Liters vs Gallons
Litres and gallons compared — and why a US gallon and a UK gallon are not the same size
Understanding the differences between Litre (L) and Gallon (gal) is essential for choosing the right system for your needs. This comparison covers the key differences, advantages of each approach, and practical guidance on when to use one over the other. Whether you are a student, engineer, scientist, or just curious about measurement systems, this guide will help you understand the trade-offs and make informed decisions.
Litre (L)
One cubic decimetre, or one thousandth of a cubic metre. The everyday metric volume unit, used for fuel, drinks and cooking in most of the world.
Gallon (gal)
A volume unit with two incompatible definitions in current use: the US liquid gallon (3.785411784 L) and the imperial gallon (4.54609 L).
Key Differences
- 1There are two gallons, and they differ by about 20%. The US liquid gallon is 3.785 L; the imperial gallon used in the UK and Canada is 4.546 L. Quoting a gallon figure without saying which one is a genuine ambiguity, not a pedantic one.
- 2This propagates into fuel economy. A car rated at 30 miles per US gallon achieves about 36 miles per imperial gallon for identical real performance — the number changes, the car does not.
- 3The litre is not strictly an SI unit; the SI volume unit is the cubic metre. The litre is accepted for use with SI and is universally used in practice for consumer volumes.
- 4The US gallon derives from the old English wine gallon of 231 cubic inches; the imperial gallon was redefined in 1824 as the volume of ten pounds of water. They were never the same and never became the same.
- 5Fluid ounces differ too, and not in the same proportion: a US fluid ounce is 29.57 mL, an imperial fluid ounce 28.41 mL — so the imperial ounce is SMALLER while the imperial gallon is LARGER, because the imperial gallon contains 160 fluid ounces to the US gallon's 128.
Background
England historically had several gallons at once — a wine gallon, an ale gallon and a corn gallon, each a different volume for a different trade. The United States inherited the wine gallon at independence and kept it. Britain swept all of them away in the Weights and Measures Act of 1824, replacing them with a single imperial gallon defined by the volume of ten pounds of distilled water. Because the two nations standardised on different ancestors at different times, the mismatch is permanent.
When to Use Litre (L) vs Gallon (gal)
Use Litre (L) when…
Use litres for fuel, drinks and recipes outside the US, all scientific work, and any international specification. It is the only one of these units with a single unambiguous definition.
Use Gallon (gal) when…
Use gallons for US fuel and large liquid volumes, and always state US or imperial. In the UK, fuel is sold in litres but fuel economy is still quoted in miles per imperial gallon, so both appear on the same forecourt.