Promoter atoms can tune a catalyst's activity and selectivity by transferring charge to and from the active site. Rational design of promoted catalysts, using density functional theory calculations, is today limited by the need to simulate many catalyst and promoter configurations. We present a simple approximation that rapidly captures some trends in promoter effects, at a cost of complexity comparable with simulating unpromoted catalysts. Negative (positive) noninteger point charges introduced into the catalyst simulate how electropositive (electronegative) promoters might affect each predicted intermediate. Calculations return Sabatier plots, relating promoters' predicted efficacy to readily measured properties such as catalyst work functions. We illustrate our approach for two reactions associated with the Fischer–Tropsch process, hydrogen–deuterium scrambling, and carbon monoxide dissociation over ruthenium. Consistent with experiment, electropositive promoters are predicted to accelerate hydrogen scrambling and unassisted CO dissociation. Simulations also provide a new prediction: electronegative promoters accelerate hydrogen‐assisted CO dissociation over hydrogen‐precovered surfaces by stabilizing the initial CO adsorption. © 2019 Wiley Periodicals, Inc.