What is the speed of light?
The speed of light in vacuum is c = 299,792,458 m/s exactly (by SI definition, zero uncertainty). In CGS units that is 2.99792458e10 cm/s.
Open the interactive PhysRef tool →Key facts
| Value | 299792458 m/s |
|---|---|
| Symbol | c |
| Exactness | Exact by SI definition (uncertainty 0) |
| CGS value | 2.99792458e10 cm/s |
| Source | CODATA 2018 |
Frequently asked questions
Is the speed of light exact?
Yes. Since 1983 c = 299,792,458 m/s is an exact defining constant of the SI, with zero uncertainty; the metre is defined from it.
What is the speed of light in km/s?
Approximately 299,792.458 km/s, often rounded to 300,000 km/s for quick estimates.
Why is c important?
It links space and time in relativity, defines the metre, and appears in E=mc^2 and Maxwell's equations as 1/sqrt(epsilon0*mu0).
Related pages
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