Litres to gallons:
two different gallons, two different answers

Why there are two gallons

A litre is defined cleanly as one cubic decimetre, so conversions to it are exact. The gallon is where confusion enters. The United States uses a gallon based on an old wine measure of 231 cubic inches, which is 3.785411784 litres exactly. The United Kingdom adopted the imperial gallon in 1824, defined as the volume of ten pounds of water, and it was later fixed as exactly 4.54609 litres.

The imperial gallon is therefore about 20 percent larger than the US gallon. One US gallon is 0.8327 of an imperial gallon, and one imperial gallon is 1.2009 US gallons. When a number crosses a border, the same word means a different amount.

A rule of thumb helps: US figures, such as American car brochures and fuel prices, use the US gallon. British figures traditionally used the imperial gallon, so a UK car rated at 40 miles per gallon is not the same as an American one rated at 40.

The two conversion formulas

Going the other way, multiply gallons by the same constant. Five US gallons equals 5 × 3.785 = 18.93 litres, and five imperial gallons equals 22.73 litres.

Worked example: a 45 litre fuel tank

A car has a 45 litre fuel tank. A US-based owner's manual quotes tank capacity in gallons, and a UK review lists it too. What would each publish?

The 20 litre example is also handy. 20 ÷ 3.785411784 = 5.283 US gallons, and 20 ÷ 4.54609 = 4.399 imperial gallons. If someone tells you 'about 5 gallons' for a 20 litre can, they are close to the US value, and 4.4 would be the British equivalent.

Reference table for common volumes

The imperial column is always about 83 percent of the US column. Round the constants too early and the error grows with the volume, so use the full constants for anything beyond a few litres.

Where the gallon type changes other figures

Fuel economy is the clearest case. A car that returns 40 miles per imperial gallon covers the same distance per litre as one returning 33.3 miles per US gallon, because 40 × 0.8327 = 33.3. Comparing the two numbers directly makes the British car look better than it is.

Prices behave the same way. A fuel price of US$3.60 per US gallon equals 3.60 ÷ 3.785 = US$0.95 per litre. A price quoted per imperial gallon would be 20 percent higher for the same fuel, so check the unit before judging whether petrol is cheap or expensive.

Cooking and brewing have their own twist: the US cup, pint and quart are fractions of the US gallon, and the imperial pint is 20 fluid ounces against the US pint of 16. An imperial pint therefore holds about 568 ml and a US pint about 473 ml.

Everyday volumes in gallons and gallons per minute

Large volumes make the gap between the two gallons obvious. A 1,000 litre household water tank holds 264.2 US gallons or 220.0 imperial gallons. A 25,000 litre pool is 6,604 US gallons, which is the figure American pool equipment manuals use for dosing chemicals.

Going the other way, a 5-gallon water bottle in the US holds 18.9 litres, and the 55-gallon drum used for oil and chemicals holds 208.2 litres. Those figures are US gallons, which is why drums sold in litres are listed as 208 or 210 litres.

Flow rates use the same conversion. A tap or shower that delivers 10 litres per minute is 10 × 0.264172 = 2.64 US gallons per minute, and 15 litres per minute is 3.96. Appliance specifications from North American manufacturers quote gpm, so converting before comparing with a local flow figure avoids buying a pump that is too small.

• Daily drinking water of 2 litres is 0.53 US gallons, a little over half a gallon.
• A 10 litre bucket is 2.64 US gallons or 2.20 imperial gallons.
• A 200 litre barrel is 52.8 US gallons or 44.0 imperial gallons.

Avoiding mistakes

• Confirm whether the source is American or British before choosing the constant.
• Do not mix the dry gallon, an old US measure of 4.405 litres, with the liquid gallon; fuel and liquids use the liquid gallon.
• Do not round 3.785 to 4 for anything where precision matters; that is an error of nearly 6 percent.
• Keep litres per 100 km and miles per gallon apart, because one is volume per distance and the other is distance per volume.

If you are working on a shipment, a tank or a purchase, ask for the stated unit in writing. A calculator that lets you choose between US and imperial gallons removes most of the risk.