Litre
A non-SI unit accepted for use · quantity: volume
01 · Definition
The litre is the only unit with two equally valid symbols. The lower-case l looks too much like the digit one and the capital I, so in 1979 the capital L was permitted as well.
One litre is a cubic decimetre — a cube ten centimetres on a side. Formally it is one thousandth of a cubic metre, and the SI recommends dm³, yet the litre took such firm hold in trade, medicine and daily life that it was kept on the list of accepted units. The measure is tied to water: a litre of water at four degrees weighs a kilogram, and all household arithmetic rests on that. From 1901 to 1964 the definition was a different one — the litre equalled the volume of a kilogram of water, and that gave a discrepancy of twenty-eight parts per million.
The American gallon is twenty per cent smaller than the imperial one: 3.785 litres against 4.546. Fuel consumption in miles per gallon cannot be compared directly between the two countries, and packaging carries litres to avoid the confusion.
02 · Conversion
Enter a volume — the sheet converts it
Volume is the one quantity where every country kept its own measures: gallons, pints, barrels, quarts. The barrel is not single either: the oil barrel is 159 litres, while the American beer barrel is 117.
From 1901 to 1964 the litre was defined differently: as the volume of a kilogram of water at maximum density. It was larger than today's by 28 parts per million, and precise measurements from that period have to be recomputed.
| Unit | Name | Value | Where it occurs |
|---|---|---|---|
| ml | millilitre | 1 000 | medicine and cooking: equals cm³ |
| l | litre | 1 | trade, daily life, fuel |
| hl | hectolitre | 0.01 | winemaking, brewing, statistics |
| m³ | cubic metre | 0.001 | the SI unit: 1 m³ = 1000 l |
| cm³ | cubic centimetre | 1 000 | engine displacement, syringes |
| dm³ | cubic decimetre | 1 | the formal definition of the litre |
The litre is a cubic decimetre: a millilitre equals a cubic centimetre, and a cubic metre holds exactly a thousand litres
03 · Orders of magnitude
from a droplet to a reservoirA milk carton with room to spare — 1.5 l
04 · Measuring instruments
What litres are measured with
A single mark on a narrow neck: the volume is exact only at the stated temperature, usually twenty degrees. Class A gives an uncertainty of 0.1 %.
They measure out microlitres and millilitres in titration. An automatic pipette takes from one microlitre upwards to within a per cent.
An impeller or ultrasound counts the volume passing through. The water meter in a flat, the pump at a filling station, the metering unit in a boiler house — all one principle.
The reference method: the vessel is weighed empty and full of water, and the volume follows from the density. More accurate than any measuring glassware.
05 · Writing rules
l and L are equally valid, but keep to one of them within a document
The SI allows both symbols: the lower-case l is the historical one, the capital L was introduced because of confusion with the digit one and the letter I. They should not be mixed within a single text. Prefixes work with either: ml and mL are equally correct. The script ℓ is not provided for by the standard.
A capital M means mega: «250 Ml» reads as two hundred and fifty megalitres, that is a quarter of a cubic kilometre. For millilitres the m is always lower-case.
06 · Neighbouring units
The litre is a cubic decimetre, so the cubic metre stands beside it as the SI unit, and the millilitre coincides with the cubic centimetre. Through density the volume is tied to the kilogram, through flow rate to the second.
1000 litres
dm³
= cm³
A cubic kilometre is a trillion litres. Lake Baikal holds 23 615 km³, that is 2.4·10¹⁶ litres of water.
07 · Historical part
How volume was measured before the litre
231 cubic inches — Queen Anne's historical wine measure. Fuel, milk and paint are still sold by it in America.
The volume of ten pounds of water. Twenty per cent larger than the American one, which is why miles per gallon cannot be compared between the two countries.
The main liquid measure: a little over twelve litres. Vodka was counted in vedros and shtofs, and ten vedros made a barrel.
42 US gallons — the size of the barrel that oil was poured into in Pennsylvania. World prices are still quoted per barrel.
For sixty-three years the litre did not coincide with the cubic decimetre
In 1901 the conference on weights and measures decided to define the litre elegantly: as the volume of one kilogram of pure water at its maximum density and under normal pressure. The idea was to tie the measure of volume to the standard of mass. The trouble emerged later: the density of water had been measured with an uncertainty, and the litre so defined turned out to be larger than the cubic decimetre by 28 parts per million. For six decades precise work had to specify which litre was meant, and «ml» and «cm³» were not interchangeable. In 1964 the definition was repealed and the simple one came back: the litre is the cubic decimetre, full stop.
Catalogue · units of measurement
A sheet for every quantity
Seven SI base units, twenty-two derived ones with special names, and those non-SI units that neither engineering nor daily life can do without. Each has its own sheet: definition, conversion, instruments, writing rules. In 36 languages.
Non-SI units
open the litre sheetSI base units
in terracotta — the one the litre is defined throughSI derived units with special names
twenty-two sheetsPassport language
36 languages. The symbols l and L are international; what is translated are instrument names, liquids and the notes to the scales.