The formula for the gravitational radius of an object is
$r_{g} \equiv \frac{GM}{c^{2}}$

where

$G$ is the gravitational constant,
$M$ is the black-hole mass, and
$c$ is the speed of light.

We can conveniently express this in terms of solar masses:

$r_{g} = 1.4822 \left(\frac{M}{M_{\odot}}\right) \ {\rm km}$

or simply,

$r_{g} \sim 1.5 \left(\frac{M}{M_{\odot}}\right) \ {\rm km}$

where $M_{\odot}$ is the mass of the sun. For much larger black-hole masses it is more convenient to write:

$r_{g} = 1.4822 \times 10^{13} \ M_{8} \ {\rm cm},$

where $M_{8}$ is the mass of the black hole in units of $10^{8}$ solar masses.

The following is also useful:

$r_{g} \sim \left(\frac{M}{10^{8}M_{\odot}}\right) \ {\rm AU} \ = \ M_{8} \ \ {\rm AU},$

where AU is the astronomical unit.

Note that the event horizon of a non-spinning black hole is at $2r_{g}$, which is also equal to the Schwarzschild radius.