Two fluid ounces, one name
The US and the UK both kept the fluid ounce after their gallons went separate ways. The US gallon is 231 cubic inches, 3.785411784 litres, and it holds 128 US fluid ounces. The imperial gallon is 4.54609 litres, set by the UK Weights and Measures Act 1985, and its pint is divided into 20 fluid ounces instead of 16. The result is that a US fluid ounce is about 4% larger than a UK one, while a US pint (16 fl oz) is about 17% smaller than a UK pint (20 fl oz).
millilitres = US fl oz x 29.5735295625
millilitres = UK fl oz x 28.4130625
Both factors are exact definitions, not measurements.
Which ounce does my label use?
- A US product, a US recipe or a US nutrition label means US fluid ounces.
- A British recipe from before metrication means imperial fluid ounces. Current UK recipes and packaging are in millilitres.
- US nutrition labels use a rounded fluid ounce: 21 CFR 101.9 says that "1 fl oz means 30 mL" for labelling. So an 8 fl oz serving on a label is 240 mL, while 8 US fl oz measured exactly is 236.6 mL.
Common conversions
| Millilitres | US fl oz | UK fl oz |
|---|---|---|
| 5 mL | 0.169 | 0.176 |
| 15 mL | 0.507 | 0.528 |
| 30 mL | 1.01 | 1.06 |
| 50 mL | 1.69 | 1.76 |
| 60 mL | 2.03 | 2.11 |
| 100 mL | 3.38 | 3.52 |
| 125 mL | 4.23 | 4.4 |
| 150 mL | 5.07 | 5.28 |
| 200 mL | 6.76 | 7.04 |
| 250 mL | 8.45 | 8.8 |
| 330 mL | 11.2 | 11.6 |
| 500 mL | 16.9 | 17.6 |
| 750 mL | 25.4 | 26.4 |
| 1,000 mL | 33.8 | 35.2 |
| Fluid ounces | US fl oz in mL | UK fl oz in mL |
|---|---|---|
| 0.5 | 14.8 | 14.2 |
| 1 | 29.6 | 28.4 |
| 2 | 59.1 | 56.8 |
| 4 | 118.3 | 113.7 |
| 6 | 177.4 | 170.5 |
| 8 | 236.6 | 227.3 |
| 12 | 354.9 | 341 |
| 16 | 473.2 | 454.6 |
| 20 | 591.5 | 568.3 |
| 32 | 946.4 | 909.2 |
Fluid ounces are volume, not weight
A fluid ounce measures how much space a liquid takes up. An ounce (oz) without "fl" is a weight. For water they are close, since 1 US fl oz of water weighs about 29.6 g against 28.3 g for 1 oz. For oil, honey or milk they differ. To turn fluid ounces of an ingredient into grams, use the grams to cups converter, which knows each ingredient's weight.
| Unit | Exact size | Definition | Source |
|---|---|---|---|
| US fluid ounces | 29.5735295625 mL | 1 US fluid ounce = 1/128 US gallon | NIST Handbook 44 (2019) |
| UK | 28.4130625 mL | 1 fluid ounce = 1/20 pint | Weights and Measures Act 1985 |
| US cups | 236.5882365 mL | 1 cup = 8 US fluid ounces = 1/2 liquid pint | NIST Handbook 44 (2019) |
| US legal cups | 240 mL | For US nutrition labels, a cup means 240 mL | 21 CFR 101.9(b)(5)(viii) |
| Metric cups | 250 mL | Metric cup used in Australia, Canada and New Zealand | Wikipedia |
| Imperial cups | 284.130625 mL | Half an imperial pint, the cup in older British recipes | Weights and Measures Act 1985 |
| US pints | 473.176473 mL | 1 liquid pint = 16 US fluid ounces | NIST Handbook 44 (2019) |
| UK | 568.26125 mL | 1 pint = 0.56826125 cubic decimetre | Weights and Measures Act 1985 |
| US gallons | 3785.411784 mL | 1 US gallon = 231 cubic inches = 3.785411784 L | NIST Handbook 44 (2019) |
| UK gallons | 4546.09 mL | 1 gallon = 4.54609 cubic decimetres | Weights and Measures Act 1985 |