# Does the event horizon of a black hole increase or decrease by adding mass?

So a black hole is a something that has enormous gravity, therefore mass accelerates faster towards it. But if the black hole double it's mass, will it's event horizon increase in length , or will it decrease?

• Did you read the Wikipedia article about the radius of black holes and its dependence on the mass? – pela Feb 14 '16 at 14:35
• Not even a minimal amount of research. As a guide - if you type your headline question into a search engine and the first link it provides directly gives you the answer, then it is not a very interesting stack exchange question. – Rob Jeffries Feb 15 '16 at 13:49

The radius of the event horizon ($r_\mathrm{s}$) is directly proportional to the mass of the black hole (M). More exactly:

$$r_\mathrm{s} = \frac{2 G M}{c^2}$$

The black holes whose merger was detected by LIGO would each have been about 90 km in radius, and after merger, a little less than 180km.

• So apparently, if my waist line is smaller than $6.5×10^{-25}$ meters, I collapse into a black hole. – SE - stop firing the good guys Feb 14 '16 at 15:05
• Additional fun fact: If you consider the density of a black hole $\rho$ as its mass over it's volume given as a sphere with a radius equal to the Schwarzschild-radius $\rho = \frac{M}{\frac{4}{3}\pi r_S^3}$, than you′ll find $\rho \sim M^{-2}$. It's getting less dense, the more massive it becomes. – rtime Feb 14 '16 at 20:29