Litres, gallons, and the cup that is not the same cup

No, and the gap is big enough to matter. A US liquid gallon is exactly 3.785411784 litres; an imperial (UK) gallon is exactly 4.54609 litres, roughly 20% larger. This is why fuel economy figures do not travel: a car quoted at 40 miles per imperial gallon does about 33 miles per US gallon on identical fuel. The car did not change — the gallon did.

A US customary cup is defined as half a US pint, which works out at 236.588 ml. The metric cup used in Australia, New Zealand and most metric cookbooks is a round 250 ml. That is about 5.6% more. It makes no difference to stock or water, and a real difference to flour, sugar and rice, where a 5% error compounds across several cups. Both are in the list under their own names so you never have to guess which one a recipe meant.

Yes. A US liquid pint is 473 ml and an imperial pint is 568 ml, so a UK pint really is about a fifth larger. Quarts follow the pints, since a quart is two pints in both systems: 946 ml against 1,137 ml.

They differ as well, and in the direction most people do not expect: the imperial fluid ounce is the smaller one, at 28.413 ml against the US 29.574 ml. The imperial pint still comes out bigger because it holds 20 fluid ounces where the US pint holds 16. Two sizes of ounce and two sizes of pint, arranged so that the smaller ounce belongs to the larger pint.

Close enough to ignore. A US teaspoon is 4.929 ml and a metric teaspoon is a round 5 ml — under 1.5% apart, which no recipe notices. Tablespoons drift further: 14.787 ml in the US, 15 ml metric, and 20 ml in Australia. An Australian tablespoon of anything strong is a third more than a US one, so that is the one to check.

Exactly 1,000. A cubic metre is a box one metre along each edge; a litre is a cube 10 cm along each edge, and a thousand of those fill it. The same relationship makes one millilitre exactly one cubic centimetre, which is why engine capacities in cc and medicine doses in ml are the same measurement wearing different names.

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