Planet Average Density Calculator

Planetary Density & Bulk Composition

Mean density is just mass over volume. Treat a planet as a sphere of radius r and mass M and you get ρ = M / V = M / (4/3 · π · r³). Put that number in g/cm³ and the families of objects in the Solar System sort themselves out almost on their own. Rocky and metallic worlds bunch up around 3–6 g/cm³. The ice giants sit near 1–2. The gas giants slip under 1.5.

Some reference values to keep handy: Earth 5.51, Mercury 5.43, Venus 5.24, Mars 3.93, Moon 3.34, Jupiter 1.33, Saturn 0.69 (less dense than water, so in principle it would float if you found an ocean big enough), Uranus 1.27, Neptune 1.64. What stands out is Mercury, oddly dense for something so small. That points to an oversized iron core, roughly 60% of the planet’s mass, and hints at a violent giant impact somewhere in its past.

Applications

When astronomers find an exoplanet, density is the first thing they reach for to guess what it is made of. Transit photometry hands you the radius, radial-velocity or transit-timing gets you the mass, and the two together give you ρ. Around 5 g/cm³ and you are probably looking at a rocky super-Earth. Closer to 1.5 and it could be a water world or a planet wrapped in a thick H/He envelope. Drop below 1 g/cm³ — the “puffy planets” like Kepler-7b — and you are seeing an atmosphere bloated by the heat of its host star.

FAQ

Would Saturn really float? On paper, going by mean density, yes. In reality it is a thought experiment and nothing more. Saturn has no solid surface; it is mostly hydrogen under crushing pressure, and it would just smear apart in any ocean you imagined for it.

Why does Mercury have such high density? The leading explanation is a giant impact early on that tore away much of the silicate mantle, leaving a planet that is mostly iron core.

Is mean density the same as core density? No, and the gap is large. Mean density blends core, mantle and atmosphere into one number. Earth’s core runs about 13 g/cm³ and its crust about 2.7, so the 5.51 g/cm³ figure is the whole-planet average rather than any one layer.