Quick answer: A UK fluid ounce is 28.4130625 ml and a US fluid ounce is 29.5735295625 ml, so the American ounce is about 4% larger, by roughly 1.16 ml. The gap is small for a single ounce but adds up: 20 UK fluid ounces make a 568 ml pint, while 20 US fluid ounces come to 591 ml.
Fluid ounces turn up on cosmetics, American cocktail recipes and older British cookbooks, and the label rarely tells you which kind it means. This explainer shows how the two ounces differ, why they differ, and how to convert either one to millilitres. Related pints, gallons and litres pages are gathered in our volume and capacity conversions hub.
What is the difference between a UK and US fluid ounce?
A US fluid ounce holds 1.16 ml more than a UK fluid ounce. In ratio terms, one US fluid ounce equals 1.0408 UK fluid ounces, and one UK fluid ounce equals 0.9608 US fluid ounces. That is the opposite of pints and gallons, where the UK version is the larger one.
| UK fluid ounce | US fluid ounce | |
|---|---|---|
| Volume | 28.4130625 ml | 29.5735295625 ml |
| Share of a gallon | 1/160 of an imperial gallon | 1/128 of a US gallon |
| Per pint | 20 to a UK pint (568 ml) | 16 to a US pint (473 ml) |
| Per cup | Not used with cups | 8 to a US cup (237 ml) |
| Fluid ounces per 100 ml | 3.52 | 3.38 |
| Abbreviation | fl oz (sometimes “imp fl oz”) | fl oz (sometimes “US fl oz”) |
Why is the US fluid ounce bigger when the US pint is smaller?
Because each country cuts a different gallon into a different number of ounces. The imperial gallon (4.546 litres) is divided into 160 fluid ounces. The US gallon (3.785 litres) is divided into only 128. Fewer slices of a smaller cake still leaves each American slice slightly bigger.
The pint shows the effect clearly. A UK pint has 20 fluid ounces and a US pint has 16, so even though each US ounce is larger, the US pint ends up about 95 ml smaller. Our explainer on the imperial gallon vs the US gallon covers the history of how the two gallons split apart.
Does a fluid ounce of water weigh one ounce?
A UK fluid ounce of water weighs almost exactly one ounce; a US fluid ounce of water weighs about 4% more. The imperial system was built around water, so 28.41 ml of water at room temperature weighs about 28.36 g, within a fraction of a gram of the 28.35 g avoirdupois ounce. A US fluid ounce of water weighs about 29.5 g, or roughly 1.04 oz.
This neat match only works for water and liquids of similar density, such as milk. Oil is lighter and honey is much heavier, so a fluid ounce of either will not weigh an ounce. For dry ingredients, ignore fluid ounces and weigh in grams; the guide to measuring 100 g of flour without scales explains the alternatives.
UK and US fluid ounces to ml chart
Values are rounded to two decimal places. The last column shows how far apart the two answers drift as the quantity grows.
| Fluid ounces | UK (ml) | US (ml) | Difference (ml) |
|---|---|---|---|
| 1 fl oz | 28.41 | 29.57 | 1.16 |
| 2 fl oz | 56.83 | 59.15 | 2.32 |
| 4 fl oz | 113.65 | 118.29 | 4.64 |
| 5 fl oz | 142.07 | 147.87 | 5.80 |
| 8 fl oz | 227.30 | 236.59 | 9.28 |
| 10 fl oz | 284.13 | 295.74 | 11.60 |
| 16 fl oz | 454.61 | 473.18 | 18.57 |
| 20 fl oz | 568.26 | 591.47 | 23.21 |
How do you convert ml to fluid ounces?
Divide millilitres by 28.413 for UK fluid ounces, or by 29.574 for US fluid ounces. For example, a 330 ml can holds 11.61 UK fl oz or 11.16 US fl oz. The ml to fl oz converter does the sum for any amount.
| Millilitres | UK fl oz | US fl oz |
|---|---|---|
| 25 ml | 0.88 | 0.85 |
| 30 ml | 1.06 | 1.01 |
| 50 ml | 1.76 | 1.69 |
| 100 ml | 3.52 | 3.38 |
| 150 ml | 5.28 | 5.07 |
| 200 ml | 7.04 | 6.76 |
| 250 ml | 8.80 | 8.45 |
| 330 ml | 11.61 | 11.16 |
| 500 ml | 17.60 | 16.91 |
Where will you see fluid ounces in the UK?
