Volume

This converter is US liquid measure throughout, and that qualifier does more work than it looks like it does. A US gallon is 3.785411784 litres; an imperial gallon is 4.54609 litres. They share a name and differ by twenty percent, which is enough to wreck a fuel-economy comparison, a brewing recipe or an engine-oil order.

Ten units are covered, from the millilitre up to the cubic metre, opening on one US gallon. The kitchen units — teaspoon, tablespoon, cup, fluid ounce — are the US customary ones, which are not the same as the Australian, British or Japanese units bearing those names.

Volume is also the place where converting the number is not enough. A cup of flour is a volume; a recipe wanting 120 g of flour is asking about mass, and no volume conversion crosses that gap without a density.

US gallons, imperial gallons, and the ounce that goes the other way

The US liquid gallon is defined as exactly 231 cubic inches, a value inherited from the English wine gallon of 1707. The imperial gallon was defined separately in 1824 as the volume of ten pounds of water, and is now fixed at exactly 4.54609 litres. The imperial gallon is 20.1% larger.

Fuel economy is where this bites hardest. A car advertised at 40 mpg in the UK is doing 33.3 mpg by US measure, because the UK gallon holds more fuel. Comparing British and American economy figures without converting overstates the British car by a fifth.

The fluid ounces run the opposite way. A US fluid ounce is 29.5735 mL and an imperial fluid ounce is 28.4131 mL, so the American ounce is 4.1% larger. The pints still favour Britain, because an imperial pint holds twenty imperial ounces (568.26 mL) while a US pint holds sixteen US ounces (473.18 mL) — a UK pint of beer is 20% more liquid than an American one.

Dry measure is a third system again. US dry pints and quarts exist and are about 16% larger than their liquid namesakes; they turn up in produce sales, where a "dry pint" of berries is a container size rather than a weight. Nothing in this converter is dry measure.

A recipe that crossed a border

Millilitres to cups. A British recipe calling for 250 mL of milk is 1.057 US customary cups — a US cup is 236.588 mL. Rounding to one cup loses about a tablespoon, which is fine for milk in a sauce and not fine for liquid in a bread dough.

The cup that is not a cup. The US legal cup used on FDA Nutrition Facts labels is 240 mL, not the 236.588 mL customary cup. A serving listed as one cup is therefore 1.4% more than what your measuring cup holds. Australian recipes use a 250 mL metric cup, and Japanese recipes a 200 mL cup, with rice measured in the 180 mL gou that rice-cooker cups are sized to.

The tablespoon that ruins baking. A US tablespoon is 14.79 mL, three teaspoons. An Australian tablespoon is 20 mL, four teaspoons. Measuring an Australian recipe’s three tablespoons of baking powder with US spoons delivers 44.4 mL where 60 mL was intended — about a quarter short, and enough to flatten a cake.

Dosing. Medical teaspoons are the metric 5 mL, not the 4.929 mL US customary teaspoon. The difference is trivial; using a kitchen spoon instead of the supplied syringe is not, because domestic teaspoons range from roughly 3 to 7 mL.

Ten units, and the ladder that links them

The US customary chain is short and worth memorising outright: 3 teaspoons make a tablespoon, 2 tablespoons make a fluid ounce, 8 fluid ounces make a cup, 2 cups make a pint, 2 pints make a quart, and 4 quarts make a gallon. That puts 128 fluid ounces in a gallon and 16 tablespoons in a cup.

On the metric side, the millilitre and litre carry everyday liquids and the cubic metre carries anything industrial — freight volumes, HVAC airflow, concrete, water utility billing. One cubic metre is 1,000 litres, and it is also 35.31 cubic feet, which is the unit air conditioning and shipping tend to use in the US.

One relationship is exact and useful: 1 mL is 1 cm³, precisely. That has been true since 1964, when the litre was redefined as exactly one cubic decimetre. Between 1901 and 1964 the litre was instead the volume occupied by one kilogram of water at maximum density, which made it 1.000028 dm³ — so pre-1964 scientific literature quoting litres is carrying a 28-parts-per-million offset.

Exact factors, with the litre as the base

• 1 US gallon = 3.785411784 L (exact, from 231 in³)
• 1 US quart = 0.946352946 L
• 1 US pint = 0.473176473 L
• 1 US cup = 0.2365882365 L
• 1 US fl oz = 0.0295735295625 L
• 1 US tablespoon = 0.01478676478125 L
• 1 US teaspoon = 0.00492892159375 L
• 1 m³ = 1,000 L

Every one of those imperial figures is exact rather than measured, because they all derive from the 231-cubic-inch gallon and the exact 25.4 mm inch. The long decimal tails are a consequence of squeezing an inch-based definition into metric, not of measurement uncertainty.

For rough work: a litre is a shade more than a quart (1.057 quarts), four litres is a little more than a gallon (1.057 gallons), and 500 mL is about 16.9 US fluid ounces — which is why a half-litre water bottle is labelled 16.9 fl oz in the United States.

Why a cup of one thing is not a cup of another

The two flour rows are the argument for weighing. Same cup, same flour, 20% difference in what ends up in the bowl depending on how it was filled — which is a bread that comes out dense rather than a bread that comes out right. This converter can turn cups into millilitres, but only a scale and the Weight Converter can turn either into grams of a specific ingredient.

Fuel, tanks and engine oil

Fuel. A 60-litre tank is 15.85 US gallons. Filling in Europe and reasoning in American units, or the reverse, is the everyday case, and the per-litre price converts by the same factor: 1.80 euros a litre is 6.81 euros a US gallon.

Engine oil. A service calling for 5 US quarts wants 4.73 L, so a 5-litre jug overfills by 0.27 L unless you stop at the dipstick. Overfilling is not harmless — it aerates the oil against the crankshaft.

Tanks and containers. Volume from dimensions goes through cubic inches: a tank with an interior of 48 × 13 × 21 inches is 13,104 in³, and dividing by 231 gives 56.7 US gallons. That is the arithmetic behind the standard "55 gallon" aquarium, and it explains why the actual water volume once substrate, rock and the gap below the rim are accounted for is closer to 45 gallons.

Shipping. Freight quotes in cubic metres; a 20-foot container holds about 33 m³, or 1,170 cubic feet. Converting the cargo, not the container, is what determines the bill.

Frequently asked questions

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