The litre (Commonwealth spelling) or liter (American spelling) (SI symbols L and l, other symbol used: ℓ) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metres (m3). A cubic decimetre (or litre) occupies a volume of 10 cm × 10 cm × 10 cm (see figure) and is thus equal to one-thousandth of a cubic metre. The original French metric system used the litre as a base unit. The word litre is derived from an older French unit, the litron, whose name came from Byzantine Greek—where it was a unit of weight, not volume—via Late Medieval Latin, and which equalled approximately 0.82 litres. The litre was also used in several subsequent versions of the metric system but is not an SI unit; the SI unit of volume is the cubic metre (m3). The spelling used by the International Bureau of Weights and Measures is "litre", a spelling which is shared by most English-speaking countries. The spelling "liter" is predominantly used in American English. One litre of liquid water has a mass of almost exactly one kilogram, because the kilogram was originally defined in 1795 as the mass of one cubic decimetre of water at the temperature of melting ice (0 °C). Subsequent redefinitions of the metre and kilogram mean that this relationship is no longer exact.
Definition
A litre is a cubic decimetre, which is the volume of a cube 10 centimetres × 10 centimetres × 10 centimetres (1 L ≡ 1 dm3 ≡ 1000 cm3). Hence 1 L ≡ 0.001 m3 ≡ 1000 cm3; and 1 m3 (i.e. a cubic metre, which is the SI unit for volume) is exactly 1000 L. From 1901 to 1964, the litre was defined as the volume of one kilogram of pure water at maximum density (+3.98 °C) and standard pressure. The kilogram was in turn specified as the mass of the International Prototype of the Kilogram (a specific platinum/iridium cylinder) and was intended to be the same mass as the 1 litre of water referred to above. It was subsequently discovered that the cylinder was around 28 parts per million too large and thus, during this time, a litre was about 1.000028 dm3. Additionally, the mass–volume relationship of water (as with any fluid) depends on temperature, pressure, purity and isotopic uniformity. In 1964, the definition relating the litre to mass was superseded. The litre is not an SI unit and is no longer described by the BIPM as accepted for use with the SI. A litre is equal in volume to the millistere, an obsolete non-SI metric unit formerly customarily used for dry measure.
Explanation Litres are most commonly used for items (such as fluids and solids that can be poured) 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 3.984 °C. It follows, therefore, that 1/1000 of a litre, known as one millilitre (1 mL), of water has a mass of about 1 g, while 1000 litres of water has a mass of about 1000 kg (1 tonne or megagram). This relationship holds 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, with pressure. The density of water also depends on the isotopic ratios of the oxygen and hydrogen atoms in a particular sample. Modern measurements of Vienna Standard Mean Ocean Water, which is pure distilled water with an isotopic composition representative of the average of the world's oceans, show that it has a density of 0.999975±0.000001 kg/L at its point of maximum density (3.984 °C) under one standard atmosphere (101.325 kPa) of pressure.
SI prefixes applied to the litre The litre, though not an official SI unit, may be used with SI prefixes. The most commonly used derived unit is the millilitre, defined as one-thousandth of a litre, and also often referred to by the SI derived unit name "cubic centimetre". It is a commonly used measure, especially in medicine, cooking and automotive engineering. Other units may be found in the table below, where the more often used terms are in bold. However, some authorities advise against some of them; for example, in the United States, NIST advocates using the millilitre or litre instead of the centilitre. There are two international standard symbols for the litre: L and l. In the United States the former is preferred because of the risk that (in some fonts) the letter l and the digit 1 may be confused.
Non-metric conversions
See also Imperial units and US customary units.
Rough conversions One litre is about 5.7% larger than a US liquid quart, and about 12% smaller than an imperial quart. A mnemonic for its volume relative to an imperial pint is "a litre of water's a pint and three-quarters"; this is very close, as a litre is about 1.760 imperial pints. A cubic foot has a volume of exactly 28.316846592 L.
Symbol Originally, the only symbol for the litre was l (lowercase letter L), following the SI convention that only those unit symbols that abbreviate the name of a person start with a capital letter. In many English-speaking countries, however, the most common shape of a handwritten Arabic digit 1 is just a vertical stroke; that is, it lacks the upstroke added in many other cultures. Therefore, the digit "1" may easily be confused with the letter "l". In some computer typefaces, the two characters are barely distinguishable. As a result, L (uppercase letter L) was adopted by the CGPM as an alternative symbol for litre in 1979. The United States National Institute of Standards and Technology recommends the use of the uppercase letter L, a practice that is also widely followed in Canada and Australia. In these countries, the symbol L is also used with prefixes, as in mL and μL, instead of the traditional ml and μl used in Europe. In the UK and Ireland, as well as the rest of Europe, lowercase l is used with prefixes, though whole litres are often written in full (so, "750 ml" on a wine bottle, but often "1 litre" on a juice carton). In 1990, the International Committee for Weights and Measures stated that it was too early to choose a single symbol for the litre.
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