US customary is not imperial, and the difference is all in the volume
The two systems share their length and weight units exactly. An inch is 25.4 millimetres on both sides of the Atlantic, and a pound is 453.59237 grams in both. Volume is where they split, and they split badly.
| Unit | US customary | Imperial | Difference |
|---|---|---|---|
| Gallon | 3.785411784 L | 4.54609 L | Imperial is 20.1% larger |
| Quart | 0.946353 L | 1.136523 L | Same 20.1% |
| Pint | 0.473176 L (16 fl oz) | 0.568261 L (20 fl oz) | Same 20.1% |
| Fluid ounce | 29.5735 mL | 28.4131 mL | Imperial is 3.9% smaller |
The fluid ounce reverses direction, which is the part that catches people. The imperial gallon is bigger but its fluid ounce is smaller, because imperial divides its gallon into 160 fluid ounces while the US divides its into 128. So an imperial pint is 20 fluid ounces and a US pint is 16, and neither the pints nor the ounces match. A recipe calling for a pint of stock means 473 mL if it was written in the United States and 568 mL if it was written in Britain, and there is nothing in the text to tell you which.
Where this costs money: fuel economy. A car rated at 30 miles per US gallon is 36 miles per imperial gallon, the same car, the same fuel. Comparing a US figure against a UK figure without converting makes the British car look twenty percent more efficient than it is.
Fluid ounces and ounces are different quantities
A fluid ounce is a volume, about 29.6 millilitres. An ounce, the avoirdupois one, is a weight, 28.35 grams. They carry the same name for a historical reason — a fluid ounce of wine was once meant to weigh about an ounce — and that near-coincidence is why the confusion survives. For water the relationship holds roughly: one US fluid ounce weighs about 1.043 ounces. For anything denser or lighter it falls apart, and eight fluid ounces of flour weighs about 4.4 ounces, not 8. That single mistake is why baking recipes converted carelessly from volume to weight produce dense results.
There is a third ounce as well. The troy ounce, used for precious metals, is 31.1035 grams, about ten percent heavier than the avoirdupois ounce. A quoted gold price per ounce is per troy ounce. This converter uses avoirdupois throughout, which is the correct one for everything except bullion.
The three tons
A short ton is 2,000 pounds, or 907.185 kilograms, and it is what an American means by a ton. A long ton is 2,240 pounds, or 1,016.047 kilograms, which is the historical British ton still used in shipping and steel. A metric tonne is 1,000 kilograms, or 2,204.62 pounds. Written in text they are also easy to confuse, since a metric tonne is often spelled ton anyway.
Twelve percent between the extremes sounds small until it is applied to something being bought by the ton. On a hundred-ton order the difference between the short ton and the long ton is twelve tons of material, and on a bridge rating it is the difference between adequate and not. When a figure crosses a border, check which ton it left home as.
What to do when the number matters
Every factor in this converter is an exact legal definition rather than a rounded approximation — the inch is exactly 0.0254 metres by international agreement, the pound exactly 0.45359237 kilograms, the US gallon exactly 231 cubic inches. So the arithmetic here is as good as the number you type in. What it cannot do is tell you which unit a source meant. That ambiguity is where nearly all real conversion errors come from, not from the arithmetic. Before converting a figure that matters, establish where it was written and which system that place uses, and if it is a volume or a ton, assume nothing.
Questions people ask
How many litres in a gallon?
It depends whose gallon. The US liquid gallon is 3.785411784 litres exactly, defined as 231 cubic inches. The imperial gallon used in the UK and some Commonwealth countries is 4.54609 litres exactly, defined in 1985 as that round metric figure. The imperial gallon is 20.1 percent larger. There is also a US dry gallon, 4.405 litres, which survives in grain and produce measurement and appears nowhere else — if you are not measuring apples wholesale you will never meet it.
Is a fluid ounce the same as an ounce?
No. A fluid ounce measures volume and an ounce measures weight. A US fluid ounce is 29.5735 millilitres; an avoirdupois ounce is 28.3495 grams. They only line up for water, and even then not exactly — a US fluid ounce of water weighs about 1.043 ounces. For anything else the ratio moves with density: a fluid ounce of honey weighs roughly 1.5 ounces, of gasoline roughly 0.78. When a label says 16 oz it is usually a weight for solids and a volume for liquids, and you have to read the context.
What is the difference between a ton and a tonne?
A tonne with the extra letter is metric, 1,000 kilograms or 2,204.62 pounds. A ton without it is a short ton in the United States, 2,000 pounds or 907.185 kilograms, and a long ton in older British usage, 2,240 pounds or 1,016.05 kilograms. The spelling is not a reliable guide because American writing often uses ton for the metric one too. The safe habit is to look for the number: if a document says 2,000 lb it is short, 2,240 lb long, 1,000 kg metric.
Why do the results show so many decimal places?
Because the conversion factors are exact and truncating early hides real differences — the gap between a US and an imperial pint is only visible if you carry enough digits to see it. The displayed precision is not a claim about your measurement. If you measured a board to the nearest sixteenth of an inch, the converted millimetre figure is good to about 1.6 mm no matter how many digits appear after it. Round the answer to the precision your input deserved, which is almost always fewer digits than shown.