Volume Conversions: Litres, Cups, Ounces and Gallons

The litre comes from length

In the metric system, volume is built from length. One litre is the volume of a cube 10 cm on each side, a cubic decimetre, and one millilitre equals a cubic centimetre, a cube 1 cm on each side. The cc used in medicine and for engine sizes is exactly this cubic centimetre, so 1 mL and 1 cc are the same amount. A cubic metre is 1,000 litres, which is why water use is often quoted in cubic metres or tonnes of water. Because every value links by powers of ten, conversion within the metric system is just a matter of moving the decimal point. Nor is it a coincidence that a litre of water has a mass of about 1 kg: water was the reference when the unit of mass was first defined.

The US customary ladder

US volume units form a ladder of twos and fours. The base is the gallon, defined as 231 cubic inches, and since the inch is exactly 2.54 cm, the gallon is exactly 3.785411784 litres. Everything else divides down from there.

• 1 gallon = 4 quarts = 3.785411784 L
• 1 quart = 2 pints = 946.352946 mL
• 1 pint = 2 cups = 473.176473 mL
• 1 cup = 8 fl oz = 236.5882365 mL
• 1 fl oz = 2 tablespoons = 29.5735295625 mL
• 1 tablespoon = 3 teaspoons = about 14.79 mL

Same names, different values in the UK

The British imperial system also uses gallons, pints and fluid ounces, but with different values. The imperial gallon is exactly 4.54609 litres, about 20 percent larger than the US gallon. The imperial pint is 568.26125 mL, well above the US pint of 473.18 mL. On top of that, an imperial pint is divided into 20 fluid ounces while a US pint has 16. Yet a single fluid ounce is actually smaller in the UK, at 28.4130625 mL, than the US 29.57 mL. That is why converting by name alone can get even the direction of the error wrong.

Cups and spoons vary by country

Kitchen cups and spoons are even less consistent. Check which country a recipe comes from first, then convert using that country's values.

• US customary cup: 236.5882365 mL (the nutrition-label cup is 240 mL)
• Metric cup: 250 mL (used in Australia and elsewhere)
• Korean and Japanese measuring cup: 200 mL
• Japanese rice measure, one go: about 180 mL
• Tablespoon: usually 15 mL, 20 mL in Australia, about 14.79 mL in US customary units

Converting volume to weight

Volume and weight are different quantities, so no unit conversion alone can turn one into the other; you have to multiply by the ingredient's density. Water makes this easy because 1 mL weighs almost exactly 1 g, but that only holds for water. Cooking oil is lighter than the same volume of water, while honey and syrup are much heavier. Powders change weight considerably in the same cup depending on how they are packed. So when converting cups in a foreign recipe to grams, use an ingredient-specific chart; applying the water figure everywhere will throw the result off.

An order for converting

When several units are mixed, it is easy to lose track halfway. Converting everything to millilitres first and then dividing into the target unit cuts down on mistakes. For example, to turn 3 US cups into imperial pints, first find 3 × 236.59 = about 709.8 mL, then divide by 568.26 to get about 1.25 pints. Round the result to match the precision of the source. Carrying 'about 2 cups' to four decimal places means nothing.

Handling large volumes

The larger volume units you meet in daily life also have exact values. They are handy when units are mixed, as with aquariums, refrigerators or fuel.

• 1 cubic metre = 1,000 L
• 1 cubic foot = 28.316846592 L
• 1 oil barrel = 42 US gallons = about 159 L
• 1 cubic inch = 16.387064 mL

Common mistakes

Most volume errors come from mixing different units that share a name. Checking the items below prevents most of them.

• Not distinguishing US gallons and pints from imperial ones
• Treating weight ounces and fluid ounces as the same thing
• Using the US cup value for a Korean recipe's cup
• Turning a volume straight into grams without a density