Klumb-Schwarz type radiometer gauge (Knudsen gauge) is an absolute gauge for high vacuum measurements, but since its theoretical sensitivity is so far unknown, in practice it needs calibration against another absolute gauge (e. g. McLeod). In this paper a simple theory to determine the sensitivity of a Klumb-Schwarz gauge is described. The optimum angle of the vane is decided as 24° by theoretical considerations. It is further shown that in this gauge the force due to electromagnetic radiations limits the lower range and is something in analogue with the soft X-ray effect in conventional hot-cathode ionization gauges.