  We propose a novel algorithm to compute the width of any generic $n$-body
decay involving multiple off-shell particles having zero and non-zero spins.
Starting from a toy example, we show the computations for three different
processes that contain spin-0, $\frac{1}{2}$, and 1 off-shell particles. We
have checked that our results match with the existing results at the analytical
level. This proposal can be automatized and should be useful to compute the
phase space for long cascade decays, without any Monte Carlo sampling.
