  Determining the final spin of a black-hole (BH) binary is a question of key
importance in astrophysics. Modelling this quantity in general is made
difficult by the fact that it depends on the 7-dimensional space of parameters
characterizing the two initial black holes. However, in special cases, when
symmetries can be exploited, the description can become simpler. For black-hole
binaries with unequal masses but with equal spins which are aligned with the
orbital angular momentum, we show that the use of recent simulations and basic
but exact constraints derived from the extreme mass-ratio limit allow to model
this quantity with a simple analytic expression. Despite the simple dependence,
the expression models very accurately all of the available estimates, with
errors of a couple of percent at most. We also discuss how to use the fit to
predict when a Schwarzschild BH is produced by the merger of two spinning BHs,
when the total angular momentum of the spacetime ``flips'' sign, or under what
conditions the final BH is ``spun-up'' by the merger. Finally, suggest an
extension of the fit to include unequal-spin binaries, thus potentially
providing a complete description of the final spin from the coalescence of
generic black-hole binaries with spins aligned to the orbital angular momentum.
