  The mass of unresolved young star clusters derived from spectro-photometric
data may well be off by a factor of 2 or more once the migration of massive
stars driven by mass segregation is accounted for. We quantify this effect for
a large set of cluster parameters, including variations in the stellar IMF, the
intrinsic cluster mass, and mean mass density. Gas-dynamical models coupled
with the Cambridge stellar evolution tracks allow us to derive a scheme to
recover the real cluster mass given measured half-light radius, one-dimensional
velocity dispersion and age. We monitor the evolution with time of the ratio of
real to apparent mass through the parameter eta. When we compute eta for rich
star clusters, we find non-monotonic evolution in time when the IMF stretches
beyond a critical cutoff mass of 25.5 solar mass. We also monitor the rise of
color gradients between the inner and outer volume of clusters: we find trends
in time of the stellar IMF power indices overlapping well with those derived
for the LMC cluster NGC 1818 at an age of 30 Myr. We argue that the core region
of massive Antennae clusters should have suffered from much segregation despite
their low ages. We apply these results to a cluster mass function, and find
that the peak of the mass distribution would appear to observers shifted to
lower masses by as much as 0.2 dex. The star formation rate (SFR) derived for
the cluster population is then underestimated by from 20 to 50 per cent.
