Second (s)
Definition
The SI base unit of time, defined by the caesium-133 hyperfine transition.
History
The second was historically one 86400th of a day, but since 1967 it has been defined by the caesium atom, as 9 192 631 770 oscillations of its radiation. This atomic definition made it far more precise than astronomical timekeeping.
Where it is used
The second is the SI base unit of time, underlying clocks, timers and all scientific measurement.
Typical uses include timers, physics.
Conversion pages
These pages convert second to or from another unit in time.
No conversion pages use this unit yet.
Other Time units
Sources
- BIPM SI BIPM SI Brochure (2019); the second is defined by the caesium-133 transition · Dataset reviewed Sep 18, 2026