US vs imperial volume units

The gallon difference

The US liquid gallon is defined as 231 cubic inches, equal to exactly 3.785411784 litres. The imperial gallon is defined as exactly 4.54609 litres. An imperial gallon is therefore about 20% larger than a US gallon.

Both systems divide a gallon into four quarts and a quart into two pints. Because their gallons differ, their quarts and pints differ too.

Fluid ounces can be especially confusing

A US liquid pint contains 16 US fluid ounces. An imperial pint contains 20 imperial fluid ounces. The ounce counts differ, and the individual fluid ounces also differ in size.

• 1 US fluid ounce ≈ 29.5735 millilitres
• 1 imperial fluid ounce ≈ 28.4131 millilitres
• 1 US pint ≈ 473.176 millilitres
• 1 imperial pint = 568.26125 millilitres

Cups and recipes

A US legal cup used for nutrition labelling is 240 millilitres, while a US customary cup is about 236.588 millilitres. A metric cup is commonly 250 millilitres. Recipe authors do not always state which definition they use.

For everyday cooking, small differences may be acceptable. Baking and scaled recipes are less forgiving, so prefer a recipe’s stated gram or millilitre values when available and keep the original measurement system throughout.

How to avoid mistakes

Look for US, UK, imperial, or metric context before converting. Never choose a generic gallon, pint, or fluid ounce when the source system is unknown. UnitNimble lists US and imperial volume units separately so the distinction remains visible.