Schwarzschild Radius Calculator: r_s = 2GM / c^2

How small a mass must be squeezed to become a black hole.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose which quantity you want to find.

Enter the other values in the units shown.

The calculator gives the answer with the formula, the substituted numbers, and the answer in other units.

Formulas used

  • r_s = \frac{2GM}{c^2}
  • M = \frac{r_s c^2}{2G}

Worked example

With the default numbers in the calculator above:

  1. The formula, arranged to find the schwarzschild radius: schwarzschild radius = 2G / c^2 x mass
  2. Put the values in (every value in its SI unit): schwarzschild radius = 2G / c^2 x 1.989e+30
  3. The answer: schwarzschild radius: 2954.13 m
  4. The same answer in other units: 2.95413 km

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: What do you want to find? Mass; Schwarzschild radius (leave it as it is if this is what you are finding) 2953.25 m; Mass (leave it as it is if this is what you are finding) 1 kg.

  1. The formula, arranged to find the mass: mass = (schwarzschild radius) / (2G / c^2)
  2. Put the values in (every value in its SI unit): mass = (2953.25) / (2G / c^2)
  3. The answer: mass: 1.98841e+30 kg
  4. The same answer in other units: 0.999703 solar masses; 332955 Earth masses

Common questions

What is the Schwarzschild radius?

The radius of a sphere around a mass inside which the escape velocity is more than the speed of light.

What would the Sun's Schwarzschild radius be?

About 2.95 km.

Does a mass become a black hole at this radius?

Only if the whole mass is squeezed inside it.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Any physics text (general relativity, black holes).

Need a calculator we do not have?

Tell us what you would like to calculate. We read every request and build the most asked-for ones.

We store your request (and your email if you give it) to decide what to build. We do not store the numbers you type into calculators.

Embed this calculator on your own page

Copy this code into your page. The calculator runs inside the frame and links back to Drafting Central.

Need this drawn, designed or checked?

Drafting Central is a mechanical engineering and drafting business working with customers across Australia and around the world. A calculator gives you the numbers; we can turn them into drawings and designs.

Need help with a real project?

Talk to Drafting Central →

← All engineering calculators