The **litre** (US **liter**) (symbol **l** or **L**) is a metric unit of volume. The symbol **L** was introduced in 1979 to avoid confusion with 1 and I. The litre is not an SI unit, but is "accepted for use with the International System" [2].

It is equal to:

- 0.001 cubic metres,
- the volume of a cube of side 10 centimetres
- 1 cubic decimetre

A kilogram of pure water at a temperature of 4 °Celsius and standard atmospheric pressure occupies approximately 1 litre of space.

In the past, this was used to *define* the kilogram, but not anymore, partially because the volume depends ever-so-slightly on the pressure, and pressure units include mass as a factor, introducing a circular dependency in the definition of the kilogram.

The litre is subdivided into smaller units by the application of SI prefixes, making 1 litre equivalent to:

- 1,000 millilitres (ml); 1,000 cubic centimetres
- 100 centilitres (cl)
- 1 cubic decimetre

In 1901, at the 3rd CGPM conference, the litre was redefined as the space occupied by 1 kg of pure water at the temperature of its maximum density (approx. 4 °C) under a pressure of 1 atm. This was supposed to be 1 dm^{3}, but it was later discovered that the original measurement was off, at 1.000 028 dm^{3}.

In 1964, at the 12th CGPM conference, the original definition of the litre was restored. It was recommended that the unit be used for commercial purposes but not for high-precision scientific work.