RIR: reps in reserve
RIR is how many more good reps you could have done when you stopped. If you stop at 8 reps and could have done 10, that's 2 RIR.
RPE: rating of perceived exertion
RPE rates a set from 1–10. In lifting, RPE 10 = no reps left, RPE 9 = 1 left, RPE 8 = 2 left, RPE 7 = 3 left. So RPE and RIR are two ways of saying the same thing.
Why it's useful
- Auto-adjusts for good and bad days — you pick the weight that hits the target effort
- Stops beginners from always going to failure
- Stops people from 'training' far too easy
How to get better at judging it
Most people underestimate how many reps they have left. Once in a while, on a safe exercise like a machine or dumbbell curl, take a set to true failure and compare it to your guess.