Two gallons, twenty per cent apart
The gallon is one word covering two volumes that differ by more than any other pair of common units. A US liquid gallon is 3.785 L. An imperial gallon is 4.546 L, exactly 20.095% larger. Nothing in the name, the abbreviation or the context tells you which one a document means, and both are still in daily use — US gallons at every American fuel pump, imperial gallons in UK conversation, on Caribbean and some Canadian equipment plates, and throughout older British engineering literature.
The split is historical. The United States adopted the English wine gallon of 231 cubic inches, a measure that had been standardised under Queen Anne, and never changed it. Britain replaced its own tangle of wine, ale and corn gallons in 1824 with a single imperial gallon defined as the volume of ten pounds of distilled water at a specified temperature and pressure. That definition drifted with the precision of water density measurements until the Weights and Measures Act 1985 fixed it at exactly 4.54609 litres and ended the ambiguity.
The practical consequence shows up most sharply in fuel economy. A car rated at 40 miles per imperial gallon does 40 ÷ 1.20095 = 33.3 miles per US gallon, for identical fuel consumption. Comparing a British review's mpg figure with an American one without converting overstates the British car's economy by a fifth. The mpg to L/100km calculator handles that conversion directly.
Where the two factors come from
The US gallon. Federal law defines it as exactly 231 cubic inches. The inch has been exactly 2.54 cm since 1959, so a cubic inch is 2.54³ = 16.387064 cm³. Multiply: 231 × 16.387064 = 3,785.411784 cm³, or 3.785411784 L. Every digit is exact because both inputs are definitions rather than measurements. The subdivisions follow by simple halving: 4 quarts, 8 pints, 32 gills, 128 fluid ounces to the gallon.
The imperial gallon. Defined as exactly 4.54609 L by statute. Its subdivisions are 4 quarts, 8 pints and 160 fluid ounces — note the 160, not 128, which is why the imperial fluid ounce is smaller than the US one even though the imperial gallon is larger. An imperial pint is 568.26125 mL against the US pint's 473.176473 mL.
Cubic feet. A cubic foot is 1,728 cubic inches, so it holds 1,728 ÷ 231 = 7.48051948 US gallons, or 28.316846592 L. This is the conversion that links liquid measure to the volume units used for tanks, ductwork and bulk material — see the cubic yards to cubic feet calculator.
To move between the two gallons directly, multiply imperial gallons by 1.20095 to get US gallons, or US gallons by 0.832674 to get imperial. Those two numbers are reciprocals of each other, and checking that 1.20095 × 0.832674 = 1.0000 is the fastest way to confirm you have not inverted the conversion.
Worked example: filling a 15 US gallon fuel tank in Europe
You are driving a US-market car with a 15 US gallon tank and pulling into a pump that dispenses litres and charges € per litre.
- Tank capacity in litres. 15 × 3.785411784 = 56.781 L. Round to 56.8 L — and note that pumps cut off before a tank is completely full, so treat this as an upper bound.
- The same tank in imperial gallons. 56.781 ÷ 4.54609 = 12.490 imperial gallons. The same physical tank, a smaller number, because each imperial gallon holds more.
- Check the ratio. 15 ÷ 12.490 = 1.20095, which is exactly the ratio of the two gallon definitions. If your two gallon figures do not sit in that ratio, one of them is wrong.
- In cubic feet. 56.781 ÷ 28.316846592 = 2.005 ft³, which is a cube about 15.1 inches on a side (2.005 to the power of one third is 1.26 ft) — a useful reality check on how much space a fuel tank actually occupies.
- Cost comparison. At €1.75 per litre a full tank is 56.781 × 1.75 = €99.37. Expressed per US gallon that is 1.75 × 3.785411784 = €6.62 per gallon, which is the number to compare against a US pump price.
Deciding which gallon a document means
Use the source, then check the round numbers. Anything published in the United States, including fuel pumps, EPA ratings, paint cans and chemical labels, is US gallons. Anything from the UK in everyday speech — fuel economy in conversation, older recipes, boat and aircraft literature written before metrication — is usually imperial. Canada is mixed: fuel is sold in litres, but imperial gallons persist in trades and in older equipment documentation.
