Runoff curve numbers (CN) as used to estimate runoff volume (Q) from storm rainfall (P), are usually modified by handbook methods to account for the influence of watershed wetness. An alternate technique, based on the SCS rainfall-runoff formula and conservation of mass, is developed, illustrated, and discussed. The expression developed is: CN2=1,200/(1,200/CN1) +ET-(P-Q). The methodology requires inputs of the interim evapotranspiration and drainage (ET), but is not unreasonably sensitive to estimates of them.