Natural unit of velocity

$c$ = 299 792 458 m s⁻¹

Value: 299 792 458 m s⁻¹
Standard uncertainty: exact
Relative uncertainty: exact
Status: Exact
Symbol: $c$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# natural unit of velocity, CODATA 2022 (NIST)
natural_unit_of_velocity = 2.99792458e8  # m s^-1, exact

# SciPy: scipy.constants.physical_constants['natural unit of velocity']

What it is

The unit of velocity in the natural units of relativistic quantum mechanics, where ħ, c and the electron mass are set to 1.

It is built only from constants with exact values, so it is exact too.

In other units

Unit	Value
CGS (cm s⁻¹)	2.997 924 58 × 10¹⁰	exact
km/s kilometre per second	299 792.458	exact
mi/s mile per second	186 282.3971	exact
c speed of light	1	exact

Converted with ConstantID's unit table; digits are rounded to the precision of the value, and a long exact value is rounded only for display.

How the value changed, 1998–2022

CODATA	Value (uncertainty)	Relative uncertainty	Change from previous
2022	299 792 458	exact	exact
2018	299 792 458	exact	exact
2014	299 792 458	exact	exact
2010	299 792 458	exact	exact
2006	299 792 458	exact	exact
2002	299 792 458	exact	exact
1998	299 792 458	exact

“Change from previous” is the shift in the value divided by the standard uncertainty of the previous adjustment. Earlier values are converted into the current unit where NIST changed it (for example J s to J Hz⁻¹).