# Why does the metric system use “kilogram” as a base SI unit?

SI system uses all (that I know) measurement basic units as 1 (single) instance: meter, second, ampere, etc, except the KILOgram. It already defined with 1000 multiplier (kilo).

It prevents from using usual multiplier prefixes: mega, giga, tera, ... Though we sometimes use miligrams or micrograms. All points to the "gram" as a basic unit. But for some reason it isn't the case.

Does anybody have explanation of this fact? Why kilogram and not gram was decided to be a basic unit?

• Fun fact: in the waste industry, Mg (Megagram) is sometimes used instead of t (ton). This is entirely logical, somewhat counterintuitive and done only to separate the insiders from the outsiders. – mart May 14 '13 at 13:59
• @mart That would also help avoid confusion between short, long and metric tons. – Keep these mind May 14 '13 at 14:07
• What is the physics in this question? This seems way too localized to me ... – Dilaton May 14 '13 at 14:33
• This excellent video by Veritasium answers your question and goes on to discuss much more about the kilogram. – ejrb May 14 '13 at 14:35
• @Dilaton Are you kidding? When the kilogram will be redefined in terms of the Planck constant—I don't know—later this decade, then that will be physics news, as SI will rid itself of the last artefact. Some of your favourite bloggers might write about it, including even why the New SI will still use kilograms instead of grams. See here for the draft (p. 7). – Keep these mind May 14 '13 at 15:17

Among the base units of the International System, the kilogram is the only one whose name and symbol, for historical reasons, include a prefix. Names and symbols for decimal multiples and submultiples of the unit of mass are formed by attaching prefix names to the unit name "gram", and prefix symbols to the unit symbol "g" (CIPM 1967, Recommendation 2).

BIPM

The reason why "kilogram" is the name of a base unit of the SI is an artefact of history.

Louis XVI charged a group of savants to develop a new system of measurement. Their work laid the foundation for the "decimal metric system", which has evolved into the modern SI. The original idea of the king's commission (which included such notables as Lavoisier) was to create a unit of mass that would be known as the "grave". By definition it would be the mass of a litre of water at the ice point (i.e. essentially 1 kg). The definition was to be embodied in an artefact mass standard.

After the Revolution, the new Republican government took over the idea of the metric system but made some significant changes. For example, since many mass measurements of the time concerned masses much smaller than the kilogram, they decided that the unit of mass should be the "gramme". However, since a one-gramme standard would have been difficult to use as well as to establish, they also decided that the new definition should be embodied in a one-kilogramme artefact. This artefact became known as the "kilogram of the archives". By 1875 the unit of mass had been redefined as the "kilogram", embodied by a new artefact whose mass was essentially the same as the kilogram of the archives.

The decision of the Republican government may have been politically motivated; after all, these were the same people who condemned Lavoisier to the guillotine. In any case, we are now stuck with the infelicity of a base unit whose name has a "prefix".

BIPM The International Bureau of Weights and Measures (French: Bureau international des poids et mesures), is an international standards organisation, one of three such organisations established to maintain the International System of Units (SI) under the terms of the Metre Convention (Convention du Mètre). The organisation is usually referred to by its French initialism, BIPM.

Wikipedia

• This has some interesting history, but I don't think it's a correct answer to the question. The choice of a standard such as a 1 kg platinum-iridium bar rather than a 1 g bar is unrelated to the question of whether to do calculations in a system where the base unit is the kilogram or the gram. – Ben Crowell May 14 '13 at 15:21
• @Gugg: you can distinguish because many derived units such as newton, joule, pascal, volt etc are products of kilogram and other basic units. Because of this, when you do calculations writing all numbers in SI measurements, you use kilogram so it matches values with other units, and to match expectations of other people. The gram only behaves like a basic unit when you write values with prefixed units and the unit starts with kilogram. – b_jonas May 14 '13 at 15:40
• @b_jonas If the kilogram weren't the SI unit, then, counterfactually, the gram probably wouldn't be the SI unit either. Instead we would have an SI unit called somethingelsewithoutaprefix that happened to be equal to the kilogram. – Keep these mind May 14 '13 at 16:10
• @BenCrowell I come the following explanation for why we still have the kilogram as an SI base unit. I think it went like this. The kilogram was determined to be unit of mass. After that, newtons, joules, pascals, volts were derived from it. Then, there was no way back. The least messy option to get rid of the faux-prefix, would be to adopt the grave (=1 kilogram) after all. :) – Keep these mind May 14 '13 at 16:54
• @OleksandrPshenychnyy, We still use bytes in computer science despite we don't measure anything in them Say What? Obviously you are not a software developer. I specify the size of buffers and fields and the lengths of text strings in bytes on a daily basis. – Solomon Slow Aug 18 '15 at 17:38

The previous base unit, gram, was replaced by the kilogram in order to obtain coherence with the practical units ampere and volt.

In 1874 the mechanical units cm, g, s ('CGS') were adopted as the coherent system of units for science. In 1881 the CGS was coherently extended with the “absolute" electrical units abampere, abvolt, abohm. Coherence in this case primarily means that electrical energy and mechanical energy have identical units: $V\ I\ t = F \ L$. Unfortunately, the abvolt, abohm, were inconveniently small. Another inconveniently small unit was the unit of mechanical energy, the erg (=1 g⋅cm/s2).

In 1881-1889 the 'practical' units electrical units ampere, volt, ohm, and joule (1 joule = 1 V⋅A⋅s = 10^7 erg) were introduced for practical use. Their magnitude was convenient, but they were not coherent with CGS. In 1901, Giorgi pointed out that the practical units are coherent with m, kg, s.

Derivation: $[E] = [F\cdot L] = [(M\ L\ t^{-2})\ L] = [M]\ [L]^{2}\ [t]^{-2}\rightarrow$ $\rightarrow [M] = [E]\ [t]^2\ [L]^{-2} = 1\ J s^{2} m^{-2} = 1\ kilogram$

Because of this, the scientific community adopted the SI system in 1960, with the kilogram as base unit.

In summary: the kilogram became the base unit because it is coherent with the joule, which is derived from the practical units volt and ampere.

(source: Jayson, Amer. J. Phys. 82 (2014) 60)

• Note that the kilogram being a base unit has nothing to do with the kilogram being the calibration artefact. This is shown by the example of the CGS unit system. – jkien Aug 18 '15 at 10:57

There are various systems of units that fall within the family colloquially referred to as the metric system. The SI (formerly known by the more descriptive term mks) is based on the meter, kilogram, and second. The cgs system is based on the centimeter, gram, and second. As far as I know, nobody actually uses an mgs (meter-gram-second) system, although it would be logical to use a system with no prefixes.

Whether to use mks or cgs is an arbitrary convention, and it depends on the field you're in. Astronomers and particle physicists often use cgs. Engineers tend to use mks. The cgs units for force and energy are inconveniently small for many mechanical applications.

Mks and cgs are also associated with different electrical units. This causes equations such as Maxwell's equations to have different forms in the two systems. WP has the gory details.