Litre
In the laboratory a new 1000 mL volumetric flask is being calibrated. It is placed on the balance, distilled water is added up to the ring mark and the reading is taken: 997.20 g. If a litre of water is taken to weigh a kilogram, the flask is almost three millilitres short and must be rejected. In fact the flask is accurate to a tenth of a millilitre. Water at 20 °C is lighter than at 4 °C, and the air buoys it up slightly.
The litre is the special name of the cubic decimetre. Since 1964, 1 L has been exactly equal to 1 dm³, with no connection to water. Before that the litre was defined through mass: the volume of one kilogram of pure water at 4 °C. As a result the litre came out 28 millionths larger than the cubic decimetre. The litre is not part of the SI but is accepted for use with it. Milk, fuel and wine are sold by the litre, and medicines are dosed in millilitres.
| Quantity | volume, capacity; dimension L³ |
| Definition | 1 L = 1 dm³ = 0.001 m³ exactly (12th CGPM, 1964) |
| Notation | L or l; submultiples mL, µL, multiple hL; the script form ℓ is not accepted |
| Status | non-SI; in the SI Brochure, in the table of units accepted for use with the SI |
| A litre of water | 999.97 g at 4 °C · 998.21 g at 20 °C · 997.16 g on a balance in air |
| Typical values | 50 µL — a drop · 250 mL — a glass · 158.987 L — a barrel of oil |
01 · Definition
A cube with a ten-centimetre edge
A litre is the volume of a cube with a 1 dm edge. It holds a thousand cubic centimetres, and a thousand litres make a cubic metre. A millilitre and a cubic centimetre are the same thing. A doctor writes 5 mL, and in old medical texts you can find 5 cc, that is, cubic centimetre.
The volume of glassware is almost never measured with a ruler. It is found by weighing water: the mass is divided by the density. The density of water depends on temperature, so a thermometer enters the calculation. The balance is calibrated with steel weights, while the water occupies a volume eight times larger, and the air buoys it up more strongly. Hence a correction of one part in a thousand. Glass expands when heated, and the volume is referred to 20 °C. The corrections for water and air are gathered in the factor Z of ISO 4787; the correction for glass is a separate multiplier in the formula.
How much a litre weighs
Each row is a litre of liquid on a balance. The bar is the mass, the thin vertical line is 1000 g. Temperature changes the density: on heating the liquid expands, and a litre holds less matter. Choose a temperature.
Five vessels, one balance and water
The main action is the “Start calibration” button. The empty vessel is placed on the balance and the reading is noted. Then distilled water is poured in, and the last drops are added with a pipette until the bottom of the meniscus touches the mark. The vessel with water is weighed again. The difference of the readings is converted to a volume at 20 °C and compared with the vessel's tolerance. Below the scene you choose the vessel: a 1000 mL volumetric flask, a measuring cylinder, a 25 mL pipette, a 1.5 L bottle with the ℮ sign or a kitchen jug. There you also set the water temperature and the eye position when reading the meniscus. The “Count 1 kg = 1 L” button switches on the household count: grams instantly become millilitres.
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02 · Conversion
The US gallon is defined through the cubic inch: exactly 231 in³. Since 1959 the inch has been 25.4 mm, so the US gallon in litres is the exact number 3.785411784.
The imperial gallon has also been exact since 1985: 4.54609 L. Before that it was defined as the volume of 10 pounds of water at 62 °F, just like the 1901 litre, through the mass of water.
03 · Orders of magnitude
04 · Measuring instruments
05 · Writing rules
06 · Neighbouring units
07 · Historical section
In 1901 the litre was defined as the volume of one kilogram of pure water at its maximum density and normal pressure. The creators of the metric system had expected this volume to equal the cubic decimetre. Measurements made in the early twentieth century showed that the litre was larger: 1.000028 dm³. The difference is 28 millionths, which on one litre is 0.028 mL.
In precise work two units appeared: the litre and the cubic decimetre. Densities were written sometimes in g/mL, sometimes in g/cm³, and the figures diverged in the fifth digit. In 1964 the 12th CGPM abolished the definition through water and made the litre another name for the cubic decimetre. Since then the density of water has been a measured quantity, not part of the definition of the unit.
Catalogue of quantities
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