The litre (or liter in US) is a metric unit of volume. The litre is not an SI unit, but (along with units such as hours and days) is listed as one of the "units outside the SI that are accepted for use with the SI." The SI unit of volume is the cubic metre (m³).
The symbol for the litre is the lowercase letter l or the uppercase letter L. A cursive or script small letter l (ℓ) is also in use, but is not accepted by the BIPM.
A litre is equal to:
There are 1,000 litres in a cubic metre (m³). See 1 E-3 m³ for a comparison of volumes.
The litre is subdivided into smaller units by the application of SI prefixes, making 1 litre equivalent to:
- 1,000,000 microlitres (µL)
- 1,000 millilitres (mL) = 1,000 cubic centimetres (cm³),
- 100 centilitres (cL),
- 10 decilitres (dL),
- 0.01 hectolitre (hL).
Larger volumes (and capacities) can be measured using kilolitres (1 kL = 1,000 litres) or megalitres (1 ML = 1,000,000 litres).
- microlitre << millilitre < centilitre < decilitre < litre
There is no international standard regarding when to use litres and when to use cubic metres. In practice, litres are most commonly used for items measured by the capacity or size of their container (such as fluids and berries), 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.
The symbol for the litre was originally l (lowercase letter l).
In order to reduce confusion with the number 1, L (uppercase letter L) was accepted as an alternative symbol in 1979. The United States National Institute of Standards and Technology recommends the use of the uppercase letter L. Uppercase L is also preferred in Canada and in Australia. The symbols for the derived units (using uppercase L) are in the form of µL, mL, kL, ML, and so on.
Prior to 1979, the symbol ℓ (script small l, U+2113), came into common use in some countries; for example, it was recommended by the South African Bureau of Standards publication M33 in the 1970s. This symbol remains in common use, but is not officially recognised by the BIPM.
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 1879, the CIPM adopted the definition of the litre, and the symbol l (lowercase letter l).
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 (3.98 °C) under a pressure of 1 atm. This made the litre equal to about 1.000 028 dm³ (earlier reference works usually put it at 1.000 027 dm³).
In 1964, at the 12th CGPM conference, the litre was once again defined in exact relation to the metre, as another name for the cubic decimetre, that is, exactly 1 dm³. NIST Reference (http://ts.nist.gov/ts/htdocs/230/235/appxc/appxc.htm#footnote1)
In 1979, at the 16th CGPM conference, the alternative symbol L (uppercase letter L) was adopted. It also expressed a preference that in the future only one of these two symbols should be retained, but in 1990 said it was still too early to do so.
- BIPM's "SI Brochure" (http://www.bipm.org/utils/en/pdf/si-brochure.pdf)
- BIPM's "(Table 6 -) Non-SI units accepted for use with the International System" (http://www.bipm.org/en/si/si_brochure/chapter4/table6.html)
- NIST note on SI units (http://physics.nist.gov/cuu/Units/units.html)
- NIST recommends uppercase letter L (http://physics.nist.gov/cuu/Units/outside.html)
- UK National physical laboratory's "Internationally recognised non SI units" page (http://www.npl.co.uk/npl/reference/international.html)