  Galactic star clusters are known to harbour a significant amount of binary
stars, yet their role in the dynamical evolution of the cluster as a whole is
not comprehensively understood. We investigated the influence of binary stars
on the total mass estimate for the case of the moderately populated Galactic
star cluster NGC 225. The analysis of multi-epoch radial velocities of the 29
brightest cluster members, obtained over two observational campaigns, in
1990-1991 and in 2019-2020, yields a value of binary fraction of $\alpha =0.52$
(15 stars out of 29). Using theoretical isochrones and Monte Carlo simulations
we found that the cluster mass increases at least 1.23 times when binaries are
properly taken into account. By combining Gaia EDR3 photometric data with our
spectroscopic observations, we derived estimates of NGC 225 fundamental
parameters as follows: mean radial velocity $<V_r> = -9.8 \pm 0.7$ km s$^{-1}$,
$\log(\tau)$=8.0-8.2 dex, distance $ D = 676 \pm 22$ pc, and colour excess
$E(B-V) = 0.29 \pm 0.01$ mag.
