The Ounce Explained
Four different systems, one confusing word. Here is everything you need to know about the ounce.
Not all ounces are equal
An avoirdupois ounce (28.35 g) is lighter than a troy ounce (31.10 g). A fluid ounce measures volume, not weight. Mixing them up can be expensive — or dangerous.
Avoirdupois Ounce
The everyday ounce. When a recipe says "8 oz of flour" or a package lists its weight, this is the ounce in play. It is 1/16 of an avoirdupois pound (453.59 g). The name comes from the Old French "avoir de pois" — goods of weight.
Troy Ounce
Heavier than the avoirdupois ounce by about 10%. The troy ounce is the standard for precious metals worldwide. When you hear "gold is $1,920 per ounce," that's a troy ounce. The system is traditionally linked to the medieval trade fairs of Troyes, France.
Fluid Ounce
A unit of volume, not weight. The US fluid ounce equals 29.5735 mL; the Imperial (British) fluid ounce equals 28.4131 mL. Confusingly, a US fluid ounce is larger than an Imperial one, but the US gallon (128 fl oz) is smaller than the Imperial gallon (160 Imp fl oz).
Apothecary Ounce
Historically used by pharmacists for compounding medicines. The apothecary ounce has the same mass as the troy ounce but uses different subdivisions: it divides into 8 drams, and each dram into 3 scruples (about 1.296 g each). The system was officially phased out in favor of metric in most countries by the mid-20th century.
Side-by-Side Comparison
| Type | System | Grams | Grains | Measures |
|---|---|---|---|---|
| Avoirdupois oz | Avoirdupois | 28.3495 | 437.5 | Mass |
| Troy oz | Troy | 31.1035 | 480 | Mass |
| Apothecary oz | Apothecary | 31.1035 | 480 | Mass |
| US Fluid oz | US Customary | 29.5735 mL | — | Volume |
| Imperial Fluid oz | Imperial | 28.4131 mL | — | Volume |
The Water Coincidence
One US fluid ounce of pure water at room temperature weighs approximately 1.043 avoirdupois ounces — or about 29.57 grams. This near-parity is not an accident: the avoirdupois system was historically calibrated so that common measures of water would produce round, easy-to-remember weight equivalences.
This is why many people assume fluid ounces and weight ounces are interchangeable — for water, they nearly are. But for denser liquids like honey (density ~1.42 g/mL), 1 fluid ounce weighs about 1.48 avoirdupois ounces. For lighter liquids like olive oil (~0.92 g/mL), it weighs only about 0.96 avoirdupois ounces.
Fluid Oz → Weight Calculator
Enter fluid ounces and pick a liquid to see how much it weighs.