The “chirp mass” mentioned above is a practical quantity if one wants to describe the signals of coalescing black holes: https://en.wikipedia.org/wiki/Chirp_mass.
It depends on the masses of the individual black holes. If these are equal, the following applies
M = 2^(-1/5) * m ≈ 0.87 * m
or
m = 2^(1/5) * M ≈ 1.15 * M
Here, M is the chirp mass and m is the mass of on of the black holes.