Absolute Magnitude Calculator

About this calculator

Absolute magnitude (M) is the apparent brightness a star would have if placed at a standard distance of exactly 10 parsecs from Earth, which lets astronomers compare the true, intrinsic luminosity of stars regardless of how far away they actually are. It's computed from the star's observed apparent magnitude (m) and its distance d in parsecs using the distance modulus relation: M = m − 5·log10(d/10), equivalently written as M = m − 5·log10(d) + 5.

Because the magnitude scale is logarithmic and inverted (lower numbers mean brighter objects), a star that looks faint only because it's far away will have a much brighter — more negative — absolute magnitude than its apparent magnitude suggests. This calculation underlies the cosmic distance ladder: once a star or supernova's absolute magnitude is known or assumed (as with Cepheid variables and Type Ia supernovae, both “standard candles”), the same formula can be rearranged to solve for distance instead, which is how astronomers measure distances well beyond what parallax can reach.