| Reference | Liters (L) | Cubic centimeters (cm³) |
|---|---|---|
| A teaspoon | 0.005 L | 5 cm³ |
| A can of soft drink | 0.33 L | 330 cm³ |
| A wine bottle | 0.75 L | 750 cm³ |
| A bathtub | 150 L | 150000 cm³ |
| An Olympic swimming pool | 2500000 L | 2.5 × 109 cm³ |
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 centimetre is a unit of volume equal to one millionth of a cubic metre, written cm³ and often abbreviated cc. It is the volume of a cube one centimetre on each side, and since the litre was redefined in 1964 it is exactly one millilitre. The two names describe the same quantity, but they belong to different trades and are not interchangeable in every context.
Engines are its most visible home. Motorcycle and small-engine displacement is quoted in cubic centimetres, so a 125, a 600 and a litre bike are named by the swept volume of their cylinders. Licence categories in much of the world are drawn on the same scale, with a 125 cc limit for learners in many European countries, which turns the unit into a legal boundary rather than a mere description.
Car engines cross over to litres at about a thousand cubic centimetres, purely for readability, so a 1998 cc engine is advertised as a two-litre. American practice used cubic inches for the same purpose well into the 1980s, and the conversion is worth knowing: 1000 cubic centimetres is 61 cubic inches, so a 350 cubic inch engine is 5.7 litres.
Medicine has deliberately retreated from the abbreviation. Syringes were once marked in cc and clinicians spoke that way, but handwritten cc can be misread as a unit symbol or as a pair of zeros, and medication safety bodies now place it on lists of abbreviations that should not be used. The instruction is to write mL instead, even though the quantity is identical.
Materials science keeps the unit for density. Grams per cubic centimetre is the standard way to state how heavy a substance is for its size: water is 1.00, aluminium 2.70, steel about 7.85, lead 11.34 and gold 19.3. Those numbers are memorable precisely because the cubic centimetre is small enough that everyday solids land between one and twenty.
Modelling and manufacturing use it for part volume. Computer-aided design reports the volume of a component in cubic centimetres, which converts directly to mass once a density is chosen and to material cost in additive manufacturing. It is the natural unit for anything that fits in a hand.
One cubic centimetre equals 1 millilitre, 1000 cubic millimetres, 0.000001 cubic metres, or about 0.061 cubic inches.