Friday, Jul 24, 2026 The claims desk. Receipts included. POWERED BY LENZ ↗
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SCIENCE

The Claim

The metre is defined as the distance light travels in a vacuum in 1/299,792,458 of a second.

Verified by Lenz ·

The Short Version

The claim accurately describes the metre in operational terms. Current SI sources define the metre by fixing the speed of light in vacuum at exactly 299,792,458 m/s, which is exactly equivalent to saying one metre is the distance light travels in vacuum in 1/299,792,458 of a second. The only caveat is that modern formal wording states this through c and the SI second rather than the older direct sentence.

Caveats

  • The current formal SI wording does not literally use this sentence; it defines the metre by fixing the numerical value of the speed of light in vacuum.
  • The statement depends on the SI second, which is itself defined separately from the caesium-133 transition frequency.
  • User-generated or social-media sources are unnecessary here and should not be relied on over BIPM, NIST, or other national metrology institutes.

The Receipts

  1. Resolution 1 of the 17th CGPM (1983)

    BIPM

  2. SI Brochure - 9th ed./version 4.01 EN

    BIPM

  3. Mise en pratique – metre – Appendix 2 to the SI Brochure

    BIPM

  4. SI base unit: metre (m)

    BIPM

  5. The International System of Units (SI), 2019 Edition

    NIST

  6. The SI

    BIPM

  7. SI Units – Length

    NIST

  8. metre (m) - NPL - National Physical Laboratory

    National Physical Laboratory

  9. SI defining constants

    BIPM

  10. International System of Units (SI) – The Meter

    NIST

+ 19 more sources — see the full list on Lenz

Filed Under

Metre