US vs UK gallon — they're not the same
Why a US gallon and a UK (imperial) gallon differ by 20%, and how to tell which one you have.
1 US gallon = 3.785 L (231 in³)
1 UK gallon = 4.546 L (277.42 in³)
A US gallon is 3.785411784 L (231 cubic inches). The UK (imperial) gallon is 4.54609 L (about 277.42 cubic inches). That's a 20% difference — not a rounding error.
Which one to use depends on where the product is sold:
- US: petrol, milk, paint. Fuel economy typically quoted in MPG (US). - UK: pre-1985 petrol was sold in imperial gallons. After 1985, fuel switched to litres, but road economy is sometimes still quoted as "MPG (UK)" to keep the legacy. - Canadian: petrol in litres by law since 1979. Some legacy products (milk in Ontario) still appear in imperial but this is rare.
Traps:
- Recipes from US cookbooks use US gallons or cups. UK recipes use metric. Don't assume they're the same. - Vehicle fuel economy quoted in the US is MPG (US), while UK motor magazines still occasionally use "MPG (UK)". They differ by 20%. - Older UK car manuals (pre-1985) assumed imperial gallons.
The site lists both gallons as distinct units. Convert either to litres, or to each other (gallon-us to gallon-uk = 0.8327 imperial gallons, or 1 imperial = 1.201 US gallons).
Frequently Asked Questions
Which should I use?
Use the US gallon when the data or product is from the US (petrol, milk in US packaging, recipes).
When is the alternative better?
Use the UK gallon for UK imperial units, heritage recipes, and UK-spec car efficiency.
How is the conversion calculated?
Every conversion is a single factor (or an affine formula). The math is identical either direction; the difference is context.