If you follow a training program, you've probably seen prescriptions like "squat 4x6 at 80% of 1RM." The problem is that fixed number ignores something obvious: you're not the same person every day. After a bad night's sleep or a brutal work week, 80% of your max can feel like 90%. After a good rest, it can feel like 70%. That's exactly the gap that rate of perceived exertion, or RPE, and its sibling concept, RIR (Repetitions in Reserve), are built to close.
Neither one is advanced-coach jargon. They're simple tools that help you adjust training load based on how your body is actually responding that day, instead of blindly following a number on a spreadsheet. This article explains what they are, why they're worth using, how reliable your own perception really is, and how to apply them without overcomplicating your training.
What RPE and RIR actually mean
The RPE scale used for strength training runs from 1 to 10, where 10 means total muscular failure: you couldn't complete another rep with good form. RPE 9 means you could have done one more rep, RPE 8 means two more, and so on. RIR is essentially the same idea read backwards: instead of saying "RPE 8," you say "2 reps in reserve." One is the mirror image of the other.
This specific scale for resistance training was developed and validated in a study published in the Journal of Strength and Conditioning Research, which built and tested an RPE version based on reps in reserve — distinct from the original Borg scale, created for continuous cardiovascular exercise and too imprecise to describe how close a set of squats or bench press really is to failure.
Sources: PubMed, Stronger by Science
Why use RPE instead of just following a percentage of your 1RM
Prescribing load as a percentage of your 1RM starts from a shaky premise: that your max is a stable number, measured once and valid for months. In practice it isn't. Sleep, stress, nutrition, hormonal cycles, even room temperature affect how much weight you can move on a given day. A rigid percentage-based program has no way to capture that variation — an RPE-based prescription does.