Definition · Plain-language
The kilogram
The kilogram is the SI base unit of mass, redefined in 2019 by fixing the value of the Planck constant, ending more than a century of reliance on a single metal cylinder.
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The unit of mass
The kilogram is the SI base unit for mass — the amount of matter in an object — with the symbol kg. It is unusual among the base units in that its name already carries a prefix, “kilo”, meaning a thousand; the kilogram is effectively a thousand grams. From it flow many derived units: force in newtons is kilogram metres per second squared, and energy in joules is built on the same foundation. In everyday life people loosely call a body’s “weight” in kilograms, but strictly the kilogram measures mass, which stays the same everywhere, while weight is the gravitational force on that mass.
The 2019 redefinition
In 2019 the kilogram became the last base unit to be freed from a physical artefact. It is now defined by fixing the numerical value of the Planck constant, a fundamental constant linking energy and frequency, at an exact figure. In practice the kilogram is realised using an instrument called the Kibble balance, which relates mechanical and electrical power so precisely that a mass can be measured against the fixed Planck constant. Because the definition rests on a constant of nature rather than an object, the kilogram is now stable and reproducible anywhere, with no master copy that could drift.
The end of “Le Grand K”
For 130 years the kilogram was defined by a single cylinder of platinum-iridium, the International Prototype of the Kilogram, nicknamed “Le Grand K”, kept under bell jars near Paris. Every kilogram in the world was traceable to this one object. The flaw was obvious in hindsight: the prototype’s mass appeared to drift slightly relative to its official copies over the decades, meaning the world’s definition of mass was literally changing. The 2019 redefinition retired the cylinder, anchoring the kilogram instead to an unchanging constant — a milestone in making the entire SI independent of physical artefacts.
Key facts
At a glance
- Definition: the SI base unit of mass, symbol kg
- Current basis: fixing the value of the Planck constant
- Redefined: 2019 — the last base unit to lose its artefact
- Old basis: a platinum-iridium cylinder near Paris (“Le Grand K”)
- Realised by: the Kibble balance
- Note: already carries the “kilo” (×1,000) prefix
Common misconceptions
What people often get wrong
Often heard: The kilogram is still defined by a metal cylinder kept in Paris.
Actually: Not since 2019. The kilogram is now defined by fixing the Planck constant, so it no longer depends on any physical object that could gain or lose mass over time.
Often heard: The kilogram measures weight.
Actually: It measures mass — the amount of matter, the same everywhere. Weight is the gravitational force on that mass and is measured in newtons; it changes with gravity, mass does not.
Often heard: The gram is the SI base unit and the kilogram is derived from it.
Actually: It is the other way around: the kilogram is the base unit, even though its name contains the “kilo” prefix. The gram is one-thousandth of the base unit.
Going deeper
