Fluid Ounce (fl oz)

The fluid ounce measures volume, not weight. In the US it is the unit on drink cans, juice cartons, bar jiggers and liquid measuring cups.

The US and British fluid ounces come from different gallons. NIST explains that the British quart is divided into 40 fluid ounces and the US quart into 32, so the imperial fluid ounce ends up smaller even though the imperial pint is bigger (NIST Handbook 44, Appendix B).

The imperial ounce also kept a neat link to weight. Britain's imperial gallon of 1824 was based on 10 pounds of water and holds 160 fluid ounces, so an imperial fluid ounce of water weighs almost exactly an ounce (1.002 ounces); the US fluid ounce never had that match.

US food labels give liquids in fluid ounces, and federal rules even set the temperature for the measurement: 68 °F (20 °C) for most foods and 40 °F (4 °C) for refrigerated ones (21 CFR 101.7). Liquids expand slightly as they warm, which is why a reference temperature matters.

For Nutrition Facts serving sizes, a fluid ounce is rounded to 30 ml (21 CFR 101.9). That is why a label may pair 8 fl oz with 240 ml instead of 237 ml.

Use a tablespoon: 2 tablespoons make a fluid ounce, so 3 fl oz is 6 tablespoons. For bigger amounts, a 1/2 cup measure holds 4 fluid ounces.

A kitchen scale works for water: each US fluid ounce of water weighs about 29.6 grams, so 4 fl oz shows up as about 118 grams. Bar jiggers are usually marked in fluid ounces too; for drink sizes, see how many ounces are in a shot.