Light Travel Time Calculator
Enter a distance and find out how long light takes to cross it at 299,792 km per second.
Light is fast but not instant — it crosses about 300,000 km every second. One AU (the Earth–Sun distance) takes roughly eight minutes and twenty seconds, which is why we see the Sun as it looked a few minutes ago.
How it works
Light is the fastest thing there is, but it still takes time to get anywhere. It travels at 299,792.458 kilometers every second — fast enough to circle Earth about seven and a half times in a single tick of the clock.
The tool converts your distance to kilometers, then divides by that speed to get the travel time. It picks sensible units for the answer, from microseconds for short hops up to years for interstellar distances.
Try one AU, the average Earth–Sun distance: light covers it in about eight minutes and twenty seconds. That's why sunlight you feel right now actually left the Sun several minutes ago.
Frequently asked questions
How long does light take to reach us from the Sun?
About 8 minutes and 20 seconds. The Sun sits roughly one AU away, and light needs that long to cross the gap — so we always see the Sun slightly in the past.
What's a light-year?
It's a distance, not a time: how far light travels in one year, about 9.46 trillion kilometers. Enter a distance in light-years here and you'll get back that same number of years, since light covers exactly one light-year per year.
Why does light travel time matter in astronomy?
Because looking far away means looking back in time. Light from a galaxy millions of light-years off shows it as it was millions of years ago, not as it is today.