For six decades, a litre was slightly bigger than a cubic decimetre
Between 1901 and 1964, a litre was defined as the space filled by one kilogram of pure water. The trouble was that the official kilogram cylinder turned out about 28 parts per million too heavy, so every litre was fractionally oversized. In 1964 the link to water was dropped.
A litre is simply a cube 10 centimetres on each side: one cubic decimetre, 1,000 cubic centimetres, or 0.001 of a cubic metre. France's first metric scheme made it a base unit, and the name descends from an older French measure, the litron, about 0.82 of a litre, whose name came via medieval Latin from a Byzantine Greek unit of weight. The International Bureau of Weights and Measures spells it litre; American English prefers liter.
Its tie to water comes from the kilogram, first defined in 1795 as the mass of a cubic decimetre of water at the melting point of ice. That is why a litre of water weighs almost exactly a kilogram, a millilitre about a gram, and 1,000 litres about a tonne. The match is no longer exact, since water's density shifts with temperature, pressure and even the isotopes of its hydrogen and oxygen. Vienna Standard Mean Ocean Water, a reference blend matching average ocean composition, reaches 0.999975 kilograms per litre at its densest, at 3.984 degrees Celsius.
Oddly, the litre is not an SI unit, and the bureau no longer lists it as accepted for use with the SI; the official unit of volume is the cubic metre. It still takes SI prefixes, and the millilitre, also called a cubic centimetre, is common in medicine, cooking and car engines. Litres tend to measure what fills a container, while cubic metres describe dimensions or displacement. Americans favour a capital L as the symbol, because a lowercase l can be mistaken for the digit 1 in some fonts.
Against older units, a litre is roughly 5.7 percent bigger than a US liquid quart and about 12 percent smaller than an imperial quart. An old rhyme puts a litre of water at a pint and three-quarters, close to the true figure of about 1.76 imperial pints. A cubic foot holds exactly 28.316846592 litres.
Source: Litre
