there is a simple exact formula for the SURFACE AREA of the horizon of a Kerr-Newman black hole, which if I copied it right like a monkey, is: Area = 4*pi* ( 2*M^2 - Q^2 +- 2*M*sqrt(M^2-J^2-Q^2) ) where +- for the two horizons (it has two), M=mass, Q=charge, J=specific angular momentum, everything in Planck units. Also, EQ 11.1.13 and 14 gives "radii" of these horizons http://www.modernrelativitysite.com/chap11.htm but the meaning of that may not be what you think. Area A is the same in all coordinate systems, and A = 4*pi*(r^2 + J^2) so that these r values also are invariant. Their EQ 11.1.17 also mentions how to incorporate a nonzero cosmical constant as yet another parameter besides M,J,Q. "The area of a 1-solar mass black hole is about the same as the municipal area of Atlanta." -- Warren D. Smith http://RangeVoting.org <-- add your endorsement (by clicking "endorse" as 1st step)