Light travel time calculator
Enter a distance to see how long light takes to cross it in a vacuum, like sunlight reaching us in about 8 minutes 19 seconds.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter distance and distance unit.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
t = d ÷ c, where the speed of light c = 299,792,458 m/s exactly.
Example
Inputs: Distance = 1, Distance unit = Astronomical units (au). Result: Time in seconds: 499.004784 s; Duration (d:hh:mm:ss): 0:00:08:19.00; In minutes: 8.3167 min.
Limits and caveats
- Assumes travel in a vacuum; light is slower in air, water and optical fibre.
- Cosmic expansion is ignored, so it does not suit very large cosmological distances.
Questions about this tool
How long does sunlight take to reach Earth?
About 499 seconds, or 8 minutes 19 seconds, over the average distance of one astronomical unit.
How long does light from the Moon take?
About 1.28 seconds over the average distance of 384,400 km.
What is a light year?
The distance light covers in a Julian year of 365.25 days, about 9.46 trillion km.