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Light Travel Time Calculator

Find the light travel time across any distance.

How it is calculated

The distance is first converted to kilometres using the selected unit: 1 million km (Mkm) = 1,000,000 km, 1 astronomical unit (AU) = 149,597,870.7 km and 1 light-year = 9.4607 × 1012 km. The travel time is then t = d / c with the speed of light in vacuum c = 299,792.458 km/s. The result is shown in seconds, minutes (t / 60) and hours (t / 3600), each with two decimals.

The distance cannot be negative. An unknown unit falls back to kilometres. The speed is that of light in a vacuum; in air, glass or fibre light is slower, and radio signals in space also travel at this speed. For very small distances the two-decimal display rounds to 0.00.

Worked example

Distance 1 AU, the average Earth-Sun distance. In kilometres this is 149,597,870.7. Time = 149,597,870.7 / 299,792.458 ≈ 499.00 s. Minutes = 499.00 / 60 ≈ 8.32. Hours = 499.00 / 3600 ≈ 0.14. The tool shows 499.00 s, 8.32 min and 0.14 h.

Try this example →

Common mistakes

  • Leaving the unit on kilometres while typing an AU or light-year value, which makes the time far too short.
  • Confusing the light-year (a distance) with a year (a time): one light-year of distance takes one year to cross.
  • Using the result as the delay of a real network link; light in fibre is slower and routing adds time.
  • Entering distances in miles without converting to kilometres.

About this calculator

This calculator finds how long light takes to travel a given distance, using time = distance / c, where c is the speed of light (about 299,792 km/s, or exactly 1 light-year per year by definition). It accepts distances in kilometers, astronomical units (AU, the Earth-Sun distance), or light-years, making it easy to work across scales from planetary to interstellar.

Astronomers and students use it to translate huge cosmic distances into something intuitive: sunlight takes about 8 minutes to reach Earth, light from the Moon takes about 1.3 seconds, and light from the nearest star beyond the Sun (Proxima Centauri) takes about 4.2 years — meaning we always see space as it looked in the past. The travel time is shown in seconds, minutes and hours.

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Comments (1)

  • Editorial team

    Radio signals travel at this speed too, which makes the tool useful for space communication delays. Mars ranges from about 54.6 million km to 401 million km from Earth, so a one-way signal takes roughly 3.04 to 22.29 minutes with the unit set to million km. A question and its reply take twice as long, because the trip is made twice. Which spacecraft or star would you like to check next?

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