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Light-years to Parsecs Converter

Enter a value in light-years to convert it to parsecs, kilometres.

How it is calculated

The tool multiplies a distance in light-years by two constants: parsecs = ly × 0.306601 and km = ly × 9,460,730,472,580.8. The first constant is 1 divided by 3.2616 (one parsec is about 3.2616 light-years), and the second is the length of a light-year in kilometres. Parsecs are rounded to 4 decimals, but a result below 0.001 pc is printed as a power of ten instead (1e-9 ly gives 3.0660 × 10⁻¹⁰ pc). The kilometre result is printed in scientific notation with five significant figures, for example 4.0113 × 10¹³, with the decimal mark of the page language. The conversion runs one way only, from light-years. To go back, divide parsecs by 0.306601. Zero gives zero, and the tool does not reject negative numbers.

Worked example

The nearest known star is listed as about 4.24 light-years away, and a catalogue wants the distance in parsecs. Step 1: 4.24 × 0.306601 = 1.29999 → 1.3000 pc. Step 2: in kilometres, 4.24 × 9,460,730,472,580.8 = 4.0113 × 1013 km, printed by the tool as 4.0113 × 10¹³ km. The 4-decimal rounding shows 1.3000 even though the exact figure is slightly below 1.3.

Try this example →

Common mistakes

  • Reading × 10¹³ as 13 kilometres. The result 4.0113 × 10¹³ km means 40,113,000,000,000 km, roughly forty trillion.
  • Multiplying instead of dividing when going from parsecs to light-years. One parsec is larger than one light-year, so the same distance has more light-years than parsecs, about 3.26 times as many.
  • Confusing a light-year with a unit of time. It is a distance, the length light covers in one year, so do not use it for travel durations.
  • Quoting the 4-decimal parsec figure as exact. The source value (such as 4.24 ly) usually has only three significant digits, so quote the parsecs to the same precision, like 1.30.

About this calculator

A parsec is defined geometrically, not by light-travel time: it's the distance at which 1 astronomical unit subtends an angle of exactly 1 arcsecond (its name literally comes from “parallax of one arcsecond”). Working out that geometry gives 1 parsec = 3.2616 light-years = 3.0857×10¹³ km.

Professional astronomers overwhelmingly use parsecs (and kiloparsecs, megaparsecs for galactic and cosmological distances) because it falls directly out of the parallax measurements used to determine stellar distances, while light-years remain the more intuitive unit in popular science writing — this converter bridges the two.

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Comments (1)

  • Editorial team

    A cross-check from the other direction: a star's distance in parsecs is 1 divided by its parallax in arcseconds, so a parallax of 0.768 arcseconds gives about 1.30 pc, close to the 1.3000 pc shown for 4.24 light-years. Small distances do not round away to nothing: below 0.001 pc the parsec line switches to a power of ten, such as 4.9056 × 10⁻⁵ pc for 0.00016 light-years, and the kilometre line always uses that form. Which catalogue are you converting distances from?

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