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Parsec to Light-Year Converter

Enter a distance in parsecs to convert it to light-years and more.

How it is calculated

The tool multiplies the distance in parsecs by three constants: ly = pc × 3.2615637769, AU = pc × 206264.806 and km = pc × 3.0856775815×10¹³. Light-years are shown with four decimals, astronomical units with two decimals, and kilometres in scientific notation such as 4.0114 × 10¹³. Any number is accepted, including very small or very large values; for distances in kiloparsecs or megaparsecs, multiply by 1,000 or 1,000,000 before entering. The light-year here is the distance light travels in one Julian year in vacuum. Remember that the result is only as precise as the parsec distance you start with.

Worked example

A star is measured at 1.3 parsecs. Light-years: 1.3 × 3.2615637769 = 4.23999, shown as 4.2400 ly. Astronomical units: 1.3 × 206,264.806 = 268,144.25 AU. Kilometres: 1.3 × 3.0856775815×10¹³ = 4.0114 × 10¹³ km, about 40 trillion kilometres. Light from the star therefore needs a little over four years to reach us.

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Common mistakes

  • Treating the parsec as a unit of time like the light-year; both are distances, and a parsec is about 3.26 times longer.
  • Entering kiloparsecs without multiplying by 1,000 first, which makes the result 1,000 times too small.
  • Reading 4.0114 × 10¹³ as 4.0114 or as 40,114; the × 10¹³ part means multiply by 10 to the power 13.
  • Rounding 3.26 or 3.3 for catalog distances, where the extra digits matter for nearby stars.

About this calculator

A parsec (parallax-second) is the distance at which one astronomical unit — the Earth-Sun distance — subtends an angle of exactly one arcsecond, defined geometrically rather than by light travel time. It works out to about 3.2616 light-years, 206,265 astronomical units, or 3.0857×10¹³ kilometers, and it's the unit astronomers get directly from parallax measurements rather than deriving.

This converter takes a distance in parsecs and returns the equivalent in light-years, astronomical units, and kilometers using the exact IAU-defined constants, so you don't have to remember the conversion factors or risk rounding errors. Astronomy students, amateur stargazers, and science writers use it to translate catalog distances — which are usually given in parsecs or kiloparsecs from parallax or standard-candle measurements — into the light-year figures that are more intuitive for a general audience.

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Comments (1)

  • Editorial team

    Catalogues often list parallax rather than distance, and Gaia-style parallaxes come in milliarcseconds. The distance in parsecs is 1000 divided by the parallax in milliarcseconds, so a parallax of 100 mas means 10 parsecs, which you can then enter here to get light-years. The conversion is only as good as that parallax, and small parallaxes carry large relative uncertainty. Which catalogue distances do you usually work with?

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