The US fluid ounce and the Imperial (UK) fluid ounce are two different units of volume that share the same name but differ in size by 4.08%. One US fl oz equals 29.5735 mL while one Imperial fl oz equals 28.4131 mL. This comparison calculator shows both conversions side by side so you can use the correct value for American or British recipes.
View Formulas & ComparisonThe US fluid ounce is a unit of volume defined within the US Customary System. It equals exactly 29.5735295625 mL — derived from the US gallon, which is defined as 231 cubic inches. The US fluid ounce is used across the United States for cooking measurements, beverage servings, nutrition labels, and FDA-regulated food products.
In the US system, 1 US cup = 8 US fl oz = 236.588 mL, and 1 US gallon = 128 US fl oz = 3,785.41 mL. The US fluid ounce is the default "fluid ounce" on American food labels and in American recipe books.
The Imperial fluid ounce is a unit of volume defined within the British Imperial System. It equals exactly 28.4130625 mL — derived from the Imperial gallon, which is defined as the volume of 10 pounds of water at 62 °F. The Imperial fl oz is 4.08% smaller than the US fluid ounce.
In the Imperial system, 1 Imperial cup = 10 Imperial fl oz = 284.131 mL, and 1 Imperial gallon = 160 Imperial fl oz = 4,546.09 mL. The UK has largely moved to metric, but Imperial fl oz still appear on older British cookbooks and some product labels.
Drag the slider to see how US fl oz and Imperial fl oz diverge as the mL value increases. The gap grows wider at higher volumes.
There are 6 key differences between the US fluid ounce and the Imperial fluid ounce that affect recipe conversions.
| Factor | US Fluid Ounce | Imperial (UK) Fluid Ounce |
|---|---|---|
| Volume in mL | 29.5735 mL | 28.4131 mL |
| Size Difference | 4.08% larger than UK fl oz | 3.92% smaller than US fl oz |
| Cup Equivalent | 1 US cup = 8 US fl oz (236.588 mL) | 1 UK cup = 10 UK fl oz (284.131 mL) |
| Gallon Equivalent | 1 US gal = 128 US fl oz (3,785 mL) | 1 UK gal = 160 UK fl oz (4,546 mL) |
| Countries Using | United States, Liberia, Myanmar | UK (partially), older Canadian & Australian cookbooks |
| Definition Origin | 1/128 of a US gallon (231 in³) | 1/160 of an Imperial gallon (10 lb of water at 62°F) |
To convert mL to both systems, use these two conversion factors:
These 8 examples show how US and Imperial fl oz values diverge at common volumes. Click any card to load the value in the calculator above.
This table shows 15 common mL values converted to both US and Imperial fluid ounces, with the difference column highlighting how they diverge.
| Milliliters (mL) | 🇺🇸 US fl oz | 🇬🇧 UK fl oz | Difference |
|---|
No. A US fluid ounce equals 29.5735 mL and an Imperial fluid ounce equals 28.4131 mL. The US fl oz is 4.08% larger. Using the wrong system in a recipe can result in measurement errors, especially at larger volumes. Always check whether a recipe uses US or Imperial measurements.
They evolved from different definitions. The US fluid ounce comes from the US gallon (231 cubic inches), making 1 US fl oz = 1/128 gallon. The Imperial fl oz comes from the Imperial gallon (volume of 10 lbs of water at 62°F), making 1 UK fl oz = 1/160 gallon. Different starting definitions produce different ounce sizes.
For small quantities (under 100 mL) the difference is negligible in cooking. But for larger volumes — like a 750 mL wine bottle or a gallon of liquid — the difference exceeds 1 fl oz. In baking, where precision matters, always use the correct system. At 1 liter, the difference is 1.38 fl oz, enough to affect a cake recipe.
US fluid ounces are used in the United States, and on products exported from the US worldwide. Imperial fluid ounces were historically used across the British Empire — UK, Canada, Australia, New Zealand, India — but most of these countries have since metricated. The UK still uses Imperial fl oz on some product labels alongside mL.
Check the source of the recipe. American cookbooks and websites use US fl oz. British cookbooks use Imperial fl oz. Canadian recipes typically use metric (mL) but older ones may use Imperial. If a recipe says "1 cup = 10 fl oz" it's Imperial; if it says "1 cup = 8 fl oz" it's US. When in doubt, convert to mL first for absolute accuracy.
The "cup" is defined differently in each system. The US cup = 8 US fl oz = 236.588 mL, while the Imperial cup = 10 Imperial fl oz = 284.131 mL. The Imperial cup is 20% larger by volume. This is one of the most common sources of confusion when following British recipes with American measuring cups — always verify which system the recipe uses.