oz (food)

30 mL
0.03 L

Imperial pint

20 fl oz
1 imp pt
0.5 imp qt
0.125 imp gal

19.2151988081 fl oz
18.9420416667 fl oz (food)
1.20094992550 US pt
0.15011874069 US gal

1.03205674349 US dry pt
0.12900709294 US dry gal

568.26125 mL
0.56826125 L

US liquid pint

16.6534836926 imp fl oz
0.83267418463 imp pt
0.10408427308 imp gal

16 US fl oz
15.7725491 fl oz (food)
1 US pt
0.5 US qt
0.125 US gal

0.859367007375 US dry pt
0.107420875922 US dry gal

473.176473 mL
0.473176473 L

US dry pint

19.3787794384 imp fl oz
0.96893897192 imp pt
0.12111737149 imp gal

18.6183549784 US fl oz
18.3536823786 US fl oz (food)
1.16364718615 US pt
0.14545589827 US gal

1 dry pint
≡ 0.5 dry quart
≡ 0.125 dry gallon

550.6104713575 mL
0.5506104713575 L

Imperial quart

40 imp fl oz
1 imp qt
2 imp pt
0.25 imp gal

38.4303976162 US fl oz
37.8840833333 US fl oz (food)
1.20094992550 US qt
0.30023748138 US gal

1.03205674349 US dry qt
0.25801418587 US dry gal

1136.5225 mL
1.1365225 L

US liquid quart

33.3069673852 imp fl oz
0.83267418463 imp qt
0.20816854616 imp gal

32 US fl oz
31.5450982 US fl oz (food)
2 US pt
1 US qt
0.25 US gal

0.859367007375 US dry qt
0.214841751844 US dry gal

946.352946 mL
0.946352946 L

US dry quart

38.7575588768 imp fl oz
0.96893897192 imp qt
0.24223474298 imp gal

37.2367099567 US fl oz
36.7073647572 US fl oz (food)
1.16364718615 US qt
0.29091179654 US gal

1 US dry qt
≡ 2 US dry pt
≡ 0.25 US dry gal

1101.220942715 mL
1.101220942715 L

Imperial gallon

160 imp fl oz
4 imp qt
1 imp gal

153.721590465 US fl oz
151.536333333 US fl oz (food)
1.20094992550 US gal

4.12822697395 US dry qt
1.03205674349 US dry gal

4546.09 mL
4.54609 L

US liquid gallon

133.227869541 imp fl oz
3.33069673852 imp qt
0.83267418463 imp gal

128 US fl oz
126.1803928 US fl oz (food)
4 US qt
1 US gal
231 cu inches

3.437468029501 US dry qt
0.859367007375 US dry gal

3785.411784 mL
3.785411784 L

US dry gallon

155.030235507 imp fl oz
3.87575588768 imp qt
0.96893897192 imp gal

148.946839827 US fl oz
146.829459029 US fl oz (food)
4.6545887446 US qt
1.16364718615 US gal

4 US dry qt
1 US dry gal
268.8025 cubic inches

4404.88377086 mL
4.40488377086 L

(l or L or dm3)

35.1950797279 imp fl oz
0.87987699320 imp qt
0.21996924830 imp gal

33.8140227018 US fl oz
33.3333333333 US fl oz (food)
1.05668820943 US qt
0.26417205236 US gal

0.90808298427 US dry qt
0.22702074607 US dry gal

1000 mL
1 L

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