The formula for the Schwarzschild radius of a nonrotating black hole is 2GM/c².
In this expression, G is Newton’s gravitational constant, M is the object’s mass, and c is the speed of light. The result gives the radius of the event horizon in the idealized case of a spherical, nonrotating, electrically neutral black hole.
The formula comes from the Schwarzschild solution to Einstein’s field equations. It does not describe a material surface. Instead, it identifies the spherical boundary from which light cannot escape to distant observers. The radius increases directly with mass, so doubling the mass doubles the Schwarzschild radius.
For a black hole with the Sun’s mass, the Schwarzschild radius is about 3 kilometres. Real black holes may rotate, and a rotating black hole is described by the Kerr solution rather than the simplest Schwarzschild model. The formula remains a useful reference scale for comparing black-hole sizes.