Schwarzschild Radius Calculator
Enter a mass and get its Schwarzschild radius — the event-horizon size at which the object would become a black hole — in metres and kilometres.
Event horizon in one step
Enter a mass in kilograms and the calculator returns the Schwarzschild radius (2GM/c²) in metres and kilometres at once.
Use kilograms
Mass must be in kilograms. For stars and planets, scientific notation such as 1.989e30 keeps the entry short and accurate.
What is the Schwarzschild radius?
The size of the event horizon
The Schwarzschild radius calculator turns a single number — a mass in kilograms — into the radius of the event horizon that mass would have as a black hole. The Schwarzschild radius is the boundary at which gravity becomes so strong that nothing, not even light, can escape. Compress any object inside its own Schwarzschild radius and it becomes a black hole; stay larger and it stays an ordinary star, planet, or person. The radius depends on nothing but the mass, growing in direct proportion to it: double the mass and you double the horizon. Named after Karl Schwarzschild, who solved Einstein's field equations for a spherical mass in 1916, it is the cleanest single number in black-hole physics.
Enter a mass in kilograms to get its Schwarzschild radius in metres and kilometres instantly.
The Schwarzschild radius is twice the gravitational constant times the mass, divided by the speed of light squared. Because the speed of light is so large and squared, the divisor is enormous, so even a star-sized mass yields a radius of only a few kilometres.
r_s = 2 × G × M / c²Here G = 6.6743e-11 m³·kg⁻¹·s⁻² is the gravitational constant and c = 299,792,458 m/s is the speed of light. Take the Sun, with a mass of about 1.989e30 kg. Multiplying 2 × G × M gives roughly 2.655e20, and dividing by c² (about 8.988e16) leaves close to 2954 metres — just under 2.95 km. That is the entire event horizon the Sun would carry if it were ever crushed into a black hole, even though the real Sun spans nearly 1.4 million kilometres across. The formula scales linearly: a mass ten times heavier has a Schwarzschild radius ten times larger.
The formula is exact for an idealised mass, but a few points are worth keeping in mind.
Non-rotating model and the collapse condition
This is the classic Schwarzschild solution for a spherical, non-rotating, uncharged mass. Real black holes usually spin, which changes the horizon geometry (the Kerr solution), so treat the result as the baseline rather than an exact size for every black hole. The radius is always defined, but it only becomes a physical event horizon when the mass is actually compressed within it: any mass squeezed inside its own Schwarzschild radius becomes a black hole. Keep the mass in kilograms — mixing in grams or solar masses without converting will throw the result off by many orders of magnitude.