How the Kilogram Was Redefined in 2019 (And Why It Had To Be)
For 130 years the kilogram was a lump of metal in a vault near Paris. Then it started losing weight — and nobody could prove it.
On 20 May 2019 the kilogram stopped being an object and became an equation. It was the last SI base unit still defined by a physical artefact, and its retirement closed a chapter that began in 1889.
Le Grand K
The International Prototype of the Kilogram — a cylinder of platinum-iridium about 39 mm tall — was manufactured in 1879 and sanctioned as the standard in 1889. It sat under three nested glass bells in a climate-controlled vault at Saint-Cloud, near Paris, behind three independently keyed locks.
It was, by definition, exactly one kilogram. Not approximately. Its mass was the definition, so any measurement of it could only return 1.000000 kg. If it gained mass, the kilogram got heavier and everything else in the universe got proportionally lighter.
The drift nobody could explain
Forty official copies were distributed worldwide, plus six "témoins" (witnesses) kept alongside the prototype. Roughly every forty years they were compared.
The comparisons showed a problem. Over a century, the prototype appeared to have lost about 50 micrograms relative to its copies — roughly the mass of a grain of fine sand, about 5 parts in 100 million.
Likely culprits include surface contamination, absorbed atmospheric gases, mercury exposure, and material loss from the aggressive cleaning process itself. The point is that nobody could be certain, and a standard you cannot verify is not much of a standard.
Why this mattered beyond metrology
The kilogram propagated. The mole, the ampere and the candela were all defined in terms that depended on it, so uncertainty in the kilogram seeped into a large fraction of SI. Precision manufacturing, pharmaceutical dosing and fundamental physics all inherited the drift of an object in a French vault.
The Planck constant solution
The fix followed the 1983 metre precedent: fix a constant of nature and derive the unit.
The chosen constant was Planck's constant h, which relates a photon's energy to its frequency. Through mass-energy equivalence and quantum mechanics, h connects to mass. Fix h exactly, and the kilogram follows.
Since 2019, h is defined as exactly 6.62607015 × 10-34 joule-seconds. The kilogram is whatever mass makes that true, given the existing definitions of the metre and the second.
The Kibble balance
Turning that definition into a working measurement requires a Kibble balance (originally the watt balance, renamed for its inventor Bryan Kibble).
It operates in two phases. In weighing mode, the gravitational force on a test mass is balanced by the force on a current-carrying coil in a magnetic field. In velocity mode, the same coil is moved through the same field at measured speed and the induced voltage is recorded.
Combining the two eliminates the magnetic field strength and the coil geometry — the hardest quantities to measure — leaving mass expressed in terms of electrical measurements, which can be tied to h through the Josephson and quantum Hall effects.
An independent route, the X-ray crystal density method, counts the atoms in a near-perfect silicon-28 sphere. That the two entirely different approaches agree is what gave the redefinition its confidence.
What changed for you
Nothing. That was the point.
The redefinition was calibrated so the new kilogram matched the old one to within the measurement uncertainty — about one part in 108. No scale needed recalibrating, no recipe changed, no package was relabelled.
What changed is that the kilogram is now realisable in any suitably equipped national laboratory, rather than borrowed from a single object. It cannot be dropped, contaminated or stolen, and it will be identical in a thousand years.
Le Grand K still exists, still sits in its vault. It is now simply a very well-characterised one-kilogram mass — an artefact of history rather than the source of authority.