The litre (or liter - see spelling differences) is a unit of volume. There are two official symbols: the Latin letter L in lower and upper case (l and L). The lower case L is also often written as a cursive ℓ, though this symbol has no official approval by any international bureau. Although the litre is not an SI unit, it is accepted for use with the SI, and has appeared in several versions of the metric system. The official SI unit of volume is the cubic metre (m3), equivalent to 1,000 litres. One litre is equal to 1/1,000 cubic metres and is denoted as 1 cubic decimetre (dm3).
The spelling of the word used by the International Bureau of Weights and Measures is "litre". This spelling is also the usual one in English in most English speaking countries, but the usual American English spelling is "liter", which is officially endorsed by the United States.
From 1901 to 1964 the litre was defined as the volume of one kilogram of pure water at 4°C and 760 millimetres of mercury pressure. During this time, a litre was about 1.000028 dm3. In 1964 this definition was abandoned in favour of the current one.
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 measurement, especially in medicine and cooking. Other units may be found in the table below, 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).
|Multiple||Name||Symbols||Equivalent volume||Submultiple||Name||Symbols||Equivalent volume|
|100 L||litre||l (ℓ)||L||dm3||cubic decimetre|
|101 L||decalitre||dal||daL||10–1 L||decilitre||dl||dL|
|102 L||hectolitre||hl||hL||10–2 L||centilitre||cl||cL|
|103 L||kilolitre||kl||kL||m3||cubic metre||10–3 L||millilitre||ml||mL||cm3||cubic centimetre (cc)|
|106 L||megalitre||Ml||ML||dam3||cubic decametre||10–6 L||microlitre||µl||µL||mm3||cubic millimetre|
|109 L||gigalitre||Gl||GL||hm3||cubic hectometre||10–9 L||nanolitre||nl||nL||106 µm3||1 million cubic micrometres|
|1012 L||teralitre||Tl||TL||km3||cubic kilometre||10–12 L||picolitre||pl||pL||103 µm3||1 thousand cubic micrometres|
|1015 L||petalitre||Pl||PL||103 km3||1 thousand cubic kilometres||10–15 L||femtolitre||fl||fL||µm3||cubic micrometre|
|1018 L||exalitre||El||EL||106 km3||1 million cubic kilometres||10–18 L||attolitre||al||aL||106 nm3||1 million cubic nanometres|
|1021 L||zettalitre||Zl||ZL||Mm3||cubic megametre||10–21 L||zeptolitre||zl||zL||103 nm3||1 thousand cubic nanometres|
|1024 L||yottalitre||Yl||YL||103 Mm3||1 thousand cubic megametres||10–24 L||yoctolitre||yl||yL||nm3||cubic nanometre|
|1 L||≈ 0.87987699||quart||Imperial||1 quart||≡ 1.1365225 L|
|1 L||≈ 1.056688||fluid quart||U.S.||1 fluid quart||≡ 0.946352946 L|
|1 L||≈ 1.75975326||pint||Imperial||1 pint||≡ 0.56826125 L|
|1 L||≈ 2.11337641||fluid pints||U.S.||1 fluid pint||≡ 0.473176473 L|
|1 L||≈ 0.2641720523||liquid gallon||U.S.||1 liquid gallon||≡ 3.785411784 L|
|1 L||≈ 0.21997||gallon||Imperial||1 gallon||≡ 4.54609 L|
|1 L||≈ 0.0353146667||cubic foot||1 cubic foot||≡ 28.316846592 L|
|1 L||≈ 61.0237441||cubic inches||1 cubic inch||≡ 0.01638706 L|
|1 L||≈ 33.8140||customary fluid ounce||U.S.||1 customary fluid ounce||≡ 29.5735295625 mL|
|1 L||≈ 35.1950||fluid ounces||Imperial||1 fluid ounce||≡ 28.4130625 mL|
One litre is slightly more than one U.S. liquid quart and slightly less than one Imperial quart or the U.S. dry quart.
One measured cup ≈ 250 mL.
One teaspoon ≈ 5 mL (In some countries, this is an exact equivalency by definition of the teaspoon).
One tablespoon ≈ 15 mL (In some countries, this is an exact equivalency by definition of the tablespoon).
A litre is the volume of a cube with sides of 10 cm, which is slightly less than a cube of sides 4 inches (or one-third of a foot). Twenty-seven cubes "one-third of a foot on each side" would fit in one cubic foot, which is within 5% of the actual value of exactly 28.316846592 litres.
A nice aide-mémoire is: "A litre of water is a pint and three quarters" (Imperial pints, that is). Or, simpler: "A litre is a kilo of water" (the litre was once defined in a way that made this exactly true under certain conditions).
Litres are most commonly used for items (such as fluids and berries), 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 degrees Celsius. Similarly: 1 millilitre of water has about 1 g of mass; 1,000 litres of water has about 1000 kg of mass. This relationship is 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.
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, 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. On some typewriters, particularly older ones, the unshifted L key had to be used to type the numeral 1. Further, 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, 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).
Prior to 1979, the symbol ℓ (script small l, U+2113), 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 countries, 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 ㎘ (U+3395 to U+3398) for the microlitre, millilitre, decilitre, and kilolitre. Nevertheless, it is no longer used in most countries and was never officially recognised by the BIPM or the International Organization for Standardization, and is a character often not available in currently-used documentation systems.
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 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.
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.
In spoken English, the abbreviation "mL" (for millilitre) is often pronounced as "mil", which is homophonous with the colloquial term "mil", which is intended to mean "one thousandth of a metre". This generally does not create confusion, because the context is usually sufficient — one being a volume, the other a linear measurement. The colloquial use of "mil" for millimetre for an ambiguous topic as in "5 mils of rain fell since 9am" may, however, be confusing. And in the United States a term of the same spelling and pronunciation means a thousandth of an inch.
The abbreviation cc (for cubic centimetre, equal to a millilitre or mL) is a unit of the cgs system, that preceded the MKS system, that later evolved into the SI system. The abbreviation cc is still commonly used in many fields including (for example) sizing for motorcycle and related sports for small combustion engine displacement; larger engines, such as automobile engines, have their displacement measured in litres.
In European countries where the metric system was established well before the adoption of the SI standard, there is still carry-over of usage from the precursor cgs and MKS systems. In the SI system, use of prefixes for multiples of 1,000 is preferred and all other multiples discouraged. However, in countries where these 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 aka 0.5 L. Family size bottles as for soft drinks or drinking water use the litre (0.5 L, 1 L, 1.5 L, 2 L), and so do beer barrels (50 L, or the half sized 25 L). This unit is most common for all other household size containers of liquids, from thermocans, by buckets, to bath tubs; as well as for fuel tanks and consumption for heating or by vehicles.
In countries where the metric system was adopted as the official measuring system after the SI standard was established, common usage more closely follow contemporary SI conventions. For example, in Canada where the metric system is now in widespread use, consumer beverages are labelled almost exclusively using litres and millilitres. Hectolitres sometimes appear in industry, but centilitres and decilitres are rarely, if ever, used. Larger volumes are usually given in cubic metres (equivalent to 1 kL), or thousands or millions of cubic metres. The situation is similar in Australia, 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, as it is generally for all volumes of a non-liquid nature.
Although traditionally used only for the measurement of fluids, and containers for fluids, there are some fields where it has become a common measurement for volumes, based on the capacity of the container: