Comparison of the Imperial and US customary systems

The imperial and U.S. customary systems are similar, but there are some differences between them.

Volume

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 millilitres	≈ 1.041 Imperial fluid ounces
1 Imperial fluid ounce	= 28.413 062 5 millilitres	≈ 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

Length

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 1200/3937 metre.

Weight

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.