Light Travel Time Calculator
Find how long light takes to cross a distance — from a room to another star — at 299,792,458 m/s.
How it works
t = d ÷ c · c = 299,792,458 m/s
Light travels at a fixed speed in a vacuum, so the time to cross any gap is simply distance divided by c. Because c is so large, everyday distances give tiny times, but astronomical distances make the delay dramatic — sunlight is over eight minutes old when it reaches you, and a "light-year" is defined as exactly the distance light covers in one year.
Worked example
The Sun is 1 AU = 1.496×10¹¹ m away. Time = 1.496×10¹¹ ÷ 299,792,458 ≈ 499 s ≈ 8.32 minutes, so you always see the Sun as it looked over eight minutes ago.
Sources: Speed of light — Wikipedia
Frequently asked questions
Why does light from stars show the past?
Because light takes time to arrive, you see a star as it was when that light left it. A star 100 light-years away appears as it looked 100 years ago — it might not even exist any more.
How far is a light-year?
A light-year is a distance, not a time: about 9.461×10¹⁵ m, the distance light travels in one Julian year. This tool converts it back to a travel time of exactly one year.
Does light travel slower through glass or water?
Yes, light slows in a medium (its speed is c divided by the refractive index), but this calculator uses the vacuum speed c, which is the standard reference for space distances.
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This calculator is for educational purposes. Double-check important results before acting on them.