Convert US fluid ounces to imperial fluid ounces (fl oz to fl oz UK)

Because the two systems do not cut the gallon the same way. The US gallon, twenty per cent smaller, divides into 128 fluid ounces; the imperial one, larger, into 160. The division beats the size of the container: 3785.41 divided by 128 gives 29.57 milliliters, 4546.09 divided by 160 gives 28.41. The US ounce is therefore 4.1 per cent larger.

Twenty per cent, and this time the way you would expect. The US pint holds 16 fluid ounces, that is 473 milliliters; the imperial pint holds 20, that is 568. A pint of beer poured in London therefore holds nearly a fifth more than an American one, even though the British ounce is the smaller of the two.

The imperial one if the recipe is British, Irish or Australian, the American one if it comes from the United States or English-speaking Canada. The four per cent gap is harmless in a sauce, but tells in a batter or a pastry where the proportions hold together. Recent British books often print milliliters alongside, which settles the question.

Because they are two unrelated units sharing a name. The avoirdupois ounce weighs 28.35 grams, the US fluid ounce takes up 29.57 milliliters. Their values are close because of water, a given volume of which weighs about as much: the imperial fluid ounce was in fact defined as the volume of one ounce of water. For any other liquid the coincidence vanishes.

By the capacity printed next to it. An American can shows 12 fl oz and 355 ml, a British can 330 ml, a British milk bottle one pint and 568 ml. The milliliter always settles it: divide it by 29.57 for the US ounce, by 28.41 for the imperial one, and the round number tells you which system was used.