AbstractEstimates of the potential size of earthquakes on mapped active faults are generally based on regressions of earthquake magnitude (Mw) versus length (L) of fault rupture for historical earthqukes. The fault slip rate (S) has been ignored in formal prediction equations, but more accurate predictions of future earthquake magnitudes on mapped faults may be obtained when it is included. A least-squares regression for a data set of 43 earthquakes occurring on faults for which slip rates are reported shows Mw = 5.12 + 1.16 log L − 0.20 log S, where L is in units of Km and S is in units of mm/yr. The result indicates that the largest earthquakes will occur on the slowest slipping faults if the rupture length is held constant.