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 .