# Find the mass of a black hole from just its radius?

1. Dec 8, 2014

### Lamdbaenergy

I know the original schwarzschild formula for finding the radius of the black hole, but what equation can be used to find its mass if you ONLY know its radius?

2. Dec 8, 2014

### Bandersnatch

Can't you use the same formula? Only instead of using mass to find the Schwartzschild radius, use the Schwartzschild radius to find the mass.

3. Dec 8, 2014

### Lamdbaenergy

Nope, got nowhere near the right mass. I tried for a solar mass black hole, too. Any other suggestions?

4. Dec 8, 2014

### Bandersnatch

Can you show what you did exactly?

5. Dec 8, 2014

### Lamdbaenergy

2GR/c^2, then I tried R^2c^2/2G

6. Dec 8, 2014

### Bandersnatch

Neither of those is correct. Start with the Schwartzschild radius formula $r_s=\frac{2Gm}{c^2}$ and isolate the mass on one side.

7. Dec 9, 2014