For a non-rotating, uncharged black hole:

The Schwarzschild radius scales directly with mass. Double the mass and the radius doubles.

Using the Schwarzschild radius as the radius of a sphere, an educational average density is:

Because radius grows in direct proportion to mass while volume grows with radius cubed, average density scales approximately as 1/M².

Enter a mass or choose a preset, then inspect Schwarzschild radius, equivalent event-horizon volume, average density and optional Hawking-temperature context. Scientific notation is useful because the numbers span absurdly large ranges. Space remains committed to being inconvenient for spreadsheets.

A stellar-mass black hole has a small event horizon and high average density by this definition. A supermassive black hole has an enormous horizon volume, so its mass divided by that volume can yield an average density far below everyday materials.

The classical singularity is often described as infinite density in simplified treatments, but that statement refers to a breakdown of the classical theory at the singularity. The calculator does not assign that singularity a bulk material density; it computes an average over the event-horizon-scale volume.

If Earth’s mass were compressed inside its Schwarzschild radius, that radius would be on the order of millimetres. For one solar mass it is on the order of kilometres. These are thought experiments; normal Earth or the Sun are nowhere near these compact states.

Presets such as Sagittarius A*, M87* or TON 618 should be treated as approximate and date-sourced because astronomical mass estimates are refined over time. The calculation method is stable; the observational input estimate may change.

Hawking temperature decreases as black-hole mass increases. Tiny hypothetical black holes would be much hotter than astrophysical black holes. Evaporation-time calculations are extremely sensitive to mass and rely on an idealized theoretical framework.

Real astrophysical black holes can rotate. A rotating Kerr black hole has different horizon geometry from the Schwarzschild case. Charge is generally expected to be small astrophysically, but charged solutions exist theoretically. Advanced Kerr/Kerr–Newman calculations belong in a separate mode rather than being silently mixed into the default formula.

For a 10 M☉ Schwarzschild black hole, the event-horizon radius is roughly 29.5 km. Using that radius as a spherical boundary gives an average density vastly higher than water, but still finite. The same calculation at millions or billions of solar masses produces dramatically lower average density because volume grows faster than mass.

The chart is best interpreted on a logarithmic scale because stellar and supermassive black holes span many orders of magnitude. A straight-ish downward trend in log space reflects the inverse-square scaling of average density with mass under the Schwarzschild definition.

It does not calculate accretion-disk density, matter density inside a realistic rotating horizon, tidal disruption radius for an extended star, or merger dynamics. Those are separate physical questions even though “black hole density” appears in all the surrounding search terms.