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Answer: Since May 20, 2019, the kilogram has been defined by a fixed value of the Planck constant, a constant of nature, instead of by the mass of one metal cylinder kept near Paris. The change was voted on November 16, 2018, in Versailles, along with new definitions of the kelvin, ampere and mole.

The old kilogram

For more than a century, the kilogram equaled the mass of the International Prototype of the Kilogram (IPK), nicknamed Le Grand K: a golf-ball-sized platinum-iridium cylinder cast in 1879 and kept in a triple-locked vault. After 1960 it was the only SI unit still defined by a manufactured object.

National metrology institutes kept copies, calibrated against the IPK, and passed the standard down a recorded chain of comparisons. Any drift in the artifact would ripple through every mass measurement.

The new SI

The revised SI rests entirely on seven unchanging constants, including the speed of light, the elementary charge and the Planck constant. Four base units (kilogram, kelvin, ampere, mole) were redefined; the second, meter and candela were already constant-based.

Why it matters

A constant-based kilogram can, in principle, be realized anywhere with the right equipment, and it cannot drift.

Claims

• id: C1 claim: The kilogram was defined by a platinum-iridium cylinder cast in 1879. source: S1 evidence: "cast in 1879 of platinum and iridium"
• id: C2 claim: In November 2018 scientists voted to redefine the kilogram. source: S1 evidence: "In November 2018, the international scientific community voted to redefine the kilogram"
• id: C3 claim: After 1960 the kilogram was the only SI unit defined by an object. source: S1 evidence: "After 1960, the kilogram was the only SI unit still defined in terms of a"
• id: C4 claim: On May 20, 2019, four base units were redefined. source: S2 evidence: "On May 20, 2019, four of them — the kilogram, kelvin, ampere and mole —"
• id: C5 claim: The vote happened on November 16, 2018, at Versailles. source: S2 evidence: "A vote to adopt the change happened on November 16, 2018, at Versailles, France"

Sources

• [S1] Kilogram: Introduction. NIST. https://www.nist.gov/si-redefinition/kilogram-introduction . License: US government work, public domain. Accessed 2026-11-04.
• [S2] SI Redefinition. NIST. https://www.nist.gov/si-redefinition . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_vkr5jw3j,
  author = {{iinventedlight}},
  title = {{How the kilogram was redefined}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/kilogram-redefinition/},
  note = {Permanent ID iil:note:vkr5jw3j. License: CC-BY-4.0.},
}

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