The singularity at the center of any black hole has no volume. The size of the event horizon—the boundary of no return—of a black hole, on the other hand, varies depending on the black hole’s mass. The mathematical relationship between the mass of a black hole and the size of its event horizon was derived by the German astrophysicist Karl Schwarzschild (1873–1916). The radius of a black hole’s event horizon is named the Schwarzschild radius in his honor.

Generally speaking, the Schwarzschild radius of a stellar black hole is about a hundred miles, while the Schwarzschild radius of a supermassive black hole ranges from a few million to a few billion miles. (For reference, the average distance between the Sun and Pluto is about three billion miles.) If the Sun were squeezed small enough to become a black hole, its Schwarzschild radius would be about three miles; and if Earth were squeezed small enough to become a black hole, its Schwarzschild radius would be about three-quarters of an inch.