Postfire debris flows are fast-moving landslides that pose a significant risk to downstream communities around the world. Accurately predicting how large postfire debris flows will be, before they occur, allows us to better understand the potential effects of future events. In this study, we develop a new method for predicting postfire debris-flow volume in the western United States. Results show that this new method outperforms existing volume models and can improve postfire hazard assessments.