  We discuss a Weyl's law for the quasi-normal modes of black holes that
recovers the structural features of the standard Weyl's law for the eigenvalues
of the Laplacian in compact regions. Specifically, the asymptotics of the
counting function $N(\omega)$ of quasi-normal modes of $(d+1)$-dimensional
black holes follows a power-law $N(\omega)\sim
\mathrm{Vol}_d^{\mathrm{eff}}\omega^d$, with $\mathrm{Vol}_d^{\mathrm{eff}}$ an
effective volume determined by the light-trapping and decay properties of the
black hole geometry. Closed forms are presented for the Schwarzschild black
hole and a quasi-normal mode Weyl's law is proposed for generic black holes. As
an application, such Weyl's law could provide a probe into the effective
dimensionality of spacetime and the relevant resonant scales of actual
astrophysical black holes, upon the counting of sufficiently many overtones in
the observed ringdown signal of binary black hole mergers.
