Volume Converter

A US pint is 473 ml and a UK pint is 568 ml. Recipes and fuel figures both go wrong when that is missed.

Two gallons, two pints, two fluid ounces

The imperial and US customary systems share names and not values, which makes volume the most error-prone of all the common conversions.

Note the reversal on the last row. The imperial gallon is larger, but the imperial fluid ounce is smaller — because an imperial gallon is divided into 160 fluid ounces while a US gallon is divided into 128. The imperial system was built around the fluid ounce of water weighing one ounce; the US system descends from the older English wine gallon.

This is why a US car's fuel economy in mpg looks better than a UK car's for identical consumption, and why halving a US recipe using a UK measuring jug produces the wrong result.

Cups and cooking measures

The cup is worse still, because there are at least four:

• US customary cup: 236.6 ml (8 US fl oz) — the one in American recipes.
• US legal cup: 240 ml, used on nutrition labels.
• Metric cup: 250 ml, standard in Australia, New Zealand and Canada.
• Imperial cup: 284 ml, largely historical.

The deeper problem with cups is that they measure volume while baking depends on mass. A cup of flour can weigh anywhere from 120 to 150 grams depending on whether it was scooped, spooned or sifted — a 25 % variation that ruins bread and pastry. Serious baking recipes give grams for precisely this reason.

Handy reference points

The relationship between the litre and the kilogram is not a coincidence: the litre was originally defined as the volume of one kilogram of water at maximum density. That definition was abandoned in 1964 in favour of exactly one cubic decimetre, but the two differ by only 28 parts per million, so “a millilitre of water weighs a gram” remains reliable for any practical purpose.