  Because of their cosmological origin, gamma-ray burst (GRB) optical
afterglows are attenuated when they pass intergalactic absorbers in the GRB
line-of-sight. Without the knowledge of the number of absorbers and their
physical properties, the effect of absorption on the observed magnitudes can
not be determined precisely. Different methods have been applied in order to
correct for this effect statistically, either using semi-analytical
calculations or numerical simulations. We follow these works and present the
expected magnitude corrections as a function of redshift for a set of filters
most commonly used in the scientific community. The results are publically
available on the web (http://igmac.fmf.uni-lj.si).
