Definition · Plain-language
Volume
Volume is the amount of three-dimensional space that an object or quantity of substance occupies, measured in units such as cubic metres and litres.
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The space something occupies
Volume measures how much three-dimensional space a thing takes up. Where length is one dimension and area two, volume is the full three: length, width and depth together. A larger box holds more because it encloses more space. Volume applies to solids, liquids and gases alike, and to empty space as well as filled — the capacity of a container is its internal volume. Because it is built from length measured in three directions, volume is a derived quantity rather than a base one, with units that are simply units of length cubed.
Units of volume
The SI unit of volume is the cubic metre, the space inside a cube one metre on each side. For most everyday purposes that is too large, so the litre is used: one litre is a thousandth of a cubic metre, equal to a cube ten centimetres on a side. The litre divides into a thousand millilitres, and one millilitre equals one cubic centimetre exactly, which is why kitchen and laboratory measures move easily between them. Imperial volume units — pints, quarts and gallons — persist in everyday British and American use alongside the metric ones.
How volume is measured
For a regular solid, volume is calculated from its dimensions: a rectangular box is length times width times height, a cylinder is the area of its circular base times its height. For an irregular object, the elegant method is displacement, attributed to Archimedes: submerge the object in water and measure how much the water level rises, since the rise equals the object’s volume. For liquids, graduated containers such as measuring cylinders read the volume directly. Knowing volume alongside mass is what allows density to be calculated, linking the two quantities.
Key facts
At a glance
- Definition: the amount of three-dimensional space occupied
- SI unit: the cubic metre (m³)
- Common unit: the litre (1 L = 0.001 m³) and millilitre
- Equivalence: 1 millilitre = 1 cubic centimetre exactly
- Regular shapes: calculated from dimensions (e.g. l × w × h)
- Irregular shapes: measured by water displacement
Common misconceptions
What people often get wrong
Often heard: Volume and mass are the same thing.
Actually: They are different. Volume is the space a thing occupies; mass is the amount of matter in it. A balloon has a large volume but very little mass.
Often heard: A litre is a base SI unit of volume.
Actually: The SI unit of volume is the cubic metre. The litre is an accepted non-SI unit, defined as one-thousandth of a cubic metre, used for convenience.
Often heard: You cannot find the volume of an irregularly shaped object.
Actually: You can, by displacement: submerge it and measure the rise in water level. The volume of water displaced equals the volume of the object.
Going deeper
