Stellar Parallax Distance Calculator

How it is calculated

Distance follows directly from the parallax angle: d (pc) = 1 ÷ p, where p is the parallax in arcseconds, that is, half of the star's apparent shift between two observations six months apart. The parsec is defined by this relation. The tool then converts: light-years = d × 3.26156 and AU = d × 206265. Parsecs and light-years are shown with three decimals, and astronomical units as a whole number. The parallax must be greater than zero, because zero or negative values have no distance in this simple model. Measurement uncertainty is not considered, so very small parallaxes give large but uncertain distances.

Common mistakes

• Entering the parallax in milliarcseconds, as many catalogues list it: the field is in arcseconds, so 768 mas must be entered as 0.768, otherwise the distance comes out a thousand times too small.
• Entering the full angular shift between two observations half a year apart instead of half of it: the parallax is the shift measured against a 1 AU baseline, so the full shift gives half the true distance.
• Expecting distance to grow with parallax: they are inversely related, so the larger the parallax, the closer the star.
• Trusting the distance of a star with a tiny parallax as exact: the formula is exact, but a small measurement error in a small angle moves the distance a lot.