liter

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,^[1] and has appeared in several versions of the metric system. The official SI unit of volume is the cubic metre (m³). One litre is equal to 0.001 cubic metre and is denoted as 1 cubic decimetre (dm³).

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

The spelling of the word used by the International Bureau of Weights and Measures is "litre".^[1] 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.^[2]

Definition

A litre is defined as a special name for a cubic decimetre (1 L = 1 dm³ = 10³ cm³). Hence 1 L ≡ 0.001 m³ (exactly). So 1000 L = 1 m³

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 favor of the current one.

SI prefixes applied to the litre

The litre may be used with any SI prefix. The most commonly used is the millilitre, defined as one-thousandth of a litre, and also often referred to using the non-SI name of "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.

Non-metric conversions

Rough conversions

One litre is slightly more than one U.S. liquid quart and slightly less than one Imperial quart or the less common 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-memoire is: "A litre of water's 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).

Explanation

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 1,000 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, pressure.

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, 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.

History

In 1795, the litre was introduced in France as one of the new "Republican Measures", and defined as one cubic decimetre.

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.^[3]

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]

Colloquial and practical usage

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, do 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, etc) and for measuring the size of the catch and quotas for fishing boats; decilitres are 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.

Use for non-liquid volumes

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:

• berries and solid items that can be poured and are measured by their containers
• Car boot/trunk size [1][2]
• Climbing packs [3][4]
• Computer cases [5][6]
• Microwave ovens [7][8]
• Refrigerators [9][10]
• Sleeping bags - packed volume [11][12]

The litre is a particularly convenient and easily visualised unit for this range of volumes - a backpack could be as small as 2 litres, and a refrigerator as large as 300 litres. Expressed as cubic metres these same volumes would be 0.002 and 0.3 (or as cubic centimetres 2,000 and 300,000)—much less convenient numbers.

See also

• Claude Émile Jean-Baptiste Litre
• Cubic metre
• Gallon
• Kilogram
• Pint

Notes

References

• Bureau International des Poids et Mesures (2006). "The International System of Units (SI)" (PDF). http://www.bipm.fr/utils/common/pdf/si_brochure_8_en.pdf. Retrieved on 2008-08-18. 
• Bureau International des Poids et Mesures. (2006). "The International System of Units (SI)" (on-line browser):
  • Table 6 (Non-SI units accepted for use with the International System). Retrieved 24 August 2008
• National Institute of Standards and Technology (11 November 2000). "Appendix C: General tables of units of measurement". NIST Handbook 44: Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices. National Institute of Standards and Technology. http://ts.nist.gov/WeightsAndMeasures/Publications/appxc.cfm#4e. Retrieved on 2006-10-09. 
• National Institute of Standards and Technology. (December 2003). The NIST Reference on Constants, Units, and Uncertainty: International System of Units (SI) (web site):
  • Note on SI units. Retrieved 24 August 2008.
  • Recommending uppercase letter L. Retrieved 24 August 2008.
• Taylor, B.N. and Thompson, A. (Eds.). (2008a). The International System of Units (SI). United States version of the English text of the eighth edition (2006) of the International Bureau of Weights and Measures publication Le Système International d’ Unités (SI) (Special Publication 330). Gaithersburg, MD: National Institute of Standards and Technology. Retrieved 18 August 2008.
• Taylor, B.N. and Thompson, A. (2008b). Guide for the Use of the Internatioal System of Units (Special Publication 811). Gaithersburg, MD: National Institute of Standards and Technology. Retrieved 23 August 2008.
• Turner, J. (Deputy Director of the National Institute of Standards and Technology). (16 May 2008)."Interpretation of the International System of Units (the Metric System of Measurement) for the United States". Federal Register Vol. 73, No. 96, p. 28432-3.
• UK National Physical Laboratory. Internationally recognised non SI units