One word, three different physical quantities
"Ounce" names three units that have no fixed ratio to one another: an avoirdupois ounce and a troy ounce are both mass, but different amounts of it; a fluid ounce is not mass at all, it is volume, and US and Imperial fluid ounces are themselves different sizes. A recipe that says "one ounce of chocolate, one ounce of vanilla extract" is quietly switching from a mass unit to a volume unit between the two ingredients, and a gold dealer's "ounce" is neither.
Avoirdupois: the everyday ounce
The ounce on a kitchen scale, a nutrition label or a body-weight reading is the avoirdupois ounce, defined as exactly 1/16 of an avoirdupois pound. Since the pound was fixed at exactly 0.45359237 kilograms by the international yard and pound agreement of 1 July 1959, the avoirdupois ounce is exact by the same act: 0.45359237 kg ÷ 16 = 0.028349523125 kg = 28.349523125 g, not one significant figure more or less. "Avoirdupois" comes from Old French aveir de peis, "goods of weight" — it was the trading-weight system for wool, meat and general merchandise, as distinct from the two systems below.
Troy: the ounce that survived because bullion needed it
The troy ounce is heavier: 31.1034768 g exactly, defined as 480 grains against the same 1959-derived grain used throughout English-speaking metrology (1 grain = 1/7,000 avoirdupois pound = 0.06479891 g exactly, so 480 × 0.06479891 g = 31.1034768 g). Troy weight predates avoirdupois in English commerce and once had its own pound, divided into 12 troy ounces rather than 16 avoirdupois ounces. The UK's Weights and Measures Act 1878 abolished the troy pound as a legal unit outright — but kept the troy ounce, specifically because the bullion and gemstone trade already used it and had no interest in switching. That carve-out is why gold, silver, platinum and palladium are still quoted in troy ounces on every commodities exchange today, while a bag of sugar next to them on a US shelf is quoted in avoirdupois ounces that are about 8.85% lighter.
Fluid: not mass at all
A fluid ounce measures volume, and inherits the same split as the rest of the customary system. The US fluid ounce is defined via the US gallon, fixed at exactly 231 cubic inches (3.785411784 L exactly, since the inch is 2.54 cm exactly): one US gallon ÷ 128 = 3.785411784 L ÷ 128 = 29.5735295625 mL, conventionally written 29.5735 mL. The Imperial fluid ounce runs off a different gallon: the UK's Weights and Measures Act 1985 fixed the Imperial gallon at exactly 4.54609 L, and one Imperial fluid ounce is 1/160 of that — 4.54609 L ÷ 160 = 28.4130625 mL, conventionally 28.4131 mL. Neither figure is a rounding of the other; they come from two independently defined gallons that happen to share a name.
The four numbers side by side
| Unit | Exact value | Defined from | Where it's used |
|---|---|---|---|
| Avoirdupois ounce (oz) | 28.349523125 g | 1/16 lb; lb = 0.45359237 kg (1959) | Food labels, body weight, general retail |
| Troy ounce (oz t) | 31.1034768 g | 480 grains (grain = 1/7,000 lb) | Gold, silver, platinum, gemstones |
| US fluid ounce (US fl oz) | 29.5735295625 mL | 1/128 US gallon (231 in³ exactly) | US recipes, US nutrition labels |
| Imperial fluid ounce (imp fl oz) | 28.4130625 mL | 1/160 Imperial gallon (4.54609 L exactly, 1985 Act) | Pre-metric UK/Commonwealth recipes, spirits measures |
The two fluid ounces differ from each other by (29.5735295625 − 28.4130625) ÷ 28.4130625 ≈ 4.08% — a gap large enough to matter in baking, where liquid ratios affect texture, but small enough that a cook who doesn't know two different fluid ounces exist has no reason to suspect it.
The kitchen consequence
An American recipe calling for "1 cup (8 fl oz) of milk" and a British one calling for "8 fl oz of milk" are not asking for the same amount of milk. Eight US fluid ounces is 8 × 29.5735295625 mL = 236.588 mL, which a recipe converter rounds to 237 mL. Eight Imperial fluid ounces is 8 × 28.4130625 mL = 227.30 mL, rounding to 227 mL. A baker in Boston and a baker in Birmingham, both following an "8 fl oz" instruction from their own country's convention, pour a 4% difference in milk before either of them opens a scale — small for a glass of milk, potentially noticeable in a hydration-sensitive bread dough scaled up to litres.
A coincidence that causes real confusion
Because water has a density close to 1 g/mL, one US fluid ounce of water weighs close to 29.57 g — not 28.35 g, the avoirdupois ounce. The two numbers are near enough (about 4.3% apart) that recipes written loosely, "an ounce of water," can pass unnoticed whichever unit was intended, while the same substitution with a denser liquid, honey at roughly 1.42 g/mL, or a lighter one, vegetable oil at roughly 0.92 g/mL, would produce a mass nowhere close to 28.35 g. The apparent near-equivalence for water is a coincidence of density, not a relationship between the units, and it does not hold for anything else in the kitchen.
Rule of thumb
Fluid ounce means volume; plain ounce means avoirdupois mass unless the item is a precious metal, in which case it means troy mass. Three definitions, three purposes, one overloaded word.