Speed of Light Calculator
The speed of light in vacuum is exactly 299,792,458 m/s (~300,000 km/s). In any transparent medium it slows to c/n, where n is the refractive index. Enter n and a distance to find the speed and the travel time of light between any two points.
Distance unit
c / n — light slows in any medium with refractive index n > 1
- 1
Speed of light in vacuum (c)
299,792,458Exact SI definition since 1983 - 2
Speed in medium (c ÷ n)
299,792,458 ÷ 1 = 299,792,458
How does this calculator work?
Speed of light in vacuum c = 299,792,458 m/s (exact). In a medium with refractive index n, light travels at v = c/n. Travel time t = d/v. Water (n ≈ 1.333): ~75 % of c. Diamond (n ≈ 2.417): ~41 % of c. Earth–Moon light travel time: ~1.28 s.
Formula
How this is calculated
Light in vacuum travels at exactly c = 299,792,458 m/s — a value fixed by the 1983 SI definition of the metre, making it a defined constant rather than a measured one. In a transparent medium, photons interact with atoms and re-emit at each one; the net result is a reduced phase velocity v = c/n, where n ≥ 1 is the refractive index of the medium. In air n ≈ 1.0003 (negligibly slower than vacuum); in water n ≈ 1.333 (25 % slower); in crown glass n ≈ 1.52; in diamond n ≈ 2.417 (light travels at about 41 % of c). The refractive index is also wavelength-dependent (dispersion): violet light bends more than red through a prism, which is why white light splits into a spectrum.
The travel-time formula t = d/v lets you calculate how long light takes to traverse any distance. Earth to Moon (384,400 km) takes about 1.28 s in vacuum; Earth to Sun (1 AU ≈ 149.6 million km) about 499 s (8 min 19 s); a round-trip signal to Mars at closest approach (~56 million km) takes about 6.3 min; light from our nearest stellar neighbour, Proxima Centauri (4.24 ly), takes 4.24 years.
Note: the speed here is the phase velocity. The group velocity — at which a light pulse and its energy actually propagate — equals the phase velocity in non-dispersive media but can differ significantly near absorption resonances. No causal signal or massive particle can exceed c in vacuum, but phase velocity in certain anomalously dispersive media can appear to exceed c without violating special relativity.
Frequently asked questions
The speed of light in vacuum (c) is a fundamental defined constant — 299,792,458 m/s exactly by SI definition. Light slows in any transparent medium (v = c/n), but this is not a violation of relativity: it is the macroscopic phase velocity that slows, and no causal information or energy exceeds c.
Vacuum: 1.000 exactly. Air: ≈ 1.0003. Ice: ≈ 1.31. Water: ≈ 1.333. Crown glass: ≈ 1.52. Flint glass: ≈ 1.70. Diamond: ≈ 2.417. Silicon (infrared): ≈ 3.45. Values vary with wavelength (dispersion) and temperature.
The mean Earth–Sun distance is 149.6 million km (1 AU). At c, light covers this in 499 s — about 8 minutes 19 seconds. We therefore see the Sun as it was ~8 min ago; if the Sun vanished, we would not know for over 8 minutes.
Also known as
TG we-Calculate Editorial Team. (2026). Speed of Light Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/speed-of-light-calculator
TG we-Calculate Editorial Team. "Speed of Light Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/speed-of-light-calculator.
TG we-Calculate Editorial Team, "Speed of Light Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/speed-of-light-calculator
@misc{wecalculate_speed_of_light_calculator, title = {Speed of Light Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/speed-of-light-calculator}}, year = {2026}, note = {TG we-Calculate} }
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