Schwarzschild Radius Calculator
The Schwarzschild radius is the size of the event horizon a mass would have if it collapsed into a non-rotating black hole, given by r = 2GM/c^2. Enter a mass in kilograms or in solar masses and the calculator returns that radius in metres, kilometres, and solar radii. Because radius scales linearly with mass, a black hole twice as heavy is exactly twice as wide.
How to use the Schwarzschild radius calculator
- Optionally click a preset such as Sun, Earth, Sgr A*, or M87* to load a known mass.
- Type the mass into the Mass box (scientific notation like 5.972e24 is accepted).
- Set the Unit dropdown to Solar masses or Kilograms to match your number.
- Read the horizon radius on the three cards: Radius (metres), Radius (km), and Solar radii.
- Click Copy to grab the radius in metres and kilometres as text.
Examples
One solar mass (Sun preset)
Mass = 1, Unit = Solar masses
radius = 2953.99 m, 2.95399 km, 4.24607e-6 solar radii
Earth's mass
Mass = 5.972e24, Unit = Kilograms
radius = 0.008869 m (8.86900e-6 km), about 8.9 mm
Sagittarius A*, the Milky Way's central black hole
Mass = 4.3e6, Unit = Solar masses
radius = 1.27022e10 m, 1.27022e7 km, 18.2581 solar radii
Frequently asked questions
What is the Schwarzschild radius?
It is the radius of the event horizon of a non-rotating, uncharged black hole. Compress a mass inside this radius and its escape velocity reaches the speed of light, so nothing, not even light, can get back out.
What is the Schwarzschild radius formula?
r = 2 x G x M / c^2, where G is the gravitational constant, M is the mass, and c is the speed of light. Radius is directly proportional to mass, so doubling M doubles r.
What constants does this calculator use?
It uses G = 6.674e-11 N m^2 / kg^2, c = 299792458 m/s, a solar mass of 1.989e30 kg, and a solar radius of 6.957e8 m. Those fixed values determine the exact figures it reports.
What is the Schwarzschild radius of the Sun?
About 2953.99 metres, or roughly 2.95 km. The real Sun is far too spread out to be a black hole, but that is how small its horizon would be if all its mass collapsed.
What is the Schwarzschild radius of Earth?
Using Earth's mass of 5.972e24 kg, about 0.008869 metres, close to 8.9 millimetres. You would have to squeeze the whole planet into a sphere smaller than a marble for it to become a black hole.
Does the formula account for rotation or charge?
No. This is the simple Schwarzschild solution for a static, uncharged black hole. Spinning black holes use the Kerr solution and charged ones the Reissner-Nordstrom solution, both of which give a different horizon.
Can I use it for ordinary objects, not just black holes?
Yes. The formula returns a radius for any positive mass, which is why a person or a planet has a tiny Schwarzschild radius. It just tells you how small that mass would need to be packed to form a horizon.
Why can I enter mass in solar masses?
Astronomical black holes are usually quoted in solar masses because the numbers are more intuitive than huge kilogram values. The tool multiplies by 1.989e30 kg per solar mass before applying r = 2GM/c^2.
Is my input sent anywhere?
No. The calculation runs locally in your browser with the same engine used in the tests, so the mass you type is never uploaded or stored.
Learn more
- How black hole size scales with mass
Why the event horizon grows in a straight line with mass, from stellar black holes to Sgr A* and the giant M87*, using r = 2GM/c^2.
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