Absolute Magnitude Calculator
Apparent magnitude tells you how bright a star looks from Earth, but that brightness mixes the star's true luminosity with its distance. Absolute magnitude strips out distance by asking how bright the star would look from a standard 10 parsecs away, giving a fair comparison of intrinsic brightness. This calculator applies the distance modulus relation to convert an apparent magnitude and a distance into an absolute magnitude.
Absolute magnitude formula
M = m - 5 * log10(d) + 5
distance modulus = m - M = 5 * log10(d) - 5
where m is apparent magnitude and d is distance in parsecs
Magnitudes are a reverse logarithmic scale: smaller (more negative) numbers are brighter. The standard reference distance is 10 parsecs, where apparent and absolute magnitude are equal.
Worked example
Sirius has an apparent magnitude of about -1.46 at a distance of 2.64 parsecs. M = -1.46 - 5 * log10(2.64) + 5 = -1.46 - 2.11 + 5 = 1.43. So Sirius has an absolute magnitude of about 1.43, and a distance modulus of -2.89.
Absolute magnitude: frequently asked questions
What is absolute magnitude?
Absolute magnitude is how bright a star would appear if it were placed at a standard distance of 10 parsecs (about 32.6 light years) from Earth. Because it removes the effect of distance, it lets astronomers compare the true luminosities of stars directly, unlike apparent magnitude, which depends on how far away the star is.
What is the distance modulus?
The distance modulus is the difference between apparent magnitude (m) and absolute magnitude (M): m minus M equals 5 times log10(distance in parsecs) minus 5. Rearranging gives absolute magnitude M = m minus 5 times log10(d) plus 5. This calculator applies that relation directly.
Does this account for interstellar extinction?
No. The basic distance modulus assumes light is not dimmed by dust between the star and Earth. Real measurements often add an extinction term. If you know the extinction in magnitudes for your line of sight, subtract it from the apparent magnitude before entering it here for a more accurate absolute magnitude.
Sources
- The distance modulus relation is a standard definition in stellar astronomy. Parsec definition (IAU): see NASA NASA Science: Universe for background on stellar distance scales.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.