The short answer
RIR (reps in reserve) is the number of clean reps you could still have done at the end of a set. RPE (rating of perceived exertion) puts the same idea on a 10-point scale: RPE 10 means no reps left, RPE 9 one rep left, RPE 8 two reps left. Most working sets land between RPE 7 and 9, that is one to three reps in reserve.
Two ways to describe the same effort
RIR counts what is left: 2 RIR means you could have done two more reps. RPE rates the effort: 8 out of 10 on a set means the same thing, two reps left. Half points exist: RPE 8.5 means you might have done two more reps, but surely one. Use whichever is easier for you, and stick to it.
The RPE scale for strength training
RPE 10: maximal effort, no more reps possible. RPE 9.5: no more reps, but a bit more weight would have been possible. RPE 9: one rep left. RPE 8: two reps left. RPE 7: three reps left. Below 7, the set is more like a warm-up or technique work. Zourdos and colleagues introduced this reps-in-reserve scale in 2016; ratings matched bar speed more closely in experienced lifters than in novices.
How accurate is your estimate?
People tend to underestimate how many reps they have left, especially far from failure and on sets of many reps. Accuracy improves with experience and closer to failure. To calibrate, take the last set of an exercise to failure from time to time, safely, and compare it with what you predicted.
Choose your loads with RPE
A programme can prescribe “5 reps at RPE 8” instead of a fixed weight. On a good day you will lift a little more, on a tired day a little less, for the same effort. If you know the weight, reps and RPE of a recent set, an RPE chart estimates your 1RM and the load for any other combination: that is what the RPE calculator does.
Write it down
Add the RPE or RIR next to the weight and reps of your set, for example “100 × 5 @8”. Over time, the same weight at a lower RPE is progress too, even if the number on the bar has not changed yet.
Put it into practice
About this guide
STARK editorial guide. Examples explain how to use the library and record training; they are not individual coaching.
Training principles: Zourdos et al. (2016), Journal of Strength and Conditioning Research.
The session examples come from the STARK program catalogue linked above.