I'm not sure, but you can find interesting information on this site...http://physics.stackexchange.com/questions/26696/how-do-we-determine-the-mass-of-a-black-hole
It depends on the mass of the star that formed the black hole. If you know the radius of the black hole, then you can find its mass using this formula: M = (c^2 r)/(2 G), where c=speed of light, r=radius of the black hole, G =gravitational constant.
