  We propose a zero-point photometric calibration of the data from the ACS/WFC
on board the Hubble Space Telescope, based on a spectrum of Vega and the most
up to date in-flight transmission curves of the camera. This calibration is
accurate at the level of a few hundredths of a magnitude. The main purpose of
this effort is to transform the entire set of evolutionary models by
Pietrinferni et al. (2004) into a simple observational photometric system for
ACS/WFC data, and make them available to the astronomical community. We provide
the zero points for the most used ACS/WFC bands, and give basic recipes for
calibrating both the observed data and the models.
  We also present the Colour Magnitude Diagram (CMD) from ACS data of 5
Galactic globular clusters, spanning the metallicity range -2.2<[Fe/H]<-0.04,
and provide fiducial points representing their sequences from several
magnitudes below the turnoff to the red giant branch tip. The observed
sequences are compared with the models in the newly defined photometric system.
