RIR

RIR is the flip side of RPE. If RPE 8 means you could have done 2 more reps, RIR 2 says the same thing in a more immediate way. Many coaches and apps use RIR because it's easier to understand: it simply asks how many reps you still had in the tank when you put the weight down.

Practically, RIR determines how close to failure you should push. For hypertrophy, most sets should sit between RIR 0 and 3: close enough to the limit to recruit high-threshold muscle fibers, but with enough margin to maintain good technique and accumulate volume without destroying yourself. For maximal strength, an RIR between 2 and 4 works better, because it lets you handle heavy loads without accumulating too much fatigue.

The best way to learn how to estimate RIR is to test it. Every once in a while, take a set to total failure on a safe exercise (like a machine) and count how many extra reps you did beyond where you thought you were. With practice, your estimate will get more and more precise. When you read a sheet that says 3x8 RIR 2, you know exactly what to do: three sets of eight reps with a load that would let you do ten.