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Light-Year to AU Converter

Enter a distance and direction to convert between light-years and astronomical units.

How it is calculated

Both directions use one fixed factor, 1 ly = 63,241.077 AU. Light-years to AU: AU = ly × 63241.077, shown with two decimals. AU to light-years: ly = AU / 63241.077, shown with six decimals. The tool also prints the rate line (for the reverse direction, 1 AU = 0.00001581 ly) and a check line that restates the conversion with the original number.

Choose the direction with the selector; the value is read in whichever unit that direction starts from. Negative numbers and non-numeric text return no result, and zero converts to zero. Because the AU-to-light-year answer is rounded to six decimals, very small distances lose precision: 1 AU shows as 0.000016 ly, so for small values use the rate line (0.00001581 per AU) or work in AU.

Worked example

Proxima Centauri is about 4.25 light-years away, which you want in astronomical units. In light-year to AU mode, multiply: 4.25 × 63,241.077 = 268,774.577, which the tool rounds to 268,774.58 AU. The check line repeats it as 268,774.58 AU = 4.2500 ly. Going the other way, 100 AU entered in AU to light-year mode gives 100 / 63,241.077 = 0.001581 ly.

Try this example →

Common mistakes

  • Leaving the selector on the wrong direction. Entering 4.25 while it is set to AU to light-years gives a tiny 0.000067 ly, not 268,774 AU.
  • Expecting more than six decimals when converting small AU values. A single AU shows as 0.000016 ly, which is rounded; the exact factor is 1 AU = 0.00001581 ly.
  • Typing a negative distance or including units or text. Only non-negative plain numbers are accepted.
  • Confusing light-years with parsecs or with kilometres. This tool only converts between light-years and AU; one parsec is about 3.26 light-years and needs a different factor.
  • Rounding the AU result and then converting it back by hand. The small rounding error in the printed figure is multiplied, so use the unrounded number you entered.

About this calculator

A light-year is the distance light travels in one Julian year, about 9.4607×10¹² km, while an astronomical unit (AU) is the average Earth-Sun distance, roughly 149.6 million km. The two are related by 1 light-year = 63,241.077 AU.

Astronomers reach for AU when describing distances within the solar system — planets, asteroids, spacecraft trajectories — and switch to light-years (or parsecs) once they're talking about interstellar distances, such as the ~4.25 light-years to Proxima Centauri, our nearest star beyond the Sun. This converter is a quick tool for astronomy students, science communicators, and space enthusiasts moving between these two standard distance scales.

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Comments (1)

  • Editorial team

    Light takes about 8.3 minutes to cross one AU, so a light-year is a way of saying how many of those 8.3-minute steps fit into a year of travel, roughly 63,241 of them. That makes a handy sanity check on the answer: the default of 4.2465 ly, near Proxima Centauri, comes out at 268,553.23 AU. Which distances do you most often need to put in perspective, and would other astronomical units be useful?

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