The litre or liter (see spelling differences) is a unit of volume. There are two official symbols: the Latin letter L in lower (l) (to avoid confusion with the number 1 often as cursive ℓ) and upper case (L). The litre appears in several versions of the metric system; although not an SI unit, it is accepted for use with the SI.Bureau International des Poids et Mesures, 2006, p. 124. The international unit of volume is the cubic metre (m³). One litre is equal to 0.001 cubic metre and is denoted as 1 cubic decimetre (dm³).

The word "litre" is derived from an older French unit, the litron, whose name came from Greek via Latin. The original French metric system used the litre as a base unit.

The spelling of the word recommended by the International Bureau of Weights and Measures is "litre". However, the American English spelling is "liter", which is officially endorsed by the United States.

Definition

A litre is defined as a special name for a cubic decimetre (1 L = 1 dm³). Hence 1 L ≡ 0.001 m³ (exactly). So 1000 L = 1 m³

SI prefixes applied to the litre

The litre may be used with any SI prefix. The most commonly used is the millilitre, defined as one-thousandth of a litre (one cubic centimetre). It is a commonly used measurement, especially in medicine and cooking. Other units may be found in the table below, the more often used terms are in bold.

Rough conversions

One litre is slightly more than one U.S. liquid quart and slightly less than one Imperial quart or the less common U.S. dry quart.

A measured cup is roughly 250 mL.

A litre is the volume of a cube with sides of 10 cm, which is slightly less than a cube of sides 4 inches (or one-third of a foot). Twenty-seven cubes "one-third of a foot on each side" would fit in one cubic foot, which is within 5% of the actual value of exactly 28.316846592 litres.

A nice aide-memoire is: "A litre of water's a pint and three quarters". (Imperial pints, that is)

Explanation

Litres are most commonly used for items (such as fluids and berries) which are measured by the capacity or size of their container, whereas cubic metres (and derived units) are most commonly used for items measured either by their dimensions or their displacements. The litre is often also used in some calculated measurements, such as density (kg/L), allowing an easy comparison with the density of water.

One litre of water has a mass of almost exactly one kilogram when measured at its maximal density, which occurs at about 4 degrees celsius. Similarly: 1 millilitre of water has about 1 g of mass; 1,000 litres of water has about 1,000 kg of mass. This relationship is because the gram was originally defined as the mass of 1 mL of water. However, this definition was abandoned in 1799 because the density of water changes with temperature and, very slightly, pressure.