Space · Stars
Parallax distance calculator.
Enter a star's parallax angle to get its distance in parsecs and light-years.
Inputs
Distance
Enter a parallax above zero.
How parallax gives distance
As Earth moves around the Sun, a nearby star appears to shift against distant background stars. Half of the total shift is the parallax angle. The parsec is defined so that a star with a parallax of one arcsecond is one parsec away, which gives d (parsecs) = 1 / p (arcseconds). The IAU 2015 definition fixes 1 parsec at exactly 648,000/π astronomical units, about 3.26156 light-years (Wikipedia, Parsec). One milliarcsecond is 0.001 arcsecond.
Worked example. The Gaia DR3 parallax of Proxima Centauri is 768.0665 mas (as listed in Wikipedia's Proxima Centauri article, citing the Gaia Collaboration 2023). That is 0.7680665 arcseconds, so the distance is 1 / 0.7680665 = 1.30197 parsecs, or 4.2465 light-years, matching the 1.30197 pc and 4.2465 ly in the same article.
The tool applies the plain reciprocal. It does not correct for measurement uncertainty, so treat results for very small parallaxes with caution.
Frequently asked questions
How do you convert parallax to parsecs?
Divide 1 by the parallax in arcseconds. A parallax of 0.5 arcseconds means 2 parsecs; 100 mas (0.1 arcsecond) means 10 parsecs.
How many light-years is one parsec?
About 3.26156 light-years, or exactly 648,000/π astronomical units under the IAU 2015 definition.
What is a milliarcsecond?
One thousandth of an arcsecond. Gaia reports star parallaxes in milliarcseconds, so 1000 mas equals 1 arcsecond and a distance of 1 parsec.
Why does a smaller parallax mean a farther star?
Distance is the reciprocal of parallax, so halving the angle doubles the distance.
Sources
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Scope
Simple reciprocal distance with no correction for measurement error. A zero or negative parallax gives no distance.