The litre (international spelling) or liter (American spelling) (symbols L, l ^[2] or ℓ) is an SI accepted metric system unit of volume equal to 1 cubic decimetre (dm3), 1,000 cubic centimetres (cm3) or 1/1,000 cubic metre. 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 Greek—where it was a unit of weight, not volume ^[3]—via Latin, and which equalled approximately 0.831 litres. The litre was also used in several subsequent versions of the metric system and is accepted for use with the SI,^[4] although 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",^[4] a spelling which is shared by almost all English-speaking countries. The spelling "liter" is predominantly used in American English.^[1]

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.^[5]

A litre is defined as decimetre or 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 (+4 °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 of 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 abandoned in favour of the current one. Although the litre is not an SI unit, it is accepted by the CGPM (the standards body that defines the SI) for use with the SI. CGPM defines the litre and its acceptable symbols.

A litre is equal in volume to the millistere, an obsolete non-SI metric unit customarily used for dry measure.

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 about 4 °C. Similarly: one millilitre (1 mL) of water has a mass of about 1 g; 1,000 litres of water has a mass of about 1,000 kg (1 tonne). 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.

It is now known that 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 it has a density of 0.999975 ±0.000001 kg/L at its point of maximum density (3.984 °C) under one standard atmosphere (760 Torr, 101.325 kPa) of pressure.^[6]

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.^[7]

One litre is slightly larger than a US liquid quart, and slightly less than an imperial quart or one US dry 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 actually 1.75975399 pints.

A litre is the volume of a cube with sides of 10 cm, which is slightly less than a cube of sides 4 inches (one-third of a foot).

One cubic foot would contain exactly 27 such cubes (four inches on each side), making one cubic foot approximately equal to 27 litres.

One cubic foot has an exact volume of 28.316846592 litres, which is 4.88% higher than the 27 litre approximation.

A litre of liquid water has a mass almost exactly equal to one kilogram. An early definition of the kilogram was set as the mass of one litre of water. Because volume changes with temperature and pressure, and pressure uses units of mass, the definition of a kilogram was changed. At standard pressure, one litre of water has a mass of 0.999975 kg at 4 °C, and 0.997 kg at 25 °C.^[8]

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". Further, on some typewriters, particularly older ones, the unshifted L key had to be used to type the numeral 1. Even in some computer typefaces, the two characters are barely distinguishable. This caused some concern, especially in the medical community.

As a result, L (uppercase letter L) was adopted as an alternative symbol for litre in 1979. The United States National Institute of Standards and Technology now recommends the use of the uppercase letter L,^[9] 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 CIPM stated that it was too early to choose a single symbol for the litre.^[4]

Prior to 1979, the symbol ℓ came into common use in some countries; for example, it was recommended by South African Bureau of Standards publication M33 and Canada in the 1970s. This symbol can still be encountered occasionally in some English-speaking and European countries like Germany, and its use is ubiquitous in Japan and South Korea.

Fonts covering the CJK characters usually include not only the script small ℓ but also four precomposed characters: ㎕, ㎖, ㎗ and ㎘ for the microlitre, millilitre, decilitre and kilolitre.

The first name of the litre was "cadil"; standards are shown at the Musée des Arts et Métiers in Paris.^[12]

The litre was introduced in France in 1795 as one of the new "republican units of measurement" and defined as one cubic decimetre.^[5] One litre of liquid water has a mass of almost exactly one kilogram, due to the gram being defined in 1795 as one cubic centimetre of water at the temperature of melting ice.^[5] The original decimetre length was 44.344 lignes, which was revised in 1798 to 44.3296 lignes. This made the original litre 1.000974 of today's cubic decimetre. It was against this litre that the kilogram was constructed.

In 1879, the CIPM adopted the definition of the litre, with 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 1.000028 dm3 (earlier reference works usually put it at 1.000027 dm3).

In 1964, at the 12th CGPM conference, the original definition was reverted to, and thus the litre was once again defined in exact relation to the metre, as another name for the cubic decimetre, that is, exactly 1 dm3.^[14]

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.^[4]

In spoken English, the symbol "mL" (for millilitre) is sometimes pronounced as "mil", which is potentially confusing as this could also be interpreted as:

Generally, these various meanings do not create confusion because the context is usually sufficient—one being a volume, the others linear or angular measurement.

The abbreviation cc (for cubic centimetre, equal to a millilitre or mL) is a unit of thecgs system, that preceded the MKS system, that later evolved into the SI system. The abbreviation cc is still commonly used in many fields including medical dosage and sizing for small combustion engine displacement, such as those used in motorcycles.

The microlitre (μL) has been known in the past as the lambda (λ), but this usage is now discouraged.^[15] In the medical field the microlitre is sometimes abbreviated as mcL on test results.^[16]

In the SI system, use of prefixes for powers of 1,000 is preferred and all other multiples discouraged. However, in countries where the metric system was established well before the adoption of the SI standard other multiples were already established, their use remains common. In particular, use of the centi (10−2), deci (10−1), deca (10+1) and hecto (10+2) prefixes are still common. For example, in many European countries, the hectolitre is the typical unit for production and export volumes of beverages (milk, beer, soft drinks, wine, etc.) and for measuring the size of the catch and quotas for fishing boats; decilitres are common in Switzerland and Scandinavia and sometimes found in cookbooks; centilitres indicate the capacity of drinking glasses and of small bottles. In colloquial Dutch in Belgium, a "vijfentwintiger" and a "drieëndertiger" (literally "twenty-fiver" and "thirty-threer") are the common beer glasses, the corresponding bottles mention 25 cL or 33 cL. Bottles may also be 75 cL or half size at 37.5 cL for 'artisanal' brews or 70 cL for wines or spirits. Cans come in 25 cL, 33 cL and 50 cL.

In countries where the metric system was adopted as the official measuring system after the SI standard was established, common usage more closely follows contemporary SI conventions. For example, in Canada, Australia, and New Zealand, consumer beverages are labelled almost exclusively using litres and millilitres. Hectolitres sometimes appear in industry, but centilitres and decilitres are rarely, if ever, used. An exception is in pathology, where for instance blood lead level may be measured in micrograms per decilitre. Larger volumes are usually given in cubic metres (equivalent to 1 kL), or thousands or millions of cubic metres.

Although kilolitres, megalitres, and gigalitres are commonly used for measuring water consumption, reservoir capacities and river flows, for larger volumes of fluids, such as annual consumption of tap water, lorry (truck) tanks, or swimming pools, the cubic metre is the general unit. It is also generally for all volumes of a non-liquid nature.

Fields where the litre and millilitre are used as a measurement for non-liquid volumes, where the capacity of the container is indicated, include: