2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/173394We provide a compact analytic formula to compute the spin of the black hole produced by the coalescence of two black holes. The expression, which uses an analytic fit of numerical-relativity data and relies on four assumptions, aims at modelling generic initial spin configurations and mass ratios. A comparison with numerical-relativity simulations already shows very accurate agreements with all of the numerical data available to date, but we also suggest a number of ways in which our predictions can be further improved.Extended discussion of physical assumptions to make them less cryptic; matches published versionGeneral Relativity and Quantum CosmologyAstrophysicstext