Volume Converter

Everything converts through the litre. The complication is not the arithmetic. It is that several unit names mean two different things depending on which side of the Atlantic wrote them down.

The US gallon is 3.785 litres. The imperial gallon is 4.546 litres. That is a 20% difference under one word. The US gallon derives from an old English wine gallon; the imperial gallon was redefined in 1824 as the volume of ten pounds of water. Both survived, and both are still called a gallon.

The fluid ounce differs too, and confusingly in the opposite direction — the imperial fluid ounce is smaller at 28.41 ml against the US 29.57 ml. So an imperial pint is 20 fluid ounces and a US pint is 16, making the imperial pint 20% larger despite the smaller ounces.

Both families are listed separately here rather than merged, because there is no correct way to guess which one a recipe or a specification means.

A British recipe calls for 1 pint of milk and you have an American measuring jug.

Type 1, choose pint (imperial): 568 ml, which is 1.2 US pints. Use your US pint measure and you are 20% short — enough to ruin a batter.

The reverse mistake is more common online. An American recipe calls for a cup. Type 1, choose cup (US): 236.6 ml. Many British and Australian recipes assume a 250 ml metric cup, so following a US recipe with a metric cup adds 6% to every measured ingredient — survivable for stock, not for baking.

A non-kitchen case. A car's fuel economy is quoted as 30 miles per gallon. If that is a US source it means 30 miles per 3.785 L, or 12.75 km/L. If it is a UK source it means 30 miles per 4.546 L, or 10.62 km/L. Same number, and a 20% difference in what it claims.

The rule of thumb: recipes and specifications from the United States use US units; everything from the UK, Ireland, Canada or Australia is likely imperial or metric. When it matters and you cannot tell, weigh the ingredient instead — weight has no such ambiguity.