US Gallon vs Imperial Gallon — What's the Difference?
Two countries separated by a common unit of measurement.
Both the United States and the United Kingdom use the word "gallon," but they mean different things. A US gallon is 3.785 liters. An imperial gallon is 4.546 liters. That is a 20% difference — large enough to cause real problems if you assume they are the same.
The Volume Converter handles both types of gallon alongside liters and other units. This article explains why the two exist, where each is used, and what to watch for when converting.
The Short Answer
| Unit | Volume | Used primarily in |
|---|---|---|
| US gallon | 3.785 L | USA, some Latin American countries |
| Imperial gallon | 4.546 L | UK, Canada (historically), some Caribbean countries |
One imperial gallon is about 1.2 US gallons. One US gallon is about 0.83 imperial gallons.
Why Are There Two Different Gallons?
Both trace back to English gallon measures that were in use before and during the colonial period, but they diverged after the United States gained independence.
The US gallon
The US gallon is based on the old English "wine gallon," which was defined in 1707 as 231 cubic inches. When the US became independent, it retained this older wine gallon as its standard.
In 1824, Britain reformed its measurement system and introduced the imperial gallon — defined as the volume of 10 pounds of water at a specific temperature. This was larger than the wine gallon and was applied uniformly across the British Empire.
Since the US had already broken from Britain before this reform, it kept the older wine gallon. The result is two countries that both say "gallon" but mean different amounts.
The imperial gallon
The imperial gallon was set at 10 pounds of water (at 62°F / 16.7°C), which worked out to approximately 277.42 cubic inches or 4.54609 liters. It replaced the various regional gallon measures that had existed across England and Scotland and was standardized across all British territories.
Canada adopted the imperial gallon as part of the British Empire but switched to liters in 1976 as part of metrication. The UK itself officially moved to liters for most commercial purposes in the 1990s, though informal use of "gallon" (usually meaning imperial) persists, particularly for fuel in Northern Ireland and in casual conversation.
Where Each Gallon Is Still Used Today
US gallon
The US gallon remains the standard in:
- The United States — fuel at the pump, milk jugs, paint cans, large water containers
- Some Latin American countries that adopted the US standard for fuel pricing, including Colombia, Guatemala, and Honduras
When you fill up a car in the United States, the price is per US gallon. A 5-gallon bucket at a US hardware store holds 18.93 liters.
Imperial gallon
The imperial gallon is still used in:
- The United Kingdom — informally for car fuel economy (miles per gallon), though pumps sell in liters
- Some Caribbean countries including the Cayman Islands and Antigua
- Bermuda
The UK officially uses liters for fuel sales at the pump, but fuel economy (MPG — miles per gallon) is still commonly discussed in imperial gallons. A car getting 50 mpg in the UK is achieving a different fuel economy than a car getting 50 mpg in the US — because the gallons are different sizes.
The Practical Confusion: MPG
Fuel economy is where the gallon difference creates the most confusion.
If a British car review says a car gets 50 mpg and a US review says the same car gets 42 mpg, they are likely both correct — just using different gallon sizes.
- 50 imperial mpg × 0.8327 = 41.6 US mpg
- 42 US mpg × 1.2009 = 50.4 imperial mpg
So the same vehicle achieves about 20% higher mpg figures when measured in imperial gallons vs US gallons. This matters when comparing fuel economy figures across markets or when evaluating imported vehicles.
Fluid Ounces: The Same Problem in Smaller Form
The gallon confusion extends to fluid ounces, since a fluid ounce is defined as a fraction of a gallon.
- 1 US fluid ounce = 1/128 of a US gallon = 29.574 mL
- 1 imperial (UK) fluid ounce = 1/160 of an imperial gallon = 28.413 mL
A US fluid ounce is slightly larger than an imperial fluid ounce — about 4% larger. For most cooking purposes this difference is negligible, but it becomes relevant when following old UK recipes or reading older British cookbooks that use fluid ounces.
A US pint is 16 US fluid ounces (473 mL). An imperial pint is 20 imperial fluid ounces (568 mL). If you order "a pint" of beer in the UK, you are getting about 568 mL. In the US, a pint glass is typically 16 fl oz (473 mL) — a noticeably smaller pour.
Converting Between the Two
US gallons to imperial gallons: imperial gallons = US gallons × 0.8327
Imperial gallons to US gallons: US gallons = imperial gallons × 1.2009
Both to liters:
- US gallon × 3.7854 = liters
- Imperial gallon × 4.5461 = liters
The simplest approach for any conversion is to go through liters as an intermediate step, since liter measurements are unambiguous. The Volume Converter handles any combination directly.
Quick Reference
| US gallons | Imperial gallons | Liters |
|---|---|---|
| 1 | 0.83 | 3.785 |
| 2 | 1.67 | 7.571 |
| 5 | 4.16 | 18.927 |
| 10 | 8.33 | 37.854 |
| 1 | 1.20 (imp → US) | 4.546 |
The One-Line Summary
US gallon = 3.785 L. Imperial gallon = 4.546 L. The imperial gallon is about 20% larger. When a recipe or specification says "gallon," check which system it is using — especially for anything involving fuel, large beverage quantities, or old recipes where the origin of the document is uncertain.
