How many liters in a gallon?

It depends on which gallon. 1 US gallon = 3.785 L. 1 imperial gallon = 4.546 L. If a source just says 'gallon' without specifying, check the country of origin — US sources mean US gallon, UK sources historically meant imperial gallon though the UK now commonly uses liters.

How do I convert US MPG to liters per 100 km?

Divide 235.214 by the US MPG figure. Example: 35 MPG (US) = 235.214 / 35 = 6.72 L/100km. For imperial MPG, divide 282.481 by the MPG figure instead.

Is the Canadian gallon the same as the US gallon?

Canada officially uses the liter (SI) for fuel and commerce. Historically, Canada used the imperial gallon. Today, Canadian gas stations sell fuel in liters. 'Gallon' in Canadian casual speech may mean either US or imperial depending on context and age of the speaker.

US Gallon vs Imperial Gallon: What's the Difference?

The word 'gallon' refers to two different volumes depending on where you are. The US gallon is 3.785 liters; the imperial (UK) gallon is 4.546 liters — about 20% larger. Getting this wrong matters for fuel economy comparisons, brewing recipes, shipping, and any cross-border volume calculation.

Published March 19, 2026

Key takeaways

1 US gallon = 3.785411784 liters (exact by US legal definition).
1 imperial gallon = 4.54609 liters (exact by UK definition since 1985).
The imperial gallon is approximately 20% larger than the US gallon.
Both differ from the dry gallon (4.405 L), a US agricultural unit rarely encountered today.

Why are there two gallons?

Both gallons descend from British customary measures, but they diverged after American independence. The US retained an older wine gallon (231 cubic inches = 3.785 L) that was defined in the 1706 Queen Anne Act. Britain later standardized on the imperial gallon in 1824 as 10 pounds of water, which works out to 277.42 cubic inches (4.546 L).

The two definitions coexisted for over 200 years without much confusion — until international trade, fuel economy ratings, and the internet made cross-border comparisons routine.

Exact definitions

Formula

US gallon: 231 cubic inches exactly = 3.785411784 liters exactly.
Imperial gallon: 4.54609 liters exactly (UK Weights and Measures Act 1985).
Conversion: 1 imperial gallon = 1.20095 US gallons.
Conversion: 1 US gallon = 0.832674 imperial gallons.

Where each gallon is used

US gallon: United States and several US-influenced territories for fuel, beverages, and household liquids. Also used in Liberia and some Caribbean and Latin American countries.

Imperial gallon: United Kingdom (for fuel sold in some contexts, though the UK officially moved to liters for fuel in 1995), Canada (historically), some Caribbean nations. Today the imperial gallon is rarely used in formal commerce — most countries that were once imperial have switched to liters.

Fuel economy: MPG is not comparable

Important

The most common place where the gallon difference causes real confusion is fuel economy:

A car rated at 40 MPG (imperial, UK) consumes: 1/40 imp. gal per mile = 4.54609 L / 40 = 0.1137 L/mile = 11.37 L/100km
A car rated at 40 MPG (US) consumes: 3.785411784 L / 40 = 0.0946 L/mile = 9.46 L/100km

The imperial-MPG car uses about 20% more fuel per mile. When comparing vehicle efficiency across US and UK sources, always check which gallon is being used — or convert both to liters per 100 km for a fair comparison.

Brewing and cooking: measure carefully

Homebrewing recipes are particularly susceptible to gallon confusion. A US 5-gallon batch = 18.93 L. An imperial 5-gallon batch = 22.73 L. Using the wrong gallon means brewing 20% more or less beer than intended, which throws off grain bills, hop additions, and fermentation vessel sizing.

Similarly, large-batch cooking and canning recipes often specify gallons. US cookbooks use US gallons; British cookbooks use pints (imperial pint = 568 mL; US pint = 473 mL). Always check the source country of the recipe.

US vs Imperial gallon: quick reference

Formula

Unit comparison:
1 US gallon   = 3.785411784 L  = 3,785.41 mL = 128 US fl oz
1 Imp. gallon = 4.54609 L      = 4,546.09 mL = 160 imp. fl oz
1 Imp. gallon = 1.20095 US gallons
1 US gallon   = 0.83267 imperial gallons

Fuel economy conversions:
35 US MPG     = 42.05 UK MPG   = 6.72 L/100km
40 US MPG     = 48.05 UK MPG   = 5.88 L/100km
50 US MPG     = 60.07 UK MPG   = 4.71 L/100km

The 20% difference between US and imperial gallons applies equally to all derived units — US fluid ounces, US pints, and US quarts are all smaller than their imperial equivalents.

Common mistakes and how to avoid them

Important

Mistake 1: Comparing US and UK fuel economy figures directly. A car advertised at 50 UK MPG is not as efficient as a car rated at 50 US MPG — the UK number looks better only because the imperial gallon is larger. Always convert to L/100km for a fair comparison.

Mistake 2: Assuming 'gallon' in Canadian sources means US gallon. Canada has officially used the liter since the 1970s, but older Canadians may use 'gallon' colloquially to mean either the imperial or US gallon. Context and date of the document matter.

Mistake 3: Using US gallon conversions for UK homebrewing recipes. A recipe calling for 5 gallons of wort in a UK homebrew forum means 22.73 L, not 18.93 L. The resulting alcohol percentage, bitterness, and hop utilization all shift when the volume is wrong.

Rule: whenever a source says 'gallon' without qualification, identify the country of origin before performing any calculation.