Litre

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 litre 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 metre (m³). One litre is denoted as 1 cubic decimetre (dm³).

Origin

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

Definition

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

SI prefixes applied to the litre

The litre may be used with any SI prefix. The more often used terms are in bold in the table below.

Millilitre (millilitre)

The millilitre is defined as one cubic centimetre and one-thousandth of a litre. It is a commonly used measurement, especially in medicine and cooking. Acceptable symbols for the millilitre are mL and ml.

Non-metric conversions

Rough conversions

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.

One litre 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

Litres are most commonly used for items measured by the capacity or size of their container (such as fluids and berries), 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. Similarly: 1 millilitre of water has about 1 g of mass; 1,000 litres of water has about 1,000 kg (1 tonne) 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 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, 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. Nevertheless, it is no longer used in most countries and no longer officially recognised by the BIPM, the International Organization for Standardization due to confusion and because it is often not (anyway) available in many 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. 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 (11 November 2000). Retrieved on 9 October 2006.

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.Bureau International des Poids et Mesures (2006). The International System of Units (SI), 159. 

Colloquial and practical usage

In spoken English, the abbreviation "mL" (for millilitre) is often pronounced as "mil", which is homophonous with the term "mil", meaning "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.

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 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 hectolitre is the typical unit for production and export volumes of beverages (milk, beer, soft drinks, etc); 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 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 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 for all volumes of a non-liquid nature. There are a few exceptions in which the litre is used for rather large volumes, such as the irregularly shaped boot of a car or the internal size of a microwave oven.

See also

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

References

External links

• BIPM\'s "SI Brochure"
• BIPM\'s "(Table 6 -) Non-SI units accepted for use with the International System"
• NIST note on SI units
• NIST recommends uppercase letter L
• UK National Physical Laboratory\'s "Internationally recognised non SI units" page

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