The Schwarzschild radius gives the radius at which the Schwarzschild metric becomes singular, and is therefore the "size" of a black hole. It can naively (although incorrectly) be derived by letting the escape velocity of a black hole equal to the speed of light
where G is the gravitational constant, M is the mass of the black hole, and c is the speed of light, giving
Black Hole, Escape Velocity, Schwarzschild Black Hole
