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liter (litre)

Convert liters and milliliters to other current units of volume

In SI, a special name for the cubic decimeter, and thus a unit of capacity. Symbol, l, and in the United States, L (which was approved as an alternative symbol by the CGPM in 1979, Resolution 6). One milliliter equals a cubic centimeter; 1 liter is 1000 cubic centimeters. The use of the liter in scientific work is discouraged, partly because between 1901 and 1964 the liter had a different definition.

History of the liter

In 1901 the 3rd CGPM resolved that “the unit of capacity, for high accuracy determinations, is the volume occupied by a mass of 1 kilogram of pure water, at its maximum density and at standard atmospheric pressure; this volume is called ‘liter.’” Since the kilogram had originally been conceived as the mass of a cubic decimeter of water under those conditions, one might expect that one milliliter would equal one cubic centimeter. However, the mass of the kilogram is actually defined by the mass of the platinum prototype, which was based on an 18ᵗʰ century determination of the mass of a cubic decimeter of water.

The 1901 definition, although defining a volume, made its size depend on a standard of mass (the International Prototype of the Kilogram), not on a standard of length (the International Prototype of the Meter). Its volume expressed in meters thus had to be determined experimentally, which led to the conversion factor 1 liter = 1.000027 × 10⁻³ cubic meters.¹ In 1950, the CIPM declared that 1 liter = 1.000028 cubic decimeters was the best conversion.

In 1964 the 12th CGPM (Resolution 6) abrogated the 1901 definition and declared “that the word ‘liter’ may be used as a special name for the cubic decimeter,” and recommended that it not be used to present the results of high accuracy volume measurements.

In the United States and a few other countries, the symbol for liter is “L”, to avoid confusion with the numeral “1.”² Existence of two symbols was reluctantly sanctioned by the 16ᵗʰ CGPM in 1979 (Resolution 6), although it expressed the hope of suppressing one of them in the future.

1. Ch. Ed. Guillame.
La Creation du Bureau International des Poids et Mesure et son Oeuvre.
Paris: 1927.

Pages 256-258.

2. National Bureau of Standards Miscellaneous Publication 233, 1960.
Footnote 1, page 2.

American National Standard ANSI/IEEE Standard 268-1982 Metric Practice.

Federal Register Notice of December 20, 1990.
“Metric System of Measurement; Interpretation of the International System of Units for the United States.”
(55 FR 522 42–522 45).