Guide
Volume in everyday life: litres, millilitres and gallons
Volume figures appear in the kitchen, at the fuel pump, in heating engineering and in retail. Two traps recur: the US gallon is not the imperial gallon, and a volume is not a weight.
The base unit is the litre
One litre is exactly one cubic decimetre, that is 0.001 cubic metres. One millilitre is one thousandth of a litre and therefore one cubic centimetre. The metric units therefore divide evenly: 1 m³ = 1000 l = 1,000,000 ml.
US gallon and imperial gallon
The US gallon measures 3.785411784 litres, the imperial gallon 4.54609 litres. The difference is about 20 %. A tank of 15 US gallons holds about 56.8 litres, one of 15 imperial gallons about 68.2 litres. For the barrel there are several definitions as well, of which the oil barrel at 158.987294928 litres is the most widespread.
Volume is not weight
A litre and a kilogram are numerically equal only for water. For other substances the relation depends on density: a litre of cooking oil weighs about 920 grams, a litre of honey about 1400 grams. In recipes with volume figures the ingredient is therefore decisive, not the number.
Kitchen measures are conventions
A teaspoon is usually taken as 5 ml, a tablespoon as 15 ml, a cup as 240 ml. These are conventions rather than exact definitions: spoons and cups have different capacities depending on origin, and the US cup is smaller than the metric one. For recipes a figure in grams or millilitres is therefore more reliable than one in spoons.
Matching conversions
Matching tables
Unit family: Volume
Common questions
How many millilitres is a litre?
Exactly 1000 ml.
How many litres is a US gallon?
Exactly 3.785411784 l, about 20 % less than the imperial gallon.
How many millilitres is a teaspoon?
Usually 5 ml, a tablespoon 15 ml. These are conventions, not exact definitions.
Is a litre the same as a kilogram?
Only for water. For other substances density decides.
Sources
Do not use as the sole basis for official, medical, safety-critical or contractual decisions.
Citation note: UnitCloud, guide, sources reviewed: September 2026. Source basis: BIPM SI Brochure · NIST Guide to SI Appendix B.