Kilogram

The kilogram (sometimes spelled kilogramme) is the SI base unit of mass, equivalent to 1,000 grams. Its symbol is kg. The name combines the metric prefix kilo- (meaning one thousand) with gram, and it is often shortened informally to "kilo" (plural "kilos"). As an SI base unit, the kilogram is now defined using three fundamental constants: a specific transition frequency of the caesium-133 atom, the speed of light, and the Planck constant. A metrology lab with proper equipment can use a Kibble balance to calibrate a mass measurement instrument as a primary standard for the kilogram.

Definition

Originally defined in 1795 during the French Revolution, the kilogram was the mass of one litre of water (first at 0 °C, later changed to the temperature of maximum density, about 4 °C). The current definition matches that original one to within 30 parts per million (0.003%). In 1799, the platinum Kilogramme des Archives replaced the water-based standard.

Redefinition based on fundamental constants

In 1889, a platinum–iridium cylinder called the International Prototype of the Kilogram (IPK) became the metric system’s mass standard, remaining so for 130 years until the current definition was adopted in 2019. The kilogram is defined by three constants: the atomic transition frequency ΔνCs (which sets the second), the speed of light c (which, with the second, sets the metre), and the Planck constant h (which, with the metre and second, sets the kilogram). The formal definition from the General Conference on Weights and Measures (CGPM) states: The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10⁻³⁴ when expressed in the unit J⋅s, which equals kg⋅m²⋅s⁻¹, where the metre and second are defined in terms of c and ΔνCs.

Name and terminology

This definition remains consistent with earlier ones: the kilogram is still within 30 parts per million (0.003%) of the mass of one litre of water at its maximum density (about 4 °C), where water’s density is very close to 1 kg/L. Timeline of previous definitions - 1793: The grave (precursor to the kilogram) was defined as the mass of 1 litre (dm³) of water, equal to 18,841 grains. - 1795: The gram (1/1000 of a kilogram) was provisionally defined as the mass of one cubic centimetre of water at the melting point of ice (0 °C). - 1799: The Kilogramme des Archives was made as a prototype, with a mass equal to 1 dm³ of water at its maximum density (about 4 °C). - 1875–1889: The Metre Convention was signed in 1875; the International Prototype of the Kilogram (IPK) was produced in 1879 and adopted in 1889. - 2019: The kilogram was defined in terms of the Planck constant, the speed of light, and the hyperfine transition frequency of ¹³³Cs, as approved by the CGPM on 16 November 2018. The kilogram is the only SI base unit with an SI prefix (kilo) in its name. The word kilogramme or kilogram comes from the French kilogramme, a learned coinage that prefixes the Greek khilioi ("a thousand") to gramma, a Late Latin term for a small weight (from Greek grámma). The word kilogramme was written into French law in 1795 in the Decree of 18 Germinal, which revised the provisional system of units introduced two years earlier.

In that earlier system, the gravet was defined as the weight of a cubic centimetre of water (1/1000 of a grave). The 1795 decree replaced gravet with gramme and grave with kilogramme. The French spelling was adopted in Great Britain when the word first appeared in English in 1795, while the spelling kilogram was adopted in the United States. In the UK, both spellings are used, though kilogram is now far more common; UK trading law does not forbid either.

In the 19th century, the French shortening kilo entered English, used for both kilogram and kilometre. While kilo is acceptable as an alternative (e.g., to The Economist), Canada’s Termium Plus system notes that SI usage in scientific and technical writing does not allow it, and Russ Rowlett’s Dictionary of Units of Measurement calls it a common informal name. The US Congress legalized the metric system in 1866, permitting kilo as an alternative to kilogram, but revoked that status in 1990.

The SI system was introduced in 1960, and in 1970 the BIPM began publishing the SI Brochure, which contains all CGPM decisions and recommendations on units. The Brochure states that abbreviations for unit symbols or unit names are not permissible. For use with East Asian character sets, the SI symbol is encoded as a single Unicode character, U+338F ㎏ SQUARE KG, in the CJK Compatibility block. The replacement of the International Prototype of the Kilogram (IPK) as the primary standard was driven by long-term evidence that the mass of the IPK and its replicas had been changing; the IPK had diverged from its replicas by about 50 micrograms since they were made in the late 19th century.