What speed of light value does this tool use?

The tool uses the exact defined value of 299,792.458 km/s (approximately 186,282 miles per second), which is the 1983 BIPM definition of the speed of light in a vacuum.

What is an Astronomical Unit (AU)?

One AU is the mean distance from Earth to the Sun, defined by the IAU as exactly 149,597,870.7 km. Light covers this distance in about 8 minutes and 20 seconds.

How many km is one light-year?

One light-year is exactly 9,460,730,472,580.8 km — the distance light travels in one Julian year (365.25 days) at 299,792.458 km/s.

How many km is one parsec?

One parsec equals 3.085677581×10^13 km, or about 3.26156 light-years. It is defined as the distance at which one AU subtends an angle of one arcsecond.

Why does it matter how long light takes to travel?

When you look at a star 100 light-years away, you are seeing it as it was 100 years ago. The farther the object, the older the image you observe. Light travel time is the universe's built-in time machine.

Can I use this for distances within Earth?

Yes. Enter a distance in km — for example, 40,075 km for Earth's circumference — and the tool will show that light circles Earth in about 0.134 seconds.

Light Travel Time Calculator

Convert distance to light travel time or elapsed time to distance, including one-way and round-trip signal paths.

Enter Distance

Calculation Mode

Distance

Unit

Signal Path

Time for light to travel from sender to destination.

Common Distances

Enter a distance above and press Calculate.

Method, scope, and privacy

Calculations use the exact vacuum speed of light, 299,792.458 km/s. Round trip means a signal travels the same distance out and back; in time-to-distance mode, the entered round-trip time is divided by two to find one-way range.

This is an ideal vacuum propagation result. Atmosphere, media, orbital motion, routing, equipment, processing, and relativistic spacecraft effects are not modeled. All inputs stay in your browser and no calculation request is sent to an external service.

Definitions: BIPM definition of the metre and c, IAU 2012 Resolution B2 (astronomical unit), and NIST SI length conversions.

Reference: Light Travel Times for Famous Distances

Journey	Distance	Light Travel Time

Summary

Convert distance to light travel time or elapsed time to distance, including one-way and round-trip signal paths.

How it works

Choose distance-to-time or time-to-distance mode.
Enter a distance or elapsed time and select its unit.
Choose a one-way light path or round-trip signal delay.
The tool converts your input to kilometers using exact IAU constants.
It divides the distance by the speed of light (299,792.458 km/s) to get travel time in seconds.
Results include decimal units plus a readable composite duration or converted distance.
Use the preset table to instantly load famous astronomical distances.

Use cases

Understand how "old" the light from distant stars actually is.
Teaching or studying the scale of the solar system and universe.
Comparing communication delays for hypothetical interstellar travel.
Visualizing why real-time communication with Mars probes is impossible.
Converting between distance units used in astronomy.
Checking how long ago light left a galaxy you are observing tonight.
Building intuition for cosmological distances in science writing.
Exploring how delays grow non-linearly as distances reach parsec scales.

Frequently Asked Questions

Last updated: 2026-09-19 · Reviewed by Nham Vu