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Light Travel Time Calculator

Find how long light, or a radio signal, takes to cross any distance: to the Moon, the Sun, Mars, the outer planets, Voyager 1 or a star. Enter your own distance in km, miles, AU or light years and see the one-way and round-trip delay.

Last updated:

Distance from

Radio waves and light travel at the same speed in space.

Enter your values to see the result.
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How to use the Light Travel Time Calculator

  1. Pick an object such as the Moon, Mars or Voyager 1, or switch to Enter a distance.
  2. Type the distance and unit if you entered your own.
  3. Choose the medium: space for astronomy, optical fiber for internet cables.
  4. Read the one-way and round-trip time.
  5. Compare with the other destinations in the table, or copy the results.

The light travel time formula

Light travel time is distance divided by the speed of light: t = d ÷ c, with c = 299,792.458 km/s (exact, by the definition of the meter). In a transparent material light slows to c ÷ n, where n is the refractive index. A light-second is the distance light covers in one second, 299,792.458 km; a light year is 9.4607 × 10¹² km.

DistanceLight timeNotes
1 km3.336 µsVacuum; about 4.9 µs in optical fiber
Earth's circumference (40,075 km)0.134 sLight could circle the equator 7.5 times per second
Earth–Moon (384,400 km)1.28 sAverage distance
1 AU (Earth–Sun)8 min 19 s499.005 s
Earth–Mars3 to 22 minDepends on orbital positions
Sun–Jupiter (5.2 AU)43 minAverage
Sun–Neptune (30.07 AU)4 h 10 minAverage
1 light-day24 hAbout 173 AU, roughly Voyager 1's distance in late 2026
Proxima Centauri4.25 yearsNearest star to the Sun

Why the delay matters

Every look into space is a look back in time: you see the Sun as it was about 8 minutes ago and Proxima Centauri as it was over 4 years ago. For spacecraft the delay sets how missions are run. Lunar teleoperation feels sluggish at 2.6 s round trip, Mars rovers receive a day's plan in one upload, and commands to Voyager 1 take about a day to arrive. Planet distances in the presets are averages or extremes; the real Earth–planet distance on a given date depends on the positions of both planets.

Frequently asked questions

How long does light take to reach Earth from the Sun?

About 8 minutes 19 seconds on average (499 seconds), because the mean Earth–Sun distance is 1 AU = 149,597,870.7 km and light covers 299,792.458 km every second. It varies from about 8 min 10 s in early January to 8 min 27 s in early July.

How long does it take light to reach the Moon?

About 1.28 seconds one way for the average distance of 384,400 km, so a laser or radio signal bounced off the Moon returns after about 2.56 seconds.

How long is the signal delay to Mars?

It depends on where the planets are in their orbits: from about 3 minutes (closest approach, ~54.6 million km) to about 22 minutes (farthest, ~401 million km) one way. That is why Mars rovers cannot be driven in real time.

How far away is Voyager 1 in light time?

Voyager 1 is roughly 173 AU from the Sun in late 2026, which is about one light-day: a command takes about 24 hours to arrive and the reply another 24 hours. Its distance grows by about 3.6 AU per year, and its distance from Earth changes by up to ±1 AU over the year.

What formula does the calculator use?

Time = distance ÷ speed. In vacuum the speed is c = 299,792.458 km/s exactly. In a material it is c ÷ n, where n is the refractive index, for example about 1.468 for the glass core of an optical fiber.

Do radio waves travel at the speed of light?

Yes. Radio waves, microwaves, visible light and X-rays are all electromagnetic waves and travel at c in a vacuum, so spacecraft communication delays equal the light travel time.

Why is internet latency higher than light time?

Light in fiber is about 32% slower than in vacuum, cables do not follow the straight-line path, and routers and servers add processing time. The calculator's fiber option gives the physical minimum for a given cable length.

Are all the tools on FreeToolCabin tested for quality and accuracy?

Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.

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