  We consider black holes resulting from binary black hole mergers. By fitting
to numerical results we construct analytic formulas that predict the mass and
spin of the final black hole. Our formulas are valid for arbitrary initial
spins and mass ratios and agree well with available numerical simulations. We
use our spin formula in the context of two common merger scenarios for
supermassive galactic black holes. We consider the case of isotropically
distributed initial spin orientations (when no surrounding matter is present)
and also the case when matter closely aligns the spins with the orbital angular
momentum. The spin magnitude of black holes resulting from successive
generations of mergers (with symmetric mass ratio $\eta$) has a mean of
$1.73\eta + 0.28$ in the isotropic case and 0.94 for the closely aligned case.
