Concept
RIR (Reps in Reserve)
RIR (Reps in Reserve) is the inverse of RPE — the number of additional reps a lifter could have completed before failure on a given set.
RIR maps directly to RPE: 0 RIR = RPE 10, 1 RIR = RPE 9, 2 RIR = RPE 8, etc. Some lifters find RIR easier to estimate than RPE because the question is concrete ("how many more reps could I have done?") rather than abstract ("how hard was that?").
In programming, RIR is most useful for hypertrophy work where keeping 1-3 reps in reserve preserves volume capacity across the week. For peak strength training, sets at 0-1 RIR are appropriate; for hypertrophy and accessories, 1-3 RIR is the standard prescription.
Further reading & authoritative sources
Related terms
RPE (Rating of Perceived Exertion)
RPE (Rating of Perceived Exertion) is a 1-10 scale that describes how hard a working set was relative to true failure, where 10 means no more reps were possible and 8 means 2 reps in reserve.
AMRAP (As Many Reps As Possible)
AMRAP (As Many Reps As Possible) is a set in which the lifter performs as many quality reps as possible at a prescribed weight, typically used as the final working set in strength programs.