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

**l**(lowercase L) can be confused with the digit 1, it is sometimes written in a lowercase script form. The Unicode glyph SCRIPT SMALL L (U+2113) (ℓ) can be used for this purpose.

## History

In 1793 the litre was introduced in France as one of the new "Republican Measures", and defined as one cubic decimetre. Its name derived from an older French unit, the*litron*, whose name came from Greek via Latin.

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.