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Light-Year Converter (Light-Years, Parsecs, AU, km)

Convert light-years to parsecs, astronomical units, kilometres and miles — and back — with the IAU definitions, plus how long light takes to cover the distance.

Distance

Unit

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How it works

1 light-year = 9,460,730,472,580.8 km (light speed 299,792,458 m/s × 365.25 days) · 1 parsec = 648,000/π AU = 3.26156 ly = 30,856,775,814,913.673 km · 1 AU = 149,597,870.7 km · 1 mile = 1.609344 km

A light-year is the distance light travels in one Julian year of 365.25 days in a vacuum. Astronomers usually prefer the parsec: the distance at which one astronomical unit (the mean Earth–Sun distance) subtends an angle of one arcsecond, so a star’s distance in parsecs is simply 1 ÷ its parallax in arcseconds. The International Astronomical Union fixed the AU at exactly 149,597,870,700 m in 2012 and defined the parsec as exactly 648,000/π AU in 2015, so these conversions are exact.

Worked example

One light-year is 0.306601 parsecs, 63,241.1 astronomical units, 9.46073 × 10¹² km or 5.87863 × 10¹² miles. Proxima Centauri, at 4.2465 ly, is 1.302 pc away.

Sources: IAU Resolution B2 (2012) — re-definition of the astronomical unit · IAU Resolution B2 (2015) — parsec and other units

Frequently asked questions

How many kilometres are in a light-year?

About 9.461 trillion km (9,460,730,472,580.8 km), or about 5.879 trillion miles.

How many light-years are in a parsec?

1 parsec = 3.26156 light-years, so 1 light-year = 0.3066 parsecs.

Is a light-year a unit of time?

No, it is a distance — how far light travels in a year. The light travel time equals the distance in light-years expressed in years.

Why do astronomers use parsecs?

Because distances are measured from parallax: a star with a parallax of 1 arcsecond is 1 parsec away, so distance (pc) = 1 ÷ parallax (arcsec).

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This calculator is for educational purposes. Double-check important results before acting on them.