Volume & Capacity Converter

US gallon versus imperial gallon: a 20% gap

A US liquid gallon is exactly 3.785411784 litres. An imperial gallon is exactly 4.54609 litres. That is a difference of about 20%, and it propagates straight through quarts, pints and fluid ounces. A US pint is 473 mL; a UK pint is 568 mL. A recipe, a fuel-economy figure or a dosage converted with the wrong gallon is wrong by a fifth.

Worse, the fluid ounces do not even scale consistently: there are 16 US fluid ounces in a US pint but 20 imperial fluid ounces in an imperial pint. So a US fluid ounce (29.57 mL) is actually larger than an imperial fluid ounce (28.41 mL), even though every other US unit in the family is smaller. This is the one place where guessing from the pattern gives you the wrong direction.

Cups are the least standardised unit in the kitchen

A US customary cup is 8 US fluid ounces, or 236.588 mL. A metric cup, used in Australia, New Zealand and much of Europe, is exactly 250 mL. A US “legal” cup used on nutrition labels is 240 mL. Japanese recipes use 200 mL. Both the common ones are listed above; if a recipe does not say which, the author's country is usually the best clue.

Volume is not weight

A cup of flour and a cup of water do not weigh the same, so no volume converter can turn cups into grams without knowing what is in the cup. Flour ranges from about 120 g to 150 g per cup depending on whether it was scooped or spooned. If a baking recipe gives grams, use grams — that is the recipe author protecting you from exactly this problem.

Why the oil barrel is 42 gallons

The 42-US-gallon barrel is a nineteenth-century Pennsylvania convention that stuck: it was the size that two men could still roll, and it left headroom in a 40-gallon whiskey barrel for spillage. It has no metric logic at all, and it is still the unit that global oil prices are quoted in.

Frequently Asked Questions