Black hole calculator
How big would a black hole of any mass be?
Squeeze enough mass into a tiny point and a black hole forms. The Schwarzschild radius tells you how small that must be: enter a mass — from yourself to Earth to the Sun — and see the diameter it would have to collapse to in order to become a black hole.
Pick a mass
See how small it must collapse to become a black hole.
Schwarzschild radius
2.95 km
Event horizon (diameter)
5.91 km
Formula: r = 2GM/c² (non-rotating Schwarzschild black hole).
The Schwarzschild radius
The formula is r = 2GM/c². G is the gravitational constant, M the mass, c the speed of light. Within this radius gravity is so strong that not even light escapes — that is the event horizon.
How small that really is
The entire Earth would need to be compressed to about 9 millimetres across, the Sun to just under 6 kilometres. A human body would have a radius far smaller than an atomic nucleus — which is why black holes only form from enormous stellar masses.
Frequently asked questions
How big is the Sun’s black hole?
If the Sun collapsed, its Schwarzschild radius would be just under 3 km — the event horizon about 6 km across.
Could Earth become a black hole?
Only in theory: you would have to squeeze it to 9 mm across. It never happens in nature — its mass is far too small.
What is the event horizon?
The boundary beyond which nothing escapes — not even light. It is not a solid surface but a boundary in spacetime.
How much mass does a real black hole need?
Stellar black holes form from stars above about 20–25 solar masses whose cores collapse at the end of their lives.
Is the calculation exact?
The Schwarzschild radius is exact for a non-rotating, uncharged black hole. Rotating (Kerr) holes are a bit more complex.
What is a supermassive black hole?
One with millions to billions of solar masses, like Sagittarius A* at the Milky Way’s centre. Its horizon is larger than Mercury’s orbit.
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