| Reference | Liters (L) | Hectoliters (hL) |
|---|---|---|
| A teaspoon | 0.005 L | 0.00005 hL |
| A can of soft drink | 0.33 L | 0.0033 hL |
| A wine bottle | 0.75 L | 0.0075 hL |
| A bathtub | 150 L | 1.5 hL |
| An Olympic swimming pool | 2500000 L | 25000 hL |
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 hectolitre is a unit of volume equal to one hundred litres, or a tenth of a cubic metre. Its symbol is hL. It is the trading unit of the drinks industry and of much of agriculture, and although almost nobody outside those trades ever writes it, within them it is the number that appears on every contract, licence and annual report.
Brewing runs on it entirely. A brewery's capacity is stated in hectolitres a year, duty is assessed on hectolitres produced, and the large international groups report annual volumes in hundreds of millions of them. A standard keg holds half a hectolitre, so the unit also happens to match the physical container the trade moves, which is part of why it stuck.
Wine regulation is written in the same terms. European appellation rules cap yield in hectolitres per hectare, a figure that does more than any other single number to define the character of an appellation, since a low cap concentrates the fruit and a high one dilutes it. Production statistics, stocks and the surpluses that periodically trouble the European market are all counted the same way.
Grain trading uses the unit in an unexpected direction. The hectolitre weight, or test weight, is the mass of grain that fills one hectolitre, and it is a standard quality specification: milling wheat is typically bought at 76 to 80 kilograms per hectolitre, and a load below the contracted figure is downgraded. The measurement works because well-filled, undamaged grain packs more densely than shrivelled grain does.
Dairy and other bulk liquids follow. Milk collection, fruit juice processing, edible oil and cider are all recorded in hectolitres, because the quantities involved are too large for litres to read comfortably and too small for cubic metres to be precise enough for duty and quality purposes.
For scale, a hectolitre is a large domestic water tank, a household bath filled about one and a half times, or a barrel of oil and a half. It converts to 22 imperial gallons or 26.4 United States gallons, and to 0.1 cubic metres exactly, which is the quickest route into SI-based calculations.
One hectolitre equals 100 litres, 0.1 cubic metres, about 26.4 US gallons, or about 22 imperial gallons.