Gallons to Liters

The two units

The US gallon is defined as exactly 3.785411784 liters, or 231 cubic inches. The 231-cubic-inch definition dates to Queen Anne's 1707 wine gallon, which the United States adopted at independence and kept while the UK abandoned it in 1824 in favor of a larger gallon. The US gallon is also the gallon used by a handful of Caribbean and Latin American countries.

The UK imperial gallon, by contrast, is 4.54609 liters — about 20% larger than the US gallon. When the UK reformed its weights and measures in 1824, the new imperial gallon was defined as 10 pounds of pure water at a specific temperature. The UK has since moved most consumer measurements to metric, but the imperial gallon still appears in some contexts (fuel economy in the UK is sometimes quoted in mpg using the imperial gallon, making British mpg figures look higher than US ones for the same vehicle).

The liter is one cubic decimeter, or 1,000 cubic centimeters. It's not strictly an SI unit (the SI volume unit is the cubic meter), but it's universal in cooking, fuel sales, and chemistry. One liter of water at standard conditions weighs almost exactly one kilogram.

The conversion factor

liters = US gallons × 3.78541

For the imperial gallon (UK), multiply by 4.54609 instead. The 20% difference between US and imperial gallons matters for any cross-border comparison.

When you'd actually need this conversion

Fuel economy is the most common case. US vehicles report miles per gallon; almost everywhere else reports liters per 100 kilometers (or kilometers per liter). A US car that gets 30 mpg converts to about 7.84 L/100km — but that conversion requires both a length and a volume factor.

Drink containers and beverage sizes vary widely between countries. A US gallon of milk is 3.79 liters; a European liter bottle is about 0.26 gallons. Recipes that call for "1 gallon of stock" need to become 3.79 L for a metric kitchen.

Pool and tank capacity is another routine context. US swimming pool sizes are listed in gallons (a 20,000-gallon pool); pools in metric countries list capacity in liters or cubic meters (1 m³ = 1,000 L). Filtering systems, chemical dosing, and heating calculations all flow from the underlying volume.

Common mistakes

Using the wrong gallon definition is the biggest trap. A "20 mpg" US car becomes "8.5 L/100km"; the same number in imperial gallons (a "20 mpg" UK car) becomes "14.1 L/100km" — quite different. Cross-border fuel-economy comparisons require checking which gallon the source uses.

Confusing dry gallons with liquid gallons is rare but possible. The US dry gallon (4.405 L) is used only for grains and some agricultural commodities. The everyday US gallon is the liquid gallon (3.785 L); unless the context specifies "dry," assume liquid.