Light Travel Time Calculator
Light travels at exactly 299,792,458 meters per second in a vacuum, a value that is fixed by definition in the International System of Units. Knowing this speed, you can calculate precisely how long light takes to cover any astronomical distance. This calculator accepts distances in meters, kilometers, astronomical units (AU), light-years, and parsecs. It converts the input to meters, divides by the speed of light, and presents the travel time in seconds, minutes, hours, days, and years simultaneously so you can appreciate the scale of cosmic distances.
Light travel time formula
t (seconds) = distance (meters) / 299,792,458
The speed of light c = 299,792,458 m/s is exact by SI definition (since 1983). Convert seconds to minutes by dividing by 60, to hours by dividing by 3,600, and to years by dividing by 31,557,600 (the number of seconds in a Julian year of 365.25 days).
Distance reference points
1 AU (Earth-Sun) takes about 8 minutes 19 seconds. The Moon is about 1.28 light-seconds away. Mars at closest approach is about 3 light-minutes. Proxima Centauri is 4.24 light-years. The Andromeda Galaxy is approximately 2.537 million light-years away, meaning the light we see left it over 2.5 million years ago.
Light travel time: frequently asked questions
How long does light take to reach us from the Sun?
The Sun is about 1 astronomical unit (AU) from Earth, which is approximately 149,597,870 km. At the speed of light (299,792 km/s), that is about 499 seconds, or roughly 8 minutes and 19 seconds.
How far is one light-year in kilometers?
One light-year is the distance light travels in one Julian year (365.25 days). That is approximately 9.461 trillion kilometers, or 9.461 x 10 to the power of 12 km.
What is the speed of light used in this calculator?
The exact defined value is 299,792,458 meters per second, as fixed by the International System of Units (SI). This value is exact, not a measurement.
How long does light take to travel from the nearest star?
Proxima Centauri is about 4.243 light-years from Earth. Light from Proxima Centauri takes approximately 4.24 years to reach us.
What is 1 AU in light-minutes?
1 AU is approximately 149,597,870 km. Dividing by 299,792.458 km/s gives about 499 seconds, which equals approximately 8.32 light-minutes.
Official sources
- NIST: Speed of light definition: physics.nist.gov/cgi-bin/cuu/Value?c.
- IAU 2012 resolution B2 (AU definition): iau.org.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.