How the Kilogram Was Redefined in 2019

On May 20, 2019 — World Metrology Day — the kilogram stopped being a lump of metal and became a number. After 130 years of being defined by a platinum-iridium cylinder locked in a vault near Paris, the kilogram was redefined in terms of a fundamental constant of nature: Planck's constant.

The Problem with a Physical Object

The International Prototype of the Kilogram (IPK) was created in 1889. A small cylinder about the size of a golf ball, made of 90% platinum and 10% iridium, it sat under three nested glass bells at the International Bureau of Weights and Measures (BIPM) in Sèvres, France. Forty copies were made and distributed to member nations. Legally and scientifically, the kilogram was defined as the mass of that object.

The problem: objects change. Every few decades, the national copies were brought to Paris to be compared with the original. By the 1990s, scientists noticed that the copies had drifted relative to the original — by up to 50 micrograms, roughly the mass of a fingerprint smudge.

Worse, there was no way to know whether the copies had gained mass, the original had lost mass, or both. The definition of the kilogram was literally unknowable at the level of precision that modern science needed.

Planck's Constant

The solution came from quantum physics. Planck's constant (h) is a fundamental number that relates the energy of a photon to its frequency. It appears throughout quantum mechanics and is one of the most precisely measured quantities in physics.

Scientists realized they could flip the relationship: instead of measuring Planck's constant, they could define it as an exact number and derive the kilogram from it.

Planck's constant was fixed at exactly 6.62607015 × 10⁻³⁴ joule-seconds. Combined with exact definitions of the second and the meter (themselves defined by atomic phenomena), this fixes the kilogram precisely — without reference to any physical object.

The Kibble Balance

Realizing this definition in practice required a device called the Kibble balance (formerly Watt balance). It compares mechanical power to electrical power with extraordinary precision, using the quantum properties of electrons to link mass to Planck's constant.

Any properly equipped national metrology laboratory can now realize the kilogram independently, without pilgrimage to Paris, without comparing objects that might drift, and without any physical artifact that could be lost, damaged, or stolen.

The Other Base Units

The kilogram was the last SI base unit defined by a physical artifact. In the same 2019 revision:

• The ampere was redefined via the elementary charge of an electron
• The kelvin was redefined via Boltzmann's constant
• The mole was redefined via Avogadro's number

All seven SI base units are now defined by fixed numerical values of fundamental constants. The international measurement system finally belongs to the universe — not to a vault in France.

Did Anything Change?

For everyday purposes: no. A kilogram of flour is still a kilogram of flour. The change was at the level of micrograms and below — relevant to cutting-edge science, invisible to commerce and daily life.

What changed was certainty. Scientists can now know the kilogram with unlimited precision, limited only by the quality of their instruments, not by the drift of a metal cylinder. That matters enormously for the most sensitive measurements in physics, chemistry, and engineering — measurements that will define the next generation of technology.