Volume is where unit confusion bites hardest: the US and Imperial gallons differ, and so do all the cups, pints, quarts, and fluid ounces derived from each. This reference converts any volume into every supported unit at once, keeping US and UK measures clearly separate, with exact factors — so you never mix up a recipe or a fuel-economy figure.
How it works
Each unit stores an exact number of litres per unit, with the litre as the shared base. Conversion is two steps:
litres = value × litresPerUnit[from]
result = litres ÷ litresPerUnit[to]The key fork is the gallon: 1 US gallon = 3.785411784 L but 1 Imperial gallon = 4.54609 L — about 20% larger. Each gallon family is divided into its own quarts, pints, cups, and fluid ounces, so the US and UK sub-units never share values. The SI side is clean: 1 L = 1 dm³, 1 mL = 1 cm³, and 1 m³ = 1000 L.
US vs Imperial: the full breakdown
Because the two gallon families divide differently, every sub-unit differs too:
| Unit | US measure | Imperial measure |
|---|---|---|
| Gallon | 3.7854 L | 4.5461 L |
| Quart | 0.9464 L | 1.1365 L |
| Pint | 0.4732 L | 0.5683 L |
| Fluid ounce | 29.57 mL | 28.41 mL |
| Cup | 236.6 mL | 284.1 mL (or 250 mL metric) |
The fluid ounce anomaly often surprises people: the Imperial fluid ounce is smaller than the US one, even though the Imperial gallon is larger. This is because the Imperial gallon is divided into more fluid ounces (160) than the US gallon (128).
Worked examples
Recipe scaling — cups to millilitres:
A US recipe calls for 2 cups of milk: 2 × 236.6 = 473.2 mL. Many metric recipe sites use a 250 mL metric cup instead, which is about 6% larger — enough to notice in baking. Always check which cup a recipe means.
Fuel economy comparison — US vs UK mpg:
A UK car rated at 40 mpg (Imperial) uses 40 × 4.546 = 181.8 km per gallon. The same car re-rated in US gallons would show about 33.3 mpg (US), because the US gallon is smaller. “40 mpg” means something quite different depending on which side of the Atlantic the number came from.
Fermentation — converting a recipe in US gallons to litres:
A 5 US gallon homebrew batch: 5 × 3.785411784 = 18.93 L. Most European homebrew kit is sized in 20 L or 25 L, so 5 US gallons fits neatly inside a 19 L (5 US gal) fermenter — a standard size made specifically for that reason.
Practical tips for common scenarios
- Cooking from US recipes in a metric kitchen: 1 US cup ≈ 240 mL (close enough for most recipes); for precision baking, weigh ingredients in grams instead.
- Buying fuel abroad: petrol in the UK is sold in litres; divide your car’s US gallon tank by 3.785 to know how many litres to fill. An 80-litre European tank is about 21 US gallons.
- Aquarium sizing: aquarium volumes are usually quoted in US gallons in North America. A “55-gallon tank” holds about 208 litres.
- SI precision: very small volumes in chemistry are often in microlitres (µL, 0.001 mL), but this tool covers down to the millilitre level — for sub-mL lab work, look at the mass of water instead (1 mL = 1 g at 4 °C).