The round-number test helps when the source is unclear. Container sizes cluster on round values in their own system: 55 gallon drums, 5 gallon buckets and 1 gallon jugs are US; 4.5 L and 25 L containers are metric; 4.546 L jugs printed as "1 gallon" are imperial. If a nominally round gallon figure converts to an awkward litre figure and vice versa, you have probably picked the wrong gallon.
Where the stakes are high — chemical dosing, aviation fuel, marine tankage — work in litres or in mass and convert only at the end. Aviation adds a further trap because fuel is loaded by volume and flight-planned by weight, and the density depends on temperature. The safest habit is to write the unit next to every number, every time.
Gallon, quart and pint reference in both systems
| Measure | Liters | US gallons | Imperial gallons | Cubic feet |
|---|---|---|---|---|
| 1 US fluid ounce | 0.0295735 | 0.0078125 | 0.0065053 | 0.00104438 |
| 1 US pint | 0.4731765 | 0.125 | 0.1040843 | 0.0167101 |
| 1 US quart | 0.9463529 | 0.25 | 0.2081686 | 0.0334201 |
| 1 liter | 1 | 0.2641721 | 0.2199692 | 0.0353147 |
| 1 US gallon | 3.7854118 | 1 | 0.8326742 | 0.1336806 |
| 1 imperial gallon | 4.54609 | 1.2009499 | 1 | 0.1605437 |
| 1 cubic foot | 28.3168466 | 7.4805195 | 6.2288355 | 1 |
| 1 US 55-gal drum | 208.1976481 | 55 | 45.7970797 | 7.3524306 |
| 1 cubic meter | 1,000 | 264.1720524 | 219.9692483 | 35.3146667 |
Read the top row across: a US fluid ounce is one 128th of a US gallon and one 153.7th of an imperial gallon.
Traps in gallon conversions
- Treating the two gallons as interchangeable. A 20% error in fuel, chemical dosing or tank sizing is large enough to matter and small enough to look plausible. Always name the gallon.
- Assuming the bigger gallon has the bigger ounce. It does not. The imperial gallon is divided into 160 fluid ounces against the US gallon's 128, so the imperial fluid ounce (28.413 mL) is smaller than the US one (29.574 mL). See the fluid ounces to millilitres calculator.
- Comparing mpg figures across the Atlantic. Miles per imperial gallon runs 20.1% higher than miles per US gallon for identical consumption. Convert before comparing any two economy figures.
- Confusing the US liquid gallon with the US dry gallon. A dry gallon is 268.8025 in³ and appears only in grain and produce measure. Everything on this page is the liquid gallon.
- Ignoring temperature on precise work. Liquids expand with heat, so a volume measured warm and delivered cold is not the same number of litres. Fuel and chemical trade contracts often specify a reference temperature such as 15 °C for exactly this reason.
- Reading a tank's nominal size as its usable volume. Nominal capacities are rounded, and tanks have ullage, baffles and unusable sump volume. Use the plated capacity for compliance and the measured fill for planning.
Where gallons fit among the volume units
Gallons sit in the middle of the liquid-measure ladder. Below them are quarts, pints and fluid ounces, all of which carry the same US-versus-imperial split — the fluid ounces to millilitres calculator covers that end. Above them, bulk liquid is quoted in cubic meters, barrels or tons, and the conversion needs a density once you leave volume behind.
For anything you are filling rather than buying, the geometry usually comes first. A pool, a tank or a length of pipe has a volume you compute from dimensions and then convert — see the pool volume calculator and the pipe volume calculator. For dosing that volume with a chemical, work in litres or millilitres throughout and convert only for the label, because dosing rates in ppm and mg/L are metric by construction.
One convention worth knowing: the oil barrel is 42 US gallons (158.987 L), a figure fixed by the nineteenth-century Pennsylvania oil trade and unrelated to any other barrel. Beer barrels, dry barrels and UK barrels are all different volumes again, so a barrel figure without a qualifier is not a usable number.