RPE and RIR, explained: the two numbers that replace a percentage

Most training programmes are written with a number you cannot look up: leave two reps in reserve, or RPE 8. Both mean the same thing, and both are estimates rather than measurements — which is exactly why they work as well as they do, and why they fail so predictably for beginners.

What the two scales are

RIR is reps in reserve: how many more repetitions you could have completed with good technique before failure. Zero RIR is failure. Two RIR means you stopped with two honest reps left. RPE in a lifting context is rating of perceived exertion on a ten-point scale borrowed from the RIR system rather than the older heart-rate version: RPE 10 is failure, RPE 9 is one rep left, RPE 8 is two, and so on. The two scales are the same instrument with the labels moved, which is why programmes use them interchangeably.

Why programmes use them instead of percentages

A percentage of your one-rep max assumes your strength today is the same as the day you tested it. It is not: sleep, stress, illness, travel and cumulative fatigue all move it. RIR self-corrects, because it is rated on the set you just did rather than on a number from eight weeks ago. It also makes programmes work across lifters of different levels without recalculating anything, and it lets you push hard on the days you can and pull back on the days you cannot — which is the mechanism behind most sensible autoregulation.

The honest problem: beginners are bad at it

Estimating reps in reserve requires knowing what failure feels like, and most people new to lifting have never been near it. The research consistently finds that inexperienced lifters overestimate how many reps they have left — a set they call two RIR is often four or five. The fix is unglamorous: on a small number of sets, using a machine or a lift where failing is safe, actually take a set to failure and count what the last reps felt like. Do that a handful of times and the scale starts to mean something. Until then, treat your RPE ratings as an overestimate and progress by adding load when the top of your prescribed range feels genuinely hard.

Where RPE works well, and where it does not

It works well on compound lifts in moderate rep ranges where you can judge technique breakdown, and on isolation work where failure is safe. It is weak on explosive or technical lifts — a maximal clean or a heavy deadlift single ends for reasons that have little to do with muscular reserve — and on very high-rep sets, where cardiovascular and local burn blur the judgement. It is also weak when form itself is the limiting factor, because “I could have done two more” is not the same claim as “I could have done two more well”.

Using it in a real programme

Work in bands rather than single numbers: keep most working sets between two and four RIR, take the last set of an isolation exercise closer to one, and reserve true failure for the final set of a machine or dumbbell movement where bailing is easy. Progress by increasing load when you can hit the top of the rep range at your target RPE for every set. The relationship between these numbers and raw load is covered in how progressive overload works, and the rep ranges these ratings sit inside are in how many reps for muscle.

Sources

• PubMed — rating of perceived exertion and resistance training load
• ACE — training intensity and autoregulation guidance

Every link checked 2026-09-25; sources favour primary and public-health bodies.