Previous authors have calculated the mass function of primordial black holes only on scales which are well outside the horizon at the end of inflation. Here we extend the calculation to sub-horizon scales, on which the density perturbation never becomes classical. Regarding the formation of black holes as a 'measurement' of the (high peaks) of the density perturbation, we estimate a mass function by assuming that black holes form as soon as inflation ends, in those rare regions where the Bardeen potential exceeds a threshold value of Ψc simeq 0.5.