# Defining constants

All units in the SI are defined by seven constants.

The seven constants have been chosen in such a way that any unit of the SI can be written either through a defining constant itself or through products or quotients of one or more of the defining constants.

## SI defining constants

 Defining constant Symbol Value Unit hyperfine transition frequency of Cs Î”Î½Cs 9â€¯192â€¯631â€¯770 Hz speed of light in a vacuum c 299â€¯792â€¯458 mÂ s-1 Planck constant h 6.626â€¯070â€¯15â€¯Ã—â€¯10âˆ’34 JÂ s elementary charge e 1.602â€¯176â€¯634â€¯Ã—â€¯10âˆ’19 C Boltzmann constant k 1.380â€¯649â€¯Ã—â€¯10âˆ’23 JÂ K-1 Avogadro constant NA 6.022â€¯140â€¯76â€¯Ã—â€¯1023 mol-1 luminous efficacy Kcd 683 lmÂ W-1

The numerical values of the seven defining constants are exact.

The seven SI defining constants are fundamental constants of nature, or technical constants, whose numerical values are fixed and unvarying.

For example, the speed of light in a vacuum is a constant of nature, and in the SI is defined to be exactly 299â€¯792â€¯458 metres per second. It follows that one metre is the length of the path travelled by light in a vacuum during a time interval with duration of 1/299â€¯792â€¯458 of a second.