A gauging or calibrating device for tubular films formed by a blowing process comprises a plurality of support rings surrounding the tubular film in a zone where the film is still plastic. Each support ring, which may be divided in one or two places is formed from a curved spindle and a heat-resistant and nonadhesive covering which can rotate on the spindle. The covering can be formed either from a flexible tube mounted on support rollers, or it can be formed by a number of closely spaced individual rollers. The ends of the curved spindle are held in adjustable mounts which permit the diameter of the support ring to be adjusted.