Two Different Gallons
A US gallon is 3.785 liters; an imperial (UK) gallon is 4.546 liters — about 20% larger. The difference dates to the 1824 British standardization, after the US had already adopted its own gallon. Fuel economy figures in MPG therefore differ between the US and UK even for the same car.
Smaller Units Differ Too
A US fluid ounce is 29.57 mL; an imperial fluid ounce is 28.41 mL. US and imperial pints and quarts differ as well. The metric milliliter and liter avoid this ambiguity, which is why recipes increasingly use them.
Comparison
| Unit | US | Imperial |
|---|---|---|
| Gallon | 3.785 L | 4.546 L |
| Pint | 473 mL | 568 mL |
| Fluid ounce | 29.57 mL | 28.41 mL |