Litre

Verified Timeline

Lore & Background

The symbol for the litre has a history of confusion. Originally, the only symbol 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". In some computer typefaces, the two characters are barely distinguishable. As a result, L (uppercase letter L) was adopted by the CGPM as an alternative symbol for litre in 1979. The United States National Institute of Standards and Technology recommends the use of the uppercase letter L, a practice that is also widely followed in Canada and Australia. In 1990, the International Committee for Weights and Measures stated that it was too early to choose a single symbol for the litre.

Reader's Guide

The litre is a fundamental unit of volume in the metric system, used globally for measuring liquids and pourable solids. 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). This made the litre a bridge between volume and mass, facilitating everyday measurements and calculations such as density (kg/L). Although the litre is not an SI unit—the SI unit of volume is the cubic metre—it 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. The litre's widespread use, despite not being an SI unit, underscores its importance in daily life and industry, and its symbol remains a subject of international discussion.

Did You Know?

• The word 'litre' is derived from an older French unit, the litron, which came from Byzantine Greek where it was a unit of weight, not volume, and equalled approximately 0.82 litres.
• From 1901 to 1964, the litre was defined as the volume of one kilogram of pure water at maximum density (+3.98 °C) and standard pressure, making it about 1.000028 dm3.
• A cubic foot has a volume of exactly 28.316846592 L.
• The microlitre (μL) has been known in the past as the lambda (λ), but this usage is discouraged as of 2014.
• In Japan and South Korea, the use of the script small ℓ symbol for litre is ubiquitous.

Geometric Definition and Metric Relationships

The litre is fundamentally a geometric quantity: the space occupied by a cube measuring ten centimetres on each side. This makes it precisely one cubic decimetre, or equivalently one thousand cubic centimetres, and exactly one-thousandth of a cubic metre. Because the cubic metre is the SI base unit of volume, the litre sits as a convenient decimal subdivision rather than an official SI unit itself. The International Bureau of Weights and Measures no longer describes the litre as accepted for use with the SI, yet it remains ubiquitous in everyday measurement. A cubic metre contains exactly one thousand litres, and the litre is volume-equivalent to the millistere, an obsolete non-SI metric unit once used for dry measure. The clean decimal relationships—1 L ≡ 1 dm3 ≡ 1000 cm3 and 1 L ≡ 0.001 m3—make it a natural bridge between the small-scale measurements of daily life and the larger volumes encountered in engineering and trade.

Etymology and Historical Origins

The word "litre" traces back through a layered linguistic history. It is derived from an older French unit, the litron, whose name came from Byzantine Greek—where it was a unit of weight, not volume—via Late Medieval Latin, and which equalled approximately 0.82 litres. The litre served as a base unit in the original French metric system and continued in several subsequent versions of the metric system. In 1795, the kilogram was defined as the mass of one cubic decimetre of water at the temperature of melting ice (0 °C), binding the litre to the mass system in an elegant way. The spelling "litre" is the form used by the International Bureau of Weights and Measures and is shared by most English-speaking countries, while "liter" predominates in American English.

The Water Connection and Density

The litre's most celebrated practical property is its near-perfect correspondence with the kilogram of water. One litre of liquid water has a mass of almost exactly one kilogram when measured at its maximal density, which occurs at 3.984 °C. This means a millilitre of water weighs about one gram, and a thousand litres weigh roughly one tonne. The relationship was not arbitrary: the gram was originally defined as the mass of 1 mL of water, though that definition was abandoned in 1799 because water's density shifts with temperature and, to a lesser degree, pressure. Modern precision measurements of Vienna Standard Mean Ocean Water—pure distilled water with isotopic composition representative of the average of the world's oceans—yield a density of 0.999975±0.000001 kg/L at its point of maximum density (3.984 °C) under one standard atmosphere (101.325 kPa) of pressure. The density also varies with the isotopic ratios of the oxygen and hydrogen atoms in a particular sample, reminding us that the elegant 1-to-1 relationship is an approximation, not an absolute.

Symbol, Prefixes, and Practical Usage

The litre carries two internationally recognised symbols: lowercase l and uppercase L. Originally, only the lowercase form existed, following the SI convention that capital letters are reserved for units named after people. However, in many English-speaking countries, a handwritten "1" is simply a vertical stroke, making it nearly indistinguishable from "l" in some typefaces. To resolve this ambiguity, the CGPM adopted the uppercase L as an alternative in 1979. The United States NIST recommends the uppercase form, a practice widely followed in Canada and Australia, where prefixed forms like mL and μL are standard. 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). The most frequently used SI-prefixed derivative is the millilitre—one-thousandth of a litre, also called a cubic centimetre—pervasive in medicine, cooking, and automotive engineering. In 1990, the International Committee for Weights and Measures declared it too early to mandate a single symbol.