# 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 ΔνCs 9 192 631 770 Hz speed of light in a vacuum c 299 792 458 m s-1 Planck constant h 6.626 070 15 × 10−34 J s elementary charge e 1.602 176 634 × 10−19 C Boltzmann constant k 1.380 649 × 10−23 J K-1 Avogadro constant NA 6.022 140 76 × 1023 mol-1 luminous efficacy Kcd 683 lm 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 299 792 458 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 1/299 792 458 of a second.