Absolute Magnitude Calculator

How it is calculated

The inputs are the apparent magnitude m (a pure number, negative for very bright objects) and the distance d in parsecs, which must be greater than zero. The tool computes M = m − 5 × log10(d) + 5, the same as M = m − 5 × log10(d ÷ 10). It also shows the distance modulus m − M, which equals 5 × log10(d) − 5. Both are rounded to two decimals. At exactly 10 parsecs the modulus is 0 and M equals m. The formula ignores interstellar extinction (dust dimming) and any filter correction, so for distant stars in dusty regions the true absolute magnitude is brighter than the result.

Common mistakes

• Entering the distance in light-years. The field is in parsecs; 1 parsec is about 3.26 light-years, so divide light-years by 3.26 first.
• Expecting a larger number to mean a brighter star. The scale is inverted: smaller or more negative magnitudes are brighter.
• Entering distance in kilometers or astronomical units. Only parsecs give a correct result.
• Using a 10-parsec distance and expecting a change. At 10 pc the absolute magnitude equals the apparent magnitude.
• Ignoring dust. For stars far across the galaxy, extinction makes them look fainter, so the computed M is too faint unless you correct m first.