About this tool
The Light-Year & Light Travel Time Calculator converts an astronomical distance between light-years, parsecs, astronomical units (AU), kilometres and miles, and tells you how long light itself takes to cross that gap.
It works from fixed constants: one light-year is 9.4607 × 10¹² km, one parsec is about 3.0857 × 10¹³ km (equal to 3.2616 light-years), and one AU is 149,597,871 km. Your input is first converted to kilometres, then re-expressed in every other unit. Light travel time = distance ÷ the speed of light (299,792.458 km/s), and the result is formatted into seconds, minutes, hours, days or years automatically.
For example, 4.246 light-years — the distance to Proxima Centauri, the nearest star — is about 4.02 × 10¹³ km, and its light reaches us in 4.246 years. That the distance in light-years and the travel time in years match is exactly what the light-year unit means. Everything is computed locally in your browser with no uploads.
Frequently asked questions
What is a light-year?
A light-year is a distance, not a time: how far light travels in one year, about 9.4607 trillion kilometres or 5.88 trillion miles. It is used because interstellar distances are impractically large in kilometres.
How is light travel time worked out?
The distance is converted to kilometres and divided by the speed of light, 299,792.458 km/s. The tool then formats that number of seconds into the most readable unit — minutes, hours, days or years.
What is the difference between a light-year and a parsec?
Both measure distance. One parsec equals about 3.2616 light-years (3.0857 × 10¹³ km) and comes from stellar parallax — the distance at which one AU subtends one arcsecond. Astronomers often prefer parsecs.
How big is an astronomical unit?
One AU is the average Earth–Sun distance, 149,597,871 km. It is handy inside the Solar System; light crosses one AU in about 8.3 minutes, which is why sunlight is roughly 8 minutes old when it reaches us.
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