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SESK: Stuff Everyone Should Know · sesk.info/eng-kilogram-planck-constant

Since the SI redefinition voted in 2018, what defines the kilogram?

A fixed value of the Planck constant, a constant of nature, not a metal cylinder.

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  1. A platinum-iridium cylinder kept near Paris
  2. The Planck constant, a constant of nature ✔
  3. The mass of one liter of pure water

Why: For over a century the kilogram was one metal cylinder, Le Grand K. Now it is tied to the Planck constant, so any well-equipped lab can realize it without visiting the cylinder.

Huh, didn't know that: The old artifact was about the size of a golf ball and was officially called the International Prototype of the Kilogram.

The dad joke
The old kilogram retired in 2019. It had been carrying a lot of weight for 130 years.
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References

Answer: The Planck constant, a constant of nature. For over a century the kilogram was one metal cylinder, Le Grand K. Now it is tied to the Planck constant, so any well-equipped lab can realize it without visiting the cylinder.

  1. NIST: Kilogram (SI redefinition) www.nist.gov/si-redefinition/kilogram
    redefine the kilogram in terms of an invariant of nature: the Planck constant
    Checked 2026-10-10.
  2. NIST: Kilogram (SI redefinition) www.nist.gov/si-redefinition/kilogram
    It even had a nickname: Le Grand K (The Big K).
    Checked 2026-10-10.

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