Why the kilogram is no longer a metal cylinder
Published
In a nutshell
From 1889 to 2019 the kilogram was, by definition, the mass of one object: the International Prototype of the Kilogram, a cylinder of platinum and iridium kept at the BIPM in Sèvres.
Since 20 May 2019 the kilogram is defined by fixing the value of the Planck constant. The value was chosen so that the kilogram stayed the same size, which is why nobody’s scales needed recalibrating.
One object for the whole world
The International Prototype of the Kilogram is a cylinder about 39 mm high and wide, made of 90 % platinum and 10 % iridium. The first General Conference on Weights and Measures sanctioned it in 1889, and it has been kept at the Bureau ever since.
As long as the definition rested on it, its mass was exactly one kilogram, by definition and without any uncertainty. If anything about it had changed, there would have been no way to tell from the definition alone.
A constant instead
In November 2018 the 26th General Conference fixed the Planck constant at exactly 6.626 070 15 × 10−34 J s. Together with the definitions of the second and the metre, that number defines the kilogram. The new definition came into force on 20 May 2019.
The roles swapped. The prototype’s mass is no longer exact: on the day of the change it was 1 kg with a relative uncertainty of 1 × 10−8, and future measurements will determine it.
How you get a kilogram from a constant
Two kinds of experiment do it. A Kibble balance weighs a mass against an electromagnetic force, comparing mechanical and electrical power; the Planck constant links the two. The X-ray crystal density method counts the atoms in a nearly perfect sphere of silicon-28 and works out its mass from them.
Laboratories in several countries run such experiments, and the BIPM operates a Kibble balance of its own. They compare their results in key comparisons organised by the Bureau. Until the realisations agree closely enough, the world’s kilogram is passed on from an agreed consensus value of their results.
What changed for you
Nothing you could see. The BIPM’s advice is that ordinary measurements are not affected and that measurements made under the old definition remain valid within their stated uncertainties. The change matters to the laboratories at the top of the calibration chain, and to anyone who wants to realise the kilogram without travelling to Sèvres.
The same reform redefined the ampere, the kelvin and the mole; see The seven defining constants.
Sources
- BIPM, International Prototype of the Kilogram (IPK), checked 28 October 2026
- BIPM, SI Brochure, Appendix 1: Decisions of the CGPM and the CIPM, checked 28 October 2026
- BIPM, Resolution 1 of the 26th CGPM (2018): On the revision of the International System of Units, checked 28 October 2026
- BIPM, Information for users about the redefinition of the SI, checked 28 October 2026
- BIPM, Mise en pratique for the definition of the kilogram in the SI (SI Brochure, Appendix 2), checked 28 October 2026
- BIPM, Key comparison of realizations of the kilogram (2023), checked 28 October 2026
- BIPM, The International System of Units (SI Brochure), 9th edition, checked 28 October 2026