Planet Density Calculator

About this calculator

This calculator finds a celestial body's mean density, ρ = M / ((4/3)·π·r³), by dividing its mass by the volume of a sphere with its radius — a single number that reveals a great deal about what a planet or moon is made of even when its interior can't be observed directly. Earth's mean density of about 5.51 g/cm³ points to a substantial iron-nickel core, while Saturn's density of just 0.69 g/cm³ (lower than water) reflects a body made almost entirely of hydrogen and helium gas.

Astronomers use mean density as a first diagnostic when characterizing a newly discovered exoplanet from its measured mass and radius, distinguishing likely rocky “super-Earths” from gas- or ice-dominated “mini-Neptunes” before any spectroscopic data is available. Planetary scientists studying solar-system bodies, astronomy students, and enthusiasts following exoplanet discovery announcements all use this calculator to turn a raw mass-and-radius measurement into a clue about composition.