Free · Astronomy calculators 🌌

Light Travel Time & Distance

Convert propagation time and distance at vacuum light speed or a supplied positive speed up to that limit.

  • Formula & worked example
  • Private in your browser
  • No signup
Go to tool

Calculator inputs

Loading calculator…

How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Private by default

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

At vacuum light speed, a one-way trip of 299,792,458 m takes exactly one second. One Julian year of travel is one light-year.

Distance = propagation speed × elapsed time

Worked example

Enter these known values and leave the other values blank.

One-way elapsed time
1 sec
Travel distance
299792458 m

Assumptions and limitations

  • Time and distance are nonnegative. Propagation speed must be positive and no greater than 299,792,458 m/s; the starting value is exact vacuum light speed.
  • This is one-way travel with constant speed. A round-trip echo covers twice the one-way range. In a medium, use the relevant propagation speed for your question.
  • A year is exactly 365.25 days; a month is one twelfth of that fixed year. The 13.799 Gyr selector is a fixed interval, not a current measurement of the age of the universe.
  • Large-distance results do not model cosmic expansion, curved spacetime or cosmological distance definitions. Earth and solar radius selectors use the fixed reference lengths shown.

Common questions

Is a light-year a duration?

No. It is the distance light travels in vacuum in one Julian year: exactly 9,460,730,472,580,800 meters.

How do I use a round-trip radar time?

Divide the measured round-trip duration by two to get the one-way duration, assuming the outward and return paths have the same speed and length.

References

The light-year uses exactly 299792458 m/s times a 31557600-second Julian year, replacing the rounded reference factor. The parsec uses the IAU definition 648000/π astronomical units, with one astronomical unit exactly 149597870700 meters, instead of a rounded decimal factor. A million light-years uses exactly one million Julian light-years, consistently with the reviewed single light-year unit. A megaparsec uses exactly one million IAU-defined parsecs, consistently with the reviewed single parsec unit.

Explore astronomy calculators 🌌 or browse all physics calculators.