liter
Volumeexact by definition
The litre is exactly one cubic decimetre, that is 0.001 m³, a definition fixed in 1964. It is not an SI unit but is accepted for use with SI, and it is the standard unit of liquid volume nearly everywhere. The factor is exact by definition.
Watch out: Both "L" and "l" are accepted symbols. The uppercase L was approved in 1979 because a lowercase l is easily read as the digit 1 in many typefaces, and it is now the safer choice on labels and in code.
| 1 L | 1 L |
The litre was created in the French metric system as the volume of a cube one tenth of a metre on a side, chosen so that a litre of water at its densest weighs almost exactly one kilogram. Between 1901 and 1964 the definition was reversed to make that relationship exact, defining the litre as the volume of 1 kg of water at maximum density, which made it 1.000028 dm³. The 1964 CGPM undid that and restored the clean cubic decimetre.
About the liter
The litre's real design achievement is the chain it closes. A cube 10 cm on a side holds one litre; that litre of water weighs almost exactly one kilogram; and raising that kilogram by one degree Celsius takes about 4,184 joules. Volume, mass and energy line up with no conversion factors to memorise, which is the entire reason the metric system feels effortless for anyone doing water calculations. A cubic metre holds 1,000 litres and weighs one tonne, and a swimming pool, a rainfall depth and a pump duty all connect without a lookup.
That elegance briefly broke it. In 1901 the litre was redefined as the volume of one kilogram of pure water at the temperature of its maximum density, which made the water relationship exact but detached the litre from the cubic decimetre: the new litre was 1.000028 dm³, larger by 28 parts per million. For 63 years precise work had to say which litre it meant. The CGPM abandoned that in 1964 and defined the litre as exactly one cubic decimetre, accepting that a kilogram of water is now only approximately a litre. It is a good trade, because 28 ppm matters to almost nobody and a definition that depends on the purity and temperature of a sample of water matters to everybody who has to realise it.