Light Travel Time Calculator
Calculate how long light takes to travel a given distance.
Assumptions used in this calculation
- Light travels in a vacuum: Assumes light travels through a vacuum at exactly 299,792.458 km/s, unaffected by any intervening medium.
About this calculator
The universe's distances are so vast that describing them in kilometres becomes unwieldy, and manually dividing an astronomical distance by the speed of light (299,792.458 km/s) means first converting light-years or astronomical units into kilometres, tracking a lot of zeros, and then formatting a result that might come out in seconds, minutes, or years depending on scale. This calculator takes a distance in kilometres, astronomical units, or light-years, divides by the speed of light, and automatically formats the result in whichever time unit makes it most readable, whether that's seconds for a distance across a room-sized lab setup or years for a distance to a nearby star. It's built for astronomy students and enthusiasts calculating how long light (and therefore any information or observation) takes to cross a given distance, useful for understanding why we see distant stars and galaxies as they appeared in the past rather than as they are now.
Worked example
Distance to Proxima Centauri (4.24 light-years)
Result: 4.24 years
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