Black Hole Calculator

Schwarzschild Radius and Event Horizon

A non-rotating black hole is described by its mass. The event horizon — the boundary beyond which light cannot escape — sits at the Schwarzschild radius. For a mass equal to one Sun, that radius is about 2.95 km.

Schwarzschild Radius Formula

$$ r_s = \frac{2GM}{c^2} $$

Solve for mass when the radius is known: $M = r_s c^2 / (2G)$. Constants used: $G = 6.674 \times 10^{-11}\,\mathrm{m^3\,kg^{-1}\,s^{-2}}$, $c = 2.998 \times 10^8\,\mathrm{m/s}$, solar mass $M_\odot = 1.988 \times 10^{30}\,\mathrm{kg}$.

Frequently Asked Questions

What is the Schwarzschild radius?

It is the radius of the event horizon for a non-rotating black hole. Nothing, not even light, can escape from inside this sphere.

How big is a solar-mass black hole?

About 2.95 km. Stellar black holes of a few solar masses have radii of roughly 9 to 30 km.

Can I enter mass in kilograms?

Yes. Switch the mass unit to kilograms, or use radius-to-mass mode and choose your preferred output unit.

Does this include rotating black holes?

No. This calculator uses the Schwarzschild solution for non-rotating black holes. Spin changes the horizon shape and size.

What is a supermassive black hole radius?

A 4 million solar-mass black hole like Sagittarius A* has a Schwarzschild radius of about 12 million km, roughly 0.08 AU.

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Schwarzschild Radius and Event Horizon

Schwarzschild Radius Formula

Frequently Asked Questions

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