Volume & Capacity Converter

The mistake that ruins recipes

A US gallon and a UK gallon are not the same size. The UK one is about 20% bigger. So are UK pints. But UK fluid ounces are smaller than US ones.

That combination catches almost everybody:

	US	UK	Difference
Gallon	3,785 ml	4,546 ml	UK is 20.1% bigger
Pint	473 ml	568 ml	UK is 20% bigger
Fluid ounce	29.57 ml	28.41 ml	UK is 4% smaller

The reason the ounce goes the other way: a US pint is 16 fluid ounces while a UK pint is 20. The UK squeezed more, smaller ounces into a larger pint.

This is why the converter above lists them separately rather than offering one “gallon”. Picking the wrong one is a 20% error, and in a recipe or a fuel calculation that is not a rounding difference — it is a wrong answer.

Everyday conversions

From	To	Multiply by	Example
Litres	US gallons	0.264172	50 L = 13.21 gal
US gallons	Litres	3.785412	15 gal = 56.78 L
Litres	UK gallons	0.219969	50 L = 11.00 gal
Millilitres	US fluid ounces	0.033814	500 ml = 16.91 fl oz
US cups	Millilitres	236.588	2 cups = 473.18 ml
US tablespoons	Millilitres	14.7868	3 tbsp = 44.36 ml

Cups are three different sizes

Cup	Millilitres	Where
US customary	236.59	United States
US “legal” (nutrition labels)	240	US food labelling
Metric	250	Australia, New Zealand, often the UK

A metric cup is 5.7% larger than a US cup. For soup, irrelevant. For a cake, it is the difference between a good crumb and a dense one — which is why serious baking recipes give weights, not cups.

Kitchen spoons are steadier: a US tablespoon is 14.79 ml and a teaspoon 4.93 ml, usually treated as 15 ml and 5 ml. Australia is the exception, with a 20 ml tablespoon of four teaspoons rather than three.

Litres to kilograms: only with a density

This is the most common question, and the honest answer is that volume and mass are different quantities. The bridge between them is density.

Liquid	Approximate kg per litre
Water	1.00
Milk	1.03
Petrol	0.74
Diesel	0.85
Cooking oil	0.92
Honey	1.42

One litre of water weighing one kilogram is not a coincidence — the kilogram was originally defined that way. Everything else has to be looked up, which is why a converter that offers “litres to kilograms” without asking what the liquid is should be treated with suspicion.

Fuel economy across the gallon divide

Because of the gallon gap, miles per gallon figures are not comparable between the US and the UK.

• 30 US mpg = 36 UK mpg = 7.8 litres per 100 km
• 30 UK mpg = 25 US mpg = 9.4 litres per 100 km

A car advertised at “40 mpg” is meaningfully more efficient in the United States than the same number would be in Britain. Litres per 100 km avoids the problem entirely, which is part of why most of the world uses it.