Liter

Overview
The litre or liter (see spelling differences) is a unit of volume. There are two official symbols: the Latin letter L in lower (l) and upper case (L). The liter appears in several versions of the metric system; although not an SI unit, it is accepted for use with the SI. The international unit of volume is the cubic meter (m³). One liter is denoted as 1 cubic decimeter (dm³).

Origin
The word "liter" is derived from an older French unit, the litron, whose name came from Greek via Latin. The original metric system used the liter as a base unit.

Definition
A liter is defined as  a special name for a cubic decimeter (1 L = 1 dm³). Hence 1 L ≡ 0.001 m³ (exactly). So 1000 L = 1 m3

SI prefixes applied to the liter
The liter may be used with any SI prefix. The more often used terms are in bold in the table below.

Milliliter (milliliter)
The milliliter is defined as one cubic centimeter and one-thousandth of a liter. It is a commonly used measurement, especially in medicine and cooking. Acceptable symbols for the milliliter are mL and ml.

Rough conversions
A liter 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 liters.

One liter is also slightly more than one U.S. liquid quart and slightly less than one Imperial quart or the less common U.S. dry quart.

A measured cup is roughly 237 mL. However, this number is usually rounded to 250 mL to ease metrication of recipies using English units of measurement.

Explanation
Liters are most commonly used for items measured by the capacity or size of their container (such as fluids and berries), whereas cubic meters (and derived units) are most commonly used for items measured either by their dimensions or their displacements. The liter is often also used in some calculated measurements, such as density (kg/L), allowing an easy comparison with the density of water.

One liter of water has a mass of almost exactly one kilogram. Similarly: 1 milliliter of water has about 1 g of mass; 1,000 liters of water has about 1,000 kg (1 ton) 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 1964 because the density of water changes with pressure and the units of pressure are dependent on the definition of mass.

Symbol
Originally, the only symbol for the liter 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 at all. This caused some concern, especially in the medical community. As a result, L (uppercase letter L) was adopted as an alternative symbol for liter 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, lowercase l is used with prefixes, though whole liters are often written in full (so, "750 ml" on a wine bottle, but often "1 liter" on a juice carton).

Prior to 1979, the symbol (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. Nevertheless, it is no longer used in most countries and no longer officially recognised by the Bureau International des Poids et Mesures, the International Organization for Standardization due to confusion and because it is often not (anyway) available in many documentation systems.

History
In 1795, the liter was introduced in France as one of the new "Republican Measures", and defined as one cubic decimeter.

In 1879, the Comité International des Poids et Mesures adopted the definition of the liter, and the symbol l (lowercase letter L).

In 1901, at the 3rd General Conference on Weights and Measures conference, the liter 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 liter equal to about 1.000 028 dm³ (earlier reference works usually put it at 1.000 027 dm³).

In 1964, at the 12th General Conference on Weights and Measures conference, the original definition was reverted to, and thus the liter was once again defined in exact relation to the meter, as another name for the cubic decimeter, that is, exactly 1 dm³.

In 1979, at the 16th General Conference on Weights and Measures 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.

Colloquial and practical usage
In spoken English, the abbreviation "mL" (for milliliter) is often pronounced as "mil", which is homophonous with the term "mil", meaning "one thousandth of an inch". This generally does not create confusion, because the context is usually sufficient &mdash; one being a volume, the other a linear measurement. The colloquial use of "mil" for millimeter for an ambiguous topic as in "5 mils of rain fell since 9am" may, however, be confusing.

The abbreviation cc (for cubic centimeter, equal to a milliliter 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 combustion engine displacement.

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 hectoliter is the typical unit for production and export volumes of beverages (milk, beer, soft drinks, etc); deciliters are found in cookbooks; centiliters 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&thinsp;cL or 33&thinsp;cL. Bottles may also be 75&thinsp;cL or half size at 37.5&thinsp;cL for 'artisanal' brews or 70&thinsp;cL for wines or spirits. Cans come in 25&thinsp;cL, 33&thinsp;cL and 50&thinsp;cL aka 0.5&thinsp;L. Family size bottles as for soft drinks or drinking water use the liter (0.5&thinsp;L, 1&thinsp;L, 1.5&thinsp;L, 2&thinsp;L), and so do beer barrels (50&thinsp;L, or the half sized 25&thinsp;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 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 wide-spread use, consumer beverages are labelled almost exclusively using liters and milliliters. Hectoliters sometimes appear in industry, but centiliters and deciliters are rarely, if ever, used. Larger volumes are usually given in cubic meters (equivalent to 1 kL), or thousands or millions of cubic meters. The situation is similar in Australia, although kiloliters, megaliters and gigaliters 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 meter is the general unit, as it is for all volumes of a non-liquid nature. There are a few exceptions in which the liter is used for rather large volumes, such as the irregularly shaped boot of a car or the internal size of a microwave oven.