Speed of Light Calculator
metres per second — exact, by SI definition (1983)
Convert c into any speed unit, compute how long light takes to cross any distance, solve c = λν for wavelength or frequency, and see how much materials slow light down.
1Unit converter
Enter any speed — default is c itself — and convert between all six units instantly.
c in every unit
| Unit | Value |
|---|
2Time-of-flight calculator
How long does light need to cross a distance? t = d / c
Fun check: light crosses 30 cm (about a foot) in roughly 1 nanosecond — a delay GPS satellites and chip designers must budget explicitly.
3c = λν wavelength ↔ frequency solver
Speed of light equals wavelength (λ, lambda) times frequency (ν, nu). Solve either direction.
4Light-distance ladder
How far light travels in fixed times — the basis of the light-second, light-minute and light-year.
| Light travel time | Distance (km) | Distance (miles) |
|---|
1 light-year = c × 365.25 × 86,400 = 9.4607304725808 × 1015 m (about 9.46 trillion km / 5.88 trillion miles).
5Refractive slowdown — v = c / n
Light slows down in materials. Enter a refractive index n, or pick a material.
6How fast is that, really?
Reference comparisons computed from the exact c.
| Comparison | Result |
|---|
Scope boundary
- This tool covers light only — the constant c = 299,792,458 m/s in vacuum, its unit conversions, light travel times, the c = λν relation, light-year distances and slowdown in materials (v = c/n).
- It does not cover the speed of sound (a separate, medium-dependent constant ≈ 343 m/s in air). A dedicated speed-of-sound calculator is planned as a separate tool and is not linked here yet.
- Relativistic effects (time dilation, Lorentz factor, E = mc² energy equivalents) are out of scope — this tool treats c as the constant it is, without deriving relativity.
- Astronomical distances beyond the presets are best entered in AU or light-years in the time-of-flight calculator above.
Methodology & sources
Constants. c = 299,792,458 m/s exact — the 1983 SI definition of the metre
(BIPM SI Brochure, 9th ed.). Julian year = 365.25 × 86,400 = 31,557,600 s;
1 AU = 149,597,870,700 m exact (IAU 2012). Speed of sound 343 m/s is the 20 °C dry-air reference value.
Formulas. t = d / c · λ = c / ν · ν = c / λ ·
d = c × t · v = c / n · slowdown (1 − 1/n) × 100%.
All computation is 100% client-side IEEE-754 double precision; no data leaves your browser.
Honesty notes. Refractive indices are typical values at visible wavelengths and 20 °C — real n varies with wavelength (dispersion) and temperature. "Speed of light in a material" refers to phase velocity of the wavefront; signal velocity nuances are out of scope. Unit factors used: 1 mph = 0.44704 m/s exact, 1 ft = 0.3048 m exact, 1 mi = 1,609.344 m exact.
Frequently asked questions
14 questions · answers generated from the same source as the structured data below.
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