Volume & capacity: the exact defined values
1 US liquid gallon = 3.785 411 784 litres exactly, because it is defined as exactly 231 cubic inches.
How the base unit is defined
The litre is a non-SI unit accepted for use with the SI, defined as exactly one cubic decimetre (0.001 m³).
Exact values table
"Exact" means the value is fixed by definition and carries no measurement uncertainty.
| Quantity | Exact value | Notes |
|---|---|---|
| 1 US liquid gallon | 3.785411784 L ≈ 3.785 L | 231 in³ × (25.4 mm)³. Exact. Open converter → |
| 1 US quart | 0.946352946 L ≈ 946 mL | 1/4 US gallon. Open converter → |
| 1 US fluid ounce | 29.5735295625 mL ≈ 29.57 mL | 1/128 US gallon. Open converter → |
| 1 US legal cup (nutrition labelling) | 240 mL | FDA labelling value. The US customary cup is 236.5882365 mL — see the pitfall below. Open converter → |
| 1 imperial gallon (UK) | 4.54609 L | Defined exactly in the UK Weights and Measures Act. 20% larger than a US gallon. |
| 1 litre | 1000 mL = 0.001 m³ | Exact by definition. Open converter → |
Rounding traps that actually change your result
A US gallon and a UK gallon are not the same size
US liquid gallon = 3.785411784 L. Imperial (UK) gallon = 4.54609 L. The imperial gallon is about 20.1% larger. Fuel-economy figures in mpg are therefore not comparable between the US and the UK unless the gallon is specified.
There are three different 'cups' in circulation
US legal cup (nutrition labels) = 240 mL. US customary cup = 236.5882365 mL. Metric cup (Australia, NZ) = 250 mL. Recipes rarely say which. For baking by weight this is why grams are more reliable than cups.
Where these values come from
- NIST Handbook 44, Appendix C
US liquid gallon defined as 231 cubic inches, giving 3.785411784 L exactly.
- US FDA, 21 CFR 101.9
Defines the 240 mL cup used on US nutrition labels.
- BIPM SI Brochure, 9th edition
Litre as an accepted non-SI unit equal to one cubic decimetre.
Frequently asked questions
Is a US gallon exactly 3.785411784 litres?
Yes. The US liquid gallon is defined as exactly 231 cubic inches, and the inch is exactly 25.4 mm, so the litre value is exact rather than measured.
How many millilitres in a US fluid ounce?
Exactly 29.5735295625 mL, which is 1/128 of a US gallon. Nutrition labels commonly round this to 30 mL.
Which cup should I use for a recipe?
If the recipe comes from a US nutrition label, use 240 mL. If it is an older US cookbook, 236.588 mL. Australian and New Zealand recipes use a 250 mL metric cup.
Convert volume & capacity now
These converters use the exact factors listed above.