  We numerically calculate the equivalent mode-field radius of the fundamental
mode in a photonic crystal fiber (PCF) and show that this is a function of the
V-parameter only and not the relative hole size. This dependency is similar to
what is found for graded-index standard fibers and we furthermore show that the
relation for the PCF can be excellently approximated with the same general
mathematical expression. This is to our knowledge the first semi-analytical
description of the mode-field radius of a PCF.
