Asteroid Diameter from Magnitude Calculator

How this is worked out

An asteroid's absolute magnitude H measures how bright it would appear from a standard distance, which depends on both its size and how much sunlight it reflects (its geometric albedo). Given an assumed albedo, NASA/JPL's Center for Near-Earth Object Studies (CNEOS) publishes a direct formula for the equivalent spherical diameter: d (km) = 10^(3.1236 − 0.5 log₁₀(albedo) − 0.2H). Because the real albedo of a specific asteroid is usually unknown without further observation (radar, infrared, or a spacecraft visit), this is an estimate, not a measurement — CNEOS's own page gives an example where assuming 0.15 instead of the true 0.05 makes the size come out nearly half what it should be.

Frequently asked questions

Why does a higher albedo mean a smaller estimated size?

For the same brightness, a more reflective (higher-albedo) object doesn't need to be as large to shine as brightly as a darker one — so assuming a brighter surface always shrinks the size estimate, and assuming a darker one always grows it.

Which albedo should I pick if I don't know the asteroid's type?

Roughly three-quarters of known asteroids are dark, carbonaceous C-types (albedo around 0.03–0.09), so a low value in that range is the more probable guess absent other information — but treat any unverified estimate as rough, easily out by a factor of two either way.

Related tools: Impact Energy & Crater Calculator, Planet Facts Table, Stellar Magnitude & Distance Calculator.

Source / Last verified: NASA/JPL Center for Near-Earth Object Studies (CNEOS), "Asteroid Size Estimator" (cneos.jpl.nasa.gov/tools/ast_size_est.html), formula and worked albedo-sensitivity example read 1 October 2026. This tool reproduces that page's own example exactly: at H = 22, assuming albedo 0.05 instead of 0.15 increases the estimated diameter by a factor of 1.73 (√3), matching its stated "too small ... by a factor of almost 2". The default H = 10.31 is (433) Eros's commonly-published absolute magnitude; with Eros's own actual albedo (about 0.25, S-type) this formula gives about 23 km, close to Eros's true mean diameter (its actual shape is a very elongated ~17×11×11 km, so no spherical-equivalent formula matches it exactly).