What is the difference between a US gallon and a UK gallon?

The US gallon is 3.785 liters, while the Imperial (UK) gallon is 4.546 liters — about 20% larger. This means fuel efficiency stated in 'miles per gallon' (MPG) will always show higher numbers in the UK than in the US for the same vehicle, because the UK gallon is larger. Always check which gallon is being used when comparing fuel consumption figures.

Why does the UK still use miles while using metric for most things?

The UK is officially metric but uses miles for road distances and speeds (legally required on road signs), pints for draught beer and milk, and stone for body weight in informal contexts. This mixed system (often called 'imperial for some, metric for others') is a result of gradual and incomplete metrication that began in the 1960s.

Unit Converter — Metric vs Imperial: Length, Weight, Volume by Country

Unit Converter

Convert length, weight, volume, and area between metric and imperial units. Each unit shows which countries use it — because a "pint" means different things in the US and UK.

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Why the UK Gallon and US Gallon Are Different Sizes

One of the most common unit confusion points: the US and UK both have a unit called the "gallon" but they're not the same size:

US gallon: 3.785 liters
UK (Imperial) gallon: 4.546 liters — approximately 20% larger

This matters enormously for fuel efficiency. If a UK car gets "40 mpg" and a US car gets "40 mpg", the UK car is actually more efficient because it's traveling 40 miles on more fuel (a larger gallon).

Which Countries Use Metric vs Imperial?

Only three countries have not officially adopted the metric system: the United States, Myanmar (Burma), and Liberia. However, the reality is more nuanced:

US: Uses imperial for everyday life but metric in science, medicine, military, and international trade
UK: Officially metric, but culturally mixes systems (miles on roads, pints in pubs, stone for weight)
Canada: Officially metric, but proximity to the US means imperial is still common in construction and cooking
Australia, NZ: Fully metric since the 1970s metrication

Cooking Measurements: A Common Confusion

Cooking measurements are particularly prone to international confusion. A "cup" is not standardized globally:

US cup: 236.6 mL (8 fl oz)
UK cup: 284.1 mL (10 fl oz / ½ pint)
Metric cup (Australia, Canada): 250 mL
Japanese cup (合, go): 180.39 mL