Caesium-133 hyperfine transition frequency = 9 192 631 770 Hz

Name: Caesium-133 hyperfine transition frequency (ΔνCs) — CODATA 2022 value
Creator: NIST

Symbol: ΔνCs
Value: 9 192 631 770
Unit: Hz
Uncertainty: exact (by definition)
Copy-ready: 9192631770

Defines the second: one second is 9 192 631 770 periods of this transition.

NIST, Fundamental Physical Constants — Complete Listing (2022 CODATA adjustment), retrieved 2026-07-26.

The seven SI defining constants

Caesium-133 hyperfine transition frequency Speed of light in vacuum Planck constant Elementary charge Boltzmann constant Avogadro constant Luminous efficacy of 540 THz radiation

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UsePhysics. "Caesium-133 hyperfine transition frequency — 9 192 631 770 Hz." UsePhysics.com. https://usephysics.com/reference/caesium-hyperfine-frequency. Value from the 2022 CODATA adjustment (NIST). Accessed [date].