US Customary and Imperial Units

Where the two systems are identical

Most everyday measures are not in question. The 1959 international yard and pound agreement defines one yard as exactly 0.9144 m and one pound as exactly 0.45359237 kg, and both systems build on those two definitions. A mile is therefore 1,609.344 m in New York and in London alike, and a six-foot, 180-pound description means the same thing on either side.

Everything derived from the pound follows too. An ounce of weight is one sixteenth of a pound, so 28.3495 g in both systems, and a stone is 14 pounds, or 6.35029318 kg. The stones and pounds guide sets out how those two units nest together.

Because lengths and everyday weights agree, most imperial-versus-US confusion is narrower than it looks. It affects volume and large masses, and almost nothing else.

The gallon and the pint

Volume is where the two systems genuinely part company. Each has a gallon, a pint and a quart, but no two of those are the same size.

The US gallon is fixed by definition as 231 cubic inches, which is exactly 3.785411784 litres. The imperial gallon is exactly 4.54609 litres, so it is about 20.1% larger. One imperial gallon is 1.20095 US gallons, and one US gallon is 0.832674 imperial gallons.

Ten litres is a useful test case. It is 2.64 US gallons but only 2.2 imperial gallons, and the litres to gallons converter makes the difference concrete. Going the other way, five US gallons is 18.927 litres, so the gallons to litres converter returns a metric volume with no US-versus-imperial ambiguity attached to it.

Why the US fluid ounce is the larger one

The ounce sizes and the pint sizes point in opposite directions. The US fluid ounce is about 4% larger than the imperial fluid ounce, yet the US pint is the smaller of the two, because it is built from 16 fluid ounces while an imperial pint is built from 20. Sixteen US fluid ounces give 473.176 ml; twenty imperial fluid ounces give 568.261 ml. The different part count outweighs the different ounce.

A US cup adds a third variable: it is 8 US fluid ounces, or 236.588 ml, while the metric cup is 250 ml. A metric cup is not a rounded version of a US cup.

Two tons, and a third that means neither

Large masses diverge too, and the word "ton" is the trap. The US short ton is 2,000 pounds, the imperial long ton 2,240 pounds, and the metric tonne is 1,000 kg, a unit belonging to neither imperial family. All three appear in the weight converter, so the choice has to be made explicitly.

The long ton is the odd one out: it is about 1.6% heavier than a metric tonne, or roughly 35 pounds more, while the short ton is about 9% lighter. Whenever a document says "tonne", there is nothing to guess: it is 1,000 kg. When it says "ton", the country decides.

Which system is used where

The United Kingdom uses imperial for some consumer packaging and metric almost everywhere else. Milk and beer are sold by the imperial pint, road distances and speed limits are in miles and miles per hour, and fuel is sold by the litre. The United States uses US customary units across consumer trade: gallons for fuel, milk and paint, miles for distance, and short tons for freight. Canada and Australia are metric, so a litre quoted in either country is unambiguous.

The result is that both countries say "gallons" and neither means the other's. Speed is not affected: miles per hour is the same miles per hour on both sides, so only the volume half of a fuel economy figure needs checking.

Reading a recipe, a fuel figure or a label

A US recipe that calls for a cup means 236.588 ml, and one that calls for a fluid ounce means 29.5735 ml. It will also usually be in Fahrenheit, which is a separate scale and needs the temperature converter if your oven is marked in Celsius. A metric recipe's cup is 250 ml, so transferring either figure between the two systems means checking which kind of cup was meant, not just converting the number.

Fuel economy figures are ambiguous in exactly one direction. American miles per gallon is measured in US gallons, so a US figure of 30 mpg is about 36 UK mpg. Converted the other way, 40 UK mpg is about 33.3 US mpg. Dividing a UK figure by 1.2 gives the US equivalent, or multiplying a US figure by 1.2 gives the UK one.

On a label or a contract, three checks settle most cases: whether a volume is in gallons or litres, whether a "ton" is 2,000 or 2,240 pounds, and whether a fluid ounce is spelled out. When a number matters, the exact definitions behind these conversions are set out in the methodology definitions.

Ounces: weight or volume

An ounce on its own is normally a unit of weight, 28.3495 g. A fluid ounce is a unit of volume, and because the two systems' fluid ounces differ, the same abbreviation on a US label and a UK label denotes different volumes.

UnitUtils keeps these apart. An ounce in the weight converter is mass, and fluid ounces exist only in the volume converter, where the US customary and Imperial groups are listed separately. Converting a weight of an ingredient into a volume of that ingredient needs a density for that ingredient, and no density is implied by these converters.

Results are shown to six significant digits by default, so 473.176473 ml appears as 473.176 ml. That is more precision than most household measures justify, but it keeps a converted quantity intact when it is carried into the next calculation.