The Schwarzschild radius (sometimes historically referred to as the gravitational radius) is a physical parameter that appears in the Schwarzschild solution to Einstein's field equations, corresponding to the radius defining the event horizon of a Schwarzschild black hole. It is a characteristic radius associated with any quantity of mass. The Schwarzschild radius was named after the German astronomer Karl Schwarzschild, who calculated this exact solution for the theory of general relativity in 1916.
The Schwarzschild radius is given as
r
s
=
2
G
M
c
2
,
{\displaystyle r_{s}={\frac {2GM}{c^{2}}},}
where G is the gravitational constant, M is the object mass, and c is the speed of light.