Speed of light in vacuum
= 299 792 458 m s−1
- Value
- 299 792 458 m s−1
- Standard uncertainty
- exact
- Relative uncertainty
- exact
- Status
- Exact by definition since 1983
- Symbol
- Source
- CODATA 2022 via NIST · NIST entry
Use in a calculation
# speed of light in vacuum, CODATA 2022 (NIST)
c = 2.99792458e8 # m s^-1, exact
# SciPy: scipy.constants.physical_constants['speed of light in vacuum']What it is
The speed at which light and all other electromagnetic waves travel through empty space, and the fastest anything that carries information can go. Since 1983 it has had an exact value by definition, and the metre is defined from it: one metre is the distance light travels in vacuum in 1/299 792 458 of a second.
The value has been exact since the 17th General Conference on Weights and Measures defined the metre from it in 1983.
Equations
- mass–energy equivalence
- light as an electromagnetic wave
- wavelength and frequency
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).