Speed of Light Calculator
The speed of light calculator tells you how long light takes to cross a distance, or how far it travels in a given time, in a vacuum, in air, in water and in optical fibre.
Example: Find: Distance → time · Distance 384,400 · Distance unit: Kilometres (km) · Medium: Vacuum (c) → Result: Light takes exactly 1.282 s to cover 384,400 km.. Source: BIPM – SI-Brochure: Lichtgeschwindigkeit im Vakuum c = 299 792 458 m/s (exakt). Updated: .
How it is calculated
Light needs about 1.28 seconds from the Moon to the Earth and about 8 minutes 19 seconds from the Sun. In a vacuum it covers 299,792.458 kilometres in one second.
How it is calculated
Travel time = distance / speed. The speed of light in a vacuum, c = 299,792,458 m/s, has been fixed exactly since 1983 and defines the metre. One astronomical unit (AU) is 149,597,870,700 m, and a light-year is the distance light travels in one Julian year (365.25 days), about 9.4607 trillion kilometres. In materials v = c / n with refractive index n: about 1.33 in water and 1.47 in optical fibre, so light there is roughly 25 to 32 percent slower.
Good to know
- A data packet through 1,000 km of fibre needs at least 4.9 ms, with no equipment in between. That sets a hard lower limit for latency.
- When you point a telescope at a star you see it as it looked one light travel time ago: Proxima Centauri just over 4 years ago.
- The planet values are mean distances from the Sun; the Earth–Mars distance varies a lot with position.
- Your input stays in the browser.
Frequently asked questions
How fast is the speed of light?
In a vacuum exactly 299,792,458 metres per second, about 300,000 km/s or roughly 1.08 billion km/h.
How long does light take from the Sun to the Earth?
On average 499 seconds, which is 8 minutes and 19 seconds, because the Earth is 1 AU from the Sun.
How far is a light-year?
About 9.46 trillion kilometres: the distance light travels in a vacuum in one Julian year (365.25 days).
Why is light slower in optical fibre?
In materials the refractive index n slows light: v = c / n. In fibre (n ≈ 1.47) it reaches about 204,000 km/s.
Sources and legal basis
- BIPM – SI-Brochure: Lichtgeschwindigkeit im Vakuum c = 299 792 458 m/s (exakt)
- IAU 2012 Resolution B2 – astronomical unit = 149,597,870,700 m; IAU – light-year and Julian year
- NASA – Planetary Fact Sheet (distances of the planets and the Moon)
- Wikipedia – Refractive index (Brechzahlen von Luft, Wasser, Glas)
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