How it works
RPE 10 means 0 reps in reserve, RPE 9 means 1, RPE 8 means 2, and so on. We add your reps in reserve to the reps you did to get the reps you could have done, then use the NSCA training load chart to estimate your max.
Example: 5 reps at RPE 8 is about 7 possible reps, which the NSCA chart puts at 83% of your max.
Sources: Zourdos et al., "Novel resistance training-specific RPE scale measuring repetitions in reserve," J Strength Cond Res 30(1):267-275 (2016); NSCA, "Training Load Chart" (percent of 1RM by repetitions).