| Reference | Liters (L) | Cubic meters (m³) |
|---|---|---|
| A teaspoon | 0.005 L | 0.000005 m³ |
| A can of soft drink | 0.33 L | 0.00033 m³ |
| A wine bottle | 0.75 L | 0.00075 m³ |
| A bathtub | 150 L | 0.15 m³ |
| An Olympic swimming pool | 2500000 L | 2500 m³ |
The litre is a unit of volume equal to one thousandth of a cubic metre, or exactly one cubic decimetre. Its symbol is L, and the lower-case l is also permitted. It is not part of the International System of Units, but it is accepted for use alongside it, and it is almost certainly the most widely used volume unit in the world.
It was created by the French metric law of 1795 as the volume of a cube one tenth of a metre on a side. In 1901 it was redefined as the volume occupied by one kilogram of pure water at its densest, which sounds equivalent but is not: the water definition made a litre 1.000028 cubic decimetres, and for sixty-three years precise volumetric work had to state which litre was meant. The 1964 General Conference on Weights and Measures restored the cubic decimetre and the discrepancy vanished.
The symbol has its own small history. The lower-case l is easily mistaken for the digit one in many typefaces, a genuine hazard on a medicine label or a laboratory record, so in 1979 the capital L was approved as an alternative. Both are correct, and usage splits by country rather than by discipline, with North America favouring L and much of Europe still writing l.
Its everyday reach is enormous. Fuel, milk, paint, soft drinks and engine displacement are all measured in litres, fuel consumption in most of the world is stated as litres per hundred kilometres, and cooking outside the United States works in litres and millilitres. Medicine uses it for blood volume, roughly five litres in an adult, and for lung capacity, around six.
One relationship makes it exceptionally useful. A litre of water has a mass very close to one kilogram, because the kilogram was originally defined that way, so volume and mass can be swapped in the head for most practical purposes. Rainfall follows the same logic: one millimetre of rain on one square metre is exactly one litre of water.
Against imperial and United States measures it converts untidily, which is why drink sizes differ so visibly between markets. One litre is 1.76 imperial pints but 2.11 United States liquid pints, and 0.22 imperial gallons but 0.264 United States gallons, so a gallon means two noticeably different quantities depending on where the label was printed.
One litre equals 1000 millilitres, 0.001 cubic metres, about 0.264 US gallons, or about 0.22 imperial gallons.
The cubic metre is the SI unit of volume, written m³. It is the volume of a cube one metre on each side, and because the metre is a base unit, every other volume unit in the system is derived from it. It holds a thousand litres, and a cubic metre of fresh water has a mass of very nearly one tonne, which is the single most useful fact about it.
Utilities are billed in it. Domestic water meters almost everywhere record consumption in cubic metres, and a household of four uses somewhere between 100 and 200 of them a year. Natural gas is metered the same way and then converted to energy for billing, because the heat released by a cubic metre of gas depends on its composition and has to be corrected for temperature and pressure at the meter.
Construction orders materials by it. Ready-mixed concrete is sold and delivered by the cubic metre, a standard truck carrying six to eight; excavation and earthworks are priced per cubic metre moved; and sawn timber, aggregate, topsoil and bark are all traded the same way. Because a cubic metre of concrete weighs about 2.4 tonnes, the unit also decides what a lorry or a crane can carry.
Freight uses it in a more subtle way. Shipping and removals quote volume in cubic metres, and air freight charges by whichever is greater, the actual mass or a volumetric mass calculated at a fixed ratio, commonly 167 kilograms to the cubic metre. Light bulky cargo is therefore priced by the space it occupies rather than by what it weighs, and the whole calculation runs on this unit.
Environmental measurement lives here too. River flow is given in cubic metres per second, reservoir capacity in millions of cubic metres, air quality as micrograms of pollutant per cubic metre of air, and ventilation as cubic metres per hour. A room four metres by five with a ceiling at two and a half holds fifty cubic metres of air, which is the sort of figure ventilation standards are written against.
Against imperial and United States units it converts to 35.315 cubic feet or 1.308 cubic yards. The cubic yard matters most in practice, because American concrete and excavation are ordered in it, and the two units are close enough that a mistaken figure looks plausible while being a third out.
One cubic metre equals 1000 litres, about 35.315 cubic feet, about 1.308 cubic yards, or about 264.2 US gallons.