Volume and Capacity Converter: Litres, Gallons, Pints and Ounces

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How to use this tool

Type an amount, pick what it is in, pick what you want it in. The answer appears as you type. The unit list keeps US and imperial units as separate entries rather than offering a single gallon, because there is no such thing as a single gallon and pretending otherwise is how people end up 20 percent out.

Underneath the main answer the same quantity is shown in litres, millilitres, both gallons, both fluid ounces, US cups and cubic feet at once, which saves converting twice when a recipe and a container disagree about units. If either side of your conversion is imperial, a red note appears to remind you which system you are in.

Why there are two of everything

The United States kept the older English wine gallon of 231 cubic inches. Britain replaced its own tangle of gallons in 1824 with the imperial gallon, originally defined as the volume of ten pounds of water. The two never matched, and the countries never reconciled them, so today a US gallon is 3.785411784 litres and an imperial gallon is 4.54609 litres.

Both numbers are exact. The US gallon is exactly 231 cubic inches and an inch is exactly 2.54 centimetres, so the litre figure falls out of the definition with nothing rounded. The imperial gallon was redefined in 1985 as exactly 4.54609 litres, which tidied up the old water based definition. Neither factor carries measurement error.

The nasty part is the fluid ounce. A US pint is 16 US fluid ounces and an imperial pint is 20 imperial fluid ounces, so the counts differ. But the ounces themselves also differ, and they differ in the opposite direction: a US fluid ounce is 29.5735 mL while an imperial fluid ounce is 28.4131 mL. The US ounce is about 4 percent larger, yet the imperial pint still ends up 20 percent bigger because it contains four more of them. Two errors pointing opposite ways is exactly the sort of thing that survives a quick sanity check.

A worked check you can repeat on paper

Start with the definition. A US gallon is 231 cubic inches. An inch is 2.54 cm, so a cubic inch is 2.54 times 2.54 times 2.54, which is 16.387064 cubic centimetres. Multiply by 231 and you get 3785.411784 cubic centimetres, and since a cubic centimetre is a millilitre, that is 3.785411784 litres. No rounding anywhere.

Now the ounce. Divide that gallon by 128 and you get 29.5735295625 mL for a US fluid ounce. Divide the imperial gallon of 4.54609 litres by 160 and you get 28.4130625 mL. Divide one by the other and the US ounce is 1.0408 times the imperial one, so about 4.1 percent larger.

Finally the pint. 16 US ounces at 29.5735 mL is 473.176 mL. 20 imperial ounces at 28.4131 mL is 568.261 mL. The imperial pint is 1.2009 times the US one, which is the same 20.1 percent that separates the gallons, as it must be.

Mistakes that catch people out

Treating a gallon as a gallon. A car quoted at 40 miles per imperial gallon is not doing 40 US mpg. The gap is 20 percent, which is the single largest source of confusion in fuel figures quoted across the Atlantic.

Mixing fluid ounces with ounces. A fluid ounce measures volume, an ounce measures mass, and they are only interchangeable for water and only roughly. A US fluid ounce of honey weighs about 1.4 ounces.

Assuming a cup is a cup. The US customary cup is 236.588 mL, the US legal cup used on nutrition labels is 240 mL, and the metric cup used in Australia and Canada is 250 mL. That is a 5.7 percent spread, which matters in baking and nowhere else.

Forgetting that cubic units are cubed. A cubic foot is not twelve cubic inches, it is 1728 of them, because the conversion applies three times. This catches people converting tank and container volumes far more often than it should.

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How this was checked

Every factor was derived from its definition rather than copied from a table. The US gallon came from 231 cubic inches at exactly 2.54 cm to the inch, the imperial gallon from its 1985 definition of exactly 4.54609 litres, and every smaller unit by dividing those two by the right integer: 4 for quarts, 8 for pints, 128 or 160 for fluid ounces, 256 for tablespoons, 768 for teaspoons.

Each derived figure was then checked against its published value, and all nine matched to the last digit a double can hold: the US fluid ounce at 29.5735295625 mL, the imperial at 28.4130625 mL, the US pint at 473.176473 mL, the imperial pint at 568.26125 mL, the US cup at 236.5882365 mL, the tablespoon at 14.78676478125 mL and the teaspoon at 4.92892159375 mL.

The internal consistency check matters more than any single figure. The imperial gallon divided by the US gallon is 1.200950, and the imperial pint divided by the US pint gives the same 1.200950. If the two disagreed, a factor would be wrong somewhere.

Related Calculators and Reference

• ML to Cups Converter: the kitchen version of this page, with US, metric, imperial and Japanese cups side by side and the measuring spoons to match.
• Kilograms to Pounds Converter for the mass side, and the reason a fluid ounce and an ounce are not the same thing.
• Mass and Weight Converter for troy ounces, short tons and long tons.
• Unit Price Comparison Calculator for turning a converted volume into a price per litre so two package sizes can be compared.
• Area Converter for the two dimensional version of the same imperial and metric split.
• Volume of 3D Shapes Calculator for working out the volume of a tank or stockpile before converting it.
• Pool Volume Calculator: gallons and litres for any pool shape, with fill time.
• Pool Chlorine Calculator: chlorine to add to reach your target ppm.
• Aquarium Volume Calculator: gallons and liters in a fish tank.
• Rainwater Harvesting Calculator: litres, US gallons and imperial gallons for tank sizes and yearly harvest.
• NIST Special Publication 811 is the primary reference for the exact factors used here.