The ion current, I, in an ionization gauge is given by the equation I = K i P where K is the gauge constant, i, the electron current, and P the pressure. Values of K for gauges designed for use at UHV and XHV range from 10/Torr to 10@super 6@/Torr. It is important to know whether calibration stability is sacrificed when K, and the electron path length, are large. Using a simple model, the electron path length is estimated as a function of the probability, @beta@, that an electron will make another pass through the ionizing region. An equation is obtained for K as a function of @beta@. The fractional change in K, @DELTA@K/K, is calculated for a 1% reduction in the probability that an electron will make another pass through the ionizing region. The fractional change is zero for those gauges where the electrons make a single pass, 0.015 for a B-A gauge with K = 25, and 0.91 for a gauge with K = 10@super 4@.