Schwarzschild radius calculator

The Schwarzschild formula

r_s = 2GM/c². G is Newton's gravitational constant (6.674 × 10^-11 N⋅m²/kg²), M is mass in kilograms, c is the speed of light. The result is the radius at which escape velocity equals the speed of light — the event horizon of a non-rotating Schwarzschild black hole.

What the radius means in practice

The Sun would need to be compressed into a sphere just 2.95 km across to become a black hole. Earth's Schwarzschild radius is 8.87 mm — about the size of a marble. The supermassive black hole at the center of M87 has a Schwarzschild radius of roughly 19 billion km, larger than our entire solar system. That horizon is what the Event Horizon Telescope imaged in 2019.

Rotating black holes are slightly different

Real astrophysical black holes spin. The Kerr metric, not the Schwarzschild solution, describes them. The event horizon of a maximally rotating Kerr black hole is half the Schwarzschild radius. For most estimates, however, the Schwarzschild radius is the right order-of-magnitude answer.