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

In today's SI, how precisely is the value of the Planck constant known?

Exactly: its value is fixed by definition, and units are built from it.

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  1. To about 8 digits, refined each year
  2. Only as an average of world labs
  3. Exactly, because the SI fixes its value ✔

Why: The revised SI turns the logic around. Seven constants of nature get exact fixed values, and the units are defined from them. So h has no measurement uncertainty any more.

Huh, didn't know that: The Planck constant is fixed at exactly 6.626 070 15 x 10^-34 joule seconds.

The dad joke
The Planck constant never changes its mind. It is very set in its ways.
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References

Answer: Exactly, because the SI fixes its value. The revised SI turns the logic around. Seven constants of nature get exact fixed values, and the units are defined from them. So h has no measurement uncertainty any more.

  1. BIPM: SI defining constants www.bipm.org/en/measurement-units/si-defining-constants
    The SI is defined in terms of a set of seven defining constants.
    Checked 2026-10-11.
  2. BIPM: SI defining constants www.bipm.org/en/measurement-units/si-defining-constants
    Planck constant h
    Checked 2026-10-11.

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