How the Kilogram Was Redefined in 2019
Source: NIST. Published: 2025-01-15.
The Problem with a Physical Kilogram
For 130 years, the kilogram was defined by a small cylinder of platinum-iridium alloy stored in a vault outside Paris at the International Bureau of Weights and Measures (BIPM). Known as the International Prototype of the Kilogram, or Le Grand K, this object was the kilogram — by definition, its mass was exactly one kilogram. Every scale on earth was ultimately calibrated against it. But physical objects change over time. Microscopic contamination accumulates on the surface; cleaning the prototype removes material. When Le Grand K was periodically compared against its official copies, small but measurable discrepancies emerged. By the late twentieth century, metrologists had accepted an uncomfortable truth: the world's fundamental unit of mass was drifting, and there was no way to know whether it was the master or the copies that had changed.
Planck's Constant as the New Anchor
The solution was to stop defining the kilogram in terms of an object and instead fix it to a universal physical constant. Scientists chose Planck's constant, h — a fundamental quantity in quantum mechanics that relates the energy of a photon to its frequency. Planck's constant had been measured with extraordinary precision using an instrument called a Kibble balance, and it was known to be truly invariant: it does not drift, cannot be contaminated, and is the same everywhere in the universe. By fixing the numerical value of h to exactly 6.626 070 15 × 10⁻³⁴ joule-seconds, the 2019 SI revision made it possible to realise the kilogram from first principles anywhere in the world — in Tokyo, in Nairobi, on a spacecraft — without reference to a physical artefact in a Paris suburb.
The Kibble Balance: Linking Mass to Quantum Mechanics
The Kibble balance — invented by Bryan Kibble at the UK National Physical Laboratory in 1975 — is the instrument that makes the new kilogram definition practical. It works in two phases. In the weighing phase, the gravitational force on a mass is balanced against the electromagnetic force on a current-carrying coil sitting in a magnetic field. In the velocity phase, the same coil is moved through the same field at a known speed, generating a measurable voltage. By combining measurements from both phases, the device can relate the mechanical quantity of mass directly to the electrical quantities of voltage and resistance — which are themselves defined through quantum effects that can be measured with atomic precision. NIST operates one of the world's primary Kibble balances in Gaithersburg, Maryland, contributing to the global consensus value of Planck's constant.
The Avogadro Project: A Silicon Sphere Approach
Alongside the Kibble balance, an international collaboration known as the Avogadro Project pursued an alternative route to redefining the kilogram: counting atoms. The project produced near-perfect spheres of highly enriched silicon-28, precisely polished to be among the roundest objects ever made. By measuring the sphere's diameter and crystal lattice spacing with X-ray interferometry, scientists could calculate the exact number of silicon atoms in each sphere and thereby determine Avogadro's constant — the number of atoms in one mole of a substance. Both the Kibble balance and the Avogadro Project independently arrived at consistent values for Planck's constant and Avogadro's constant respectively, giving metrologists the confidence to commit to the new definitions. The two methods served as a cross-check, dramatically reducing the risk of systematic error.
What Changed on May 20, 2019
The new definitions took effect on May 20, 2019 — World Metrology Day — chosen to coincide with the anniversary of the 1875 Metre Convention, the treaty that created the BIPM. On that date, four SI base units were simultaneously redefined: the kilogram (through Planck's constant), the ampere (through the elementary charge), the kelvin (through the Boltzmann constant), and the mole (through Avogadro's constant). Le Grand K was retired as a definition, though it remains at the BIPM as a museum piece and a calibration reference. The change was deliberately designed to be imperceptible in everyday life — scales in supermarkets and laboratories did not need to be recalibrated, because the new kilogram matched the old one to within the uncertainty of the best available measurements.
Why It Matters for Everyday Measurement
For most people, the 2019 redefinition changes nothing about weighing a bag of apples. But for advanced manufacturing, pharmaceuticals, semiconductor fabrication, and scientific research, it matters enormously. Any industry that depends on mass measurements traceable to the SI now has a stable foundation that cannot drift or be destroyed. A national metrology institute that builds a Kibble balance can realise the kilogram independently, without sending weights to Paris for comparison. This distributes metrological sovereignty globally. It also future-proofs the definition: as measurement technology improves, the realisation of the kilogram will become more precise — something impossible when the definition was a lump of metal. The 2019 reform is widely considered the most significant change to the international system of measurement since the original Metre Convention.
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