Mostly on imported or internationally sold products, and in recipes. In UK trade the millilitre is the legal unit, so any fluid ounce figure is a secondary indication. Knowing which ounce is meant helps you read these labels correctly.
- Toiletries and cosmetics. A bottle marked “100 ml / 3.4 fl oz” is using US ounces, because 100 ml is 3.38 US fl oz but 3.52 UK fl oz. Products made for both markets usually follow the US figure.
- American nutrition labels. For nutrition labelling, US rules treat 1 fl oz as 30 ml, so serving sizes on imported drinks may be rounded that way.
- Cocktail recipes. Most cocktail books and websites written in the US use US ounces. A “2 oz” pour is about 59 ml; UK bars use 25 ml or 35 ml spirit measures instead.
- Older British cookbooks. Recipes from before metrication use UK fluid ounces, often alongside pints: “½ pint (10 fl oz) milk”.
- Feeding bottles and measuring jugs. Many show both ml and fl oz. Where accuracy matters, use the millilitre scale.
How can you tell which fluid ounce a recipe uses?
Look at where the recipe was written and at the other units it uses. A recipe that mentions cups, sticks of butter or Fahrenheit oven temperatures is almost certainly American. One that mixes fluid ounces with pints, gas marks or grams is likely British. If the recipe gives a millilitre figure as well, trust that.
In most cooking the 4% difference makes no visible change. It matters more when ounces are multiplied up, for example in a large batch of syrup or brine, or when a recipe gives liquid in ounces and expects a precise ratio to dry ingredients. For pint-based British recipes, our guide to how many ml are in a UK pint has a full fractions chart.
Common mistakes to avoid
- Confusing fluid ounces with ounces of weight. A fluid ounce measures volume; an ounce (oz) measures mass. They only line up for water in the UK system.
- Using 30 ml as an exact ounce. It is a convenient US labelling figure, but it overstates a UK fluid ounce by more than 1.5 ml.
- Assuming 16 fluid ounces make a UK pint. A UK pint is 20 fluid ounces; 16 belongs to the US pint.
Frequently asked questions
Is a UK fluid ounce bigger than a US fluid ounce?
No. A UK fluid ounce is 28.41 ml and a US fluid ounce is 29.57 ml, so the US fluid ounce is about 4% bigger.
How many ml is 1 fl oz in the UK?
One UK fluid ounce is 28.4130625 ml, usually rounded to 28.4 ml. There are 20 of them in a UK pint.
Is 3.4 fl oz the same as 100 ml?
Yes, in US fluid ounces: 100 ml is 3.38 US fl oz, which labels round to 3.4. In UK fluid ounces, 100 ml is 3.52 fl oz.
How many fluid ounces are in a UK pint?
A UK pint contains 20 UK fluid ounces, which is 568.26 ml. A US pint contains 16 US fluid ounces, which is 473.18 ml.
Should I use UK or US fluid ounces for an American recipe?
Use US fluid ounces of 29.57 ml for American recipes. For most dishes the 4% difference is too small to notice, but large batches benefit from the correct figure.
Sources
- Weights and Measures Act 1985, Schedule 1 (imperial units of capacity), legislation.gov.uk
- NIST Guide to the SI, Appendix B.9 (UK and US fluid ounce conversion factors)
- US Code of Federal Regulations, 21 CFR 101.9 (household measures for nutrition labelling)
- Business Companion (trading standards guidance on spirit measures)
More Volume and Capacity guides
- How Many ml in a Tablespoon UK? 15 ml and Chart
- How Many ml in a Teaspoon UK? 5 ml Explained
- Litres to Pints UK: Converter and Printable Chart
- UK Fluid Ounce vs US Fluid Ounce: What’s the Difference?
- Imperial Gallon vs US Gallon: The Difference Explained
- UK Gallon in Litres: Converter, Chart and mpg Guide