What radius gives the event horizon size of a non-rotating black hole?

The radius that gives the event-horizon size of a non-rotating black hole is the Schwarzschild radius.

For a non-rotating, uncharged black hole, the Schwarzschild radius is given by rₛ = 2GM/c², where G is the gravitational constant, M is the black-hole mass and c is the speed of light. It marks the radius of the event horizon when the mass is compressed inside that boundary.

The scale is directly proportional to mass: a black hole with the Sun’s mass would have a Schwarzschild radius of about 3 kilometres. A ten-solar-mass black hole would therefore have a radius of roughly 30 kilometres. The radius describes the horizon’s size, not a solid surface.

The concept comes from Karl Schwarzschild’s 1916 exact solution to Einstein’s field equations. A common mix-up is the Einstein radius, which concerns gravitational lensing, while the Hubble radius is a cosmological distance scale and the Bohr radius describes the size of the hydrogen atom. Rotating black holes require the more general Kerr solution.