Kilogram
SI base unit · quantity: mass
01 · Definition
Until 2019 the kilogram was a thing: a platinum-iridium cylinder under three bell jars in a vault outside Paris. Now it is the Planck constant, recalculated through the metre and the second.
The kilogram is defined by fixing h = 6.626 070 15·10⁻³⁴ joule-seconds. A joule-second is kg·m²/s, so, knowing the metre and the second, mass follows from the Planck constant. In practice this is done by the Kibble balance: an electromagnetic force balances the load, and the mass is computed from electrical quantities that are themselves reproduced by quantum effects. The one oddity stayed in the name: the kilogram is a base unit with a prefix inside, so multiples are built from the gram, not from the kilogram.
The Kibble balance equates mechanical power m·g·v to electrical U·I, and U and I are taken from the Josephson and Hall effects — that is, from the Planck constant. Mass is measured without a single weight.
02 · Conversion
Enter a mass — the passport will convert it
In the SI, prefixes attach to the gram: milligram, microgram, megagram. In everyday life the megagram is called the tonne — it is outside the SI but accepted. Russian measures before 1918 were tied to the English pound, so the pood and the zolotnik convert exactly.
The kilogram-force (kgf) is not a mass but the weight of a kilogram: exactly 9.806 65 N. The SI has no such unit, yet old blueprints have it on every page.
| Unit | Name | Value | Where it appears |
|---|---|---|---|
| µg | microgram | 1·10⁹ | pharmacology, trace impurities |
| mg | milligram | 1 000 000 | pills, jewellery assays, analytics |
| g | gram | 1000 | kitchen, post office, laboratory |
| kg | kilogram | 1 | the base record of mass in the SI |
| t | tonne = Mg | 0.001 | cargo, machinery, industry |
| kt | kilotonne = Gg | 1·10⁻⁶ | fleets, raw materials, production statistics |
Prefixes attach to the gram: 1 t = 1 Mg, 1 kt = 1 Gg — there is no "kilokilogram" in the SI
03 · Orders of magnitude
from an electron to a starKilogram: a litre of water, a ream of paper
04 · Measuring instruments
What kilograms are measured with
The primary realization: mechanical power is equated to electrical. The kilogram follows from the Planck constant, accuracy 10⁻⁸.
Compares the standard with a working weight in vacuum: resolution 0.1 µg on a kilogram. This is how mass is passed down the calibration chain.
The XRCD method: atoms in a perfect sphere are counted via the crystal lattice. An independent route to the kilogram — through the Avogadro number.
Electromagnetic compensation instead of weights: it displays mass but measures force — which is why it is calibrated for the local g.
05 · Writing rules
Prefixes attach to the gram, not to the kilogram
The kilogram is the only base unit with a prefix in its name, so a "kilokilogram" is impossible: a thousand kilograms is a megagram, also known as the tonne. The symbol is lowercase: kg, while "Kg" and "kgs" are not SI. In English the plural is regular — five kilograms — and the symbol never takes an s.
06 · Neighbouring units
All of mechanics grows out of the kilogram: multiply by acceleration and you get the newton, by velocity squared — the joule. Thirteen SI derived units contain the kilogram; the tonne and the gram are its closest everyday relatives.
carrier of prefixes
SI base
kg·m/s²
Mass measures inertia and stored energy at once: a kilogram of matter holds 9·10¹⁶ joules.
07 · Historical section
What was weighed with before the kilogram
Forty Russian pounds. The measure of grain, pig iron and quit-rent; it held on in blueprints and waybills into the 1920s.
1/96 of the Russian pound. An apothecary and jeweller's measure: "small is the zolotnik, yet dear" — that saying is about it.
Exactly 200 milligrams since 1907. Before that every market had its own carat — from 188 to 213 mg.
A cylinder of platinum-iridium, 39 mm across, under three glass bell jars. For a hundred and thirty years it was the kilogram.
The last unit that was a thing — and the thing began to lose weight
By the 1990s it was clear: the international prototype and its official copies were diverging by about 50 micrograms per century. Which of them was "the real one" was impossible to tell — the kilogram by definition equalled the prototype, so it was not the prototype that slimmed but the rest of the world. This became the main argument of the 2019 reform: mass, energy and frequency are now linked through the Planck constant, and the Sèvres cylinder remains a standard only for the history of metrology.
Catalogue · International System of Units
A passport for every quantity
Seven base units, twenty-two derived units with names of their own, and an archive of systems withdrawn from circulation. Each is a separate sheet: definition, dimension, conversion, instruments, writing rules. In 33 languages.
Base units
open the passport of the kilogramDerived units with their own names
in terracotta — those whose definition contains the kilogramHistorical systems
archive of notationsPassport language
33 languages. The symbol kg and the formulas are not translated — they are identical in every locale; what is translated is the description, the examples and the writing rules for each script.