Convert fluid ounces to milliliters

The formula

milliliters = fluid ounces × 29.57353

fluid ounces to milliliters — quick reference

An ounce of liquid and an ounce of weight are different things

The imperial fluid ounce was defined so that one fluid ounce of water weighs almost exactly one avoirdupois ounce — 28.35 grams against 28.41 millilitres. That near-identity is why the two units share a name, and it holds for water and for nothing else. Honey, oil, flour and spirits each have their own density, so eight fluid ounces of golden syrup weighs far more than eight ounces.

American recipes exploit the ambiguity without meaning to. Eight ounces of chocolate chips is a weight, 227 grams. Eight ounces of milk is a volume, 237 millilitres. The recipe rarely says which, and the convention breaks down on anything semi-liquid: yoghurt, soured cream, tinned tomatoes. The packet usually settles it, since American groceries carry net weight for solids and fluid ounces for liquids.

Which ounce, and when the four per cent bites

US fluid ounces are 29.5735 millilitres and imperial ones 28.4131 — about four per cent apart, with the imperial ounce the smaller despite the imperial gallon being the larger. Almost anything carrying fl oz today is American; British recipes have used millilitres for a generation, so an imperial reading is only likely in an old cookbook or a historical text.

At small volumes the difference is academic: a two-ounce measure of spirit shifts by two millilitres between systems, well inside the accuracy of pouring. It bites in bulk and in ratios. A brine or syrup specified at 32 fluid ounces moves by 35 ml between readings, and anything setting that liquid against a fixed weight of flour or sugar carries the error straight into the texture.

Questions