Gram

The gram (originally gramme; SI unit symbol g) is a unit of mass in the International System of Units (SI) equal to one thousandth of a kilogram.

Originally defined in 1795 as "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre [1 cm3], and at the temperature of melting ice", the defining temperature (0 °C) was later changed to the temperature of maximum density of water (approximately 4 °C). Subsequent redefinitions agree with this original definition to within 30 parts per million (0.003%), with the maximum density of water remaining very close to 1 g/cm3, as shown by modern measurements.

By the late 19th century, there was an effort to make the base unit the kilogram and the gram a derived unit. In 1960, the new International System of Units defined a gram as one thousandth of a kilogram (i.e., one gram is 1×10−3 kg). The kilogram, as of 2019, is defined by the International Bureau of Weights and Measures from the metre, the second, and from the fixed numerical value of the Planck constant (h).

Official SI symbol

The only unit symbol for gram that is recognised by the International System of Units (SI) is "g" following the numeric value with a space, as in "640 g" to stand for "640 grams" in the English language. The SI disallows use of abbreviations such as "gr" (which is the symbol for grains), "gm" ("g⋅m" is the SI symbol for gram-metre) or "Gm" (the SI symbol for gigametre).

History

The word gramme was adopted by the French National Convention in its 1795 decree revising the metric system as replacing the gravet (introduced in 1793 simultaneously with a base measure called grave, of which gravet was a subdivision). Its definition remained that of the mass (then called weight) of a cubic centimetre of water.