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Schwarzschild Radius Calculator

Find the Schwarzschild radius — the black-hole event-horizon radius — for any mass, Rs = 2GM/c², and see how it compares to the body's actual radius.

Schwarzschild radius
2.954 km
Comparison radius ÷ Schwarzschild radius
235,501.08×
Is the comparison radius inside the Schwarzschild radius?
No — this body is far larger than its own event horizon would be

How it works

The Schwarzschild radius is the radius a given mass would have to be compressed inside of for its escape velocity to reach the speed of light — the event horizon of a (non-rotating, uncharged) black hole. General relativity gives it the remarkably simple closed form Rs = 2GM ÷ c², where G is the gravitational constant (6.6743×10⁻¹¹ m³kg⁻¹s⁻², the CODATA 2022 value, the same constant this site's own Escape Velocity calculator uses) and c is the exact speed of light, 299,792,458 m/s.

This same result actually falls straight out of Newtonian mechanics too: setting the escape-velocity formula v = √(2GM/r) equal to c and solving for r reproduces Rs exactly, which is why the two calculators share the same gravitational constant and canonical-meters conversion.

To make the number meaningful, this calculator also compares Rs against the body's actual radius you enter (defaulting to the Sun's own mass and radius) — the ratio shows how many times larger the real object is than its own event horizon would be, and the calculator flags the rare case where the actual radius is smaller, meaning a body of that mass really would be a black hole.

FAQ

Does the Sun's default example mean the Sun is close to being a black hole?

Not remotely — the Sun's Schwarzschild radius is about 2.95 km, but its actual radius is about 696,000 km, roughly 235,000 times larger. A star needs to run out of fuel and collapse under its own gravity, typically needing several times the Sun's mass, to ever get compact enough to form a black hole; the Sun itself will end as a white dwarf, nowhere near its own Rs.

Why is Earth's Schwarzschild radius so tiny?

Because Rs scales directly with mass, and Earth has vastly less mass than a star — plugging in Earth's roughly 5.97×10²⁴ kg gives a Schwarzschild radius of about 8.87 millimeters. Earth would have to be crushed down to smaller than a marble, with all of its current mass intact, to become a black hole.

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