THE SHORT ANSWER
RIR estimates how many more repetitions you could perform before the defined end of a set. RPE describes perceived exertion, but its meaning depends on the scale: a set rating and a whole-session rating answer different questions.
- Write down which scale you use. An unexplained eight does not tell the whole story.
- Repetitions in reserve are predicted, not directly measured when you stop a set early.
- Consistent descriptions and context make a log more useful than extra decimal places.
THE IDEA IN THREE PARTS
OPEN A NOTE ↓01The set
RIR refers to predicted repetitions remaining for that exercise and the stated task standard.
02The rating
A RIR-based set RPE expresses proximity to the end of a set using a rating convention.
03The session
Session RPE summarises perceived exertion across the workout. It is not the final set's RIR in another form.
A conceptual map, not measured data.
A training log says “8.” That could mean eight repetitions, an effort rating for one set, or an overall impression of the workout. Numbers only become useful when the question behind them is clear.
RPE and RIR are common ways to describe training effort. They can support a conversation and a review, but they do not remove uncertainty. Understanding what they mean is more useful than making every entry look precise.
What is RIR?
RIR stands for repetitions in reserve. If you stop a set and estimate that two more repetitions were possible, you are describing two RIR. The estimate depends on what counts as completing another repetition: the exercise, movement standard, and endpoint need to be understood.
That last condition is easy to overlook. One person’s imagined final repetition may involve a changed range or substantial assistance. Another person’s endpoint may require a more consistent task. Those are not necessarily estimates of the same thing.
The RIR-based approach described by Helms and colleagues relates a set’s perceived exertion to predicted repetitions remaining. In that convention, a rating of eight corresponds approximately to two repetitions in reserve. This is a naming convention for a set, not a universal conversion for every use of RPE. Read the explanation.
What is session RPE?
Session RPE asks about the overall perceived exertion of a workout. It can include the experience of several exercises and tasks rather than a prediction about the last set.
Foster and colleagues investigated this session-wide approach in cycling and basketball. Their work helped establish a way to describe training across different sessions. It does not turn an overall workout rating into a precise measure of how close a particular set was to failure. Read the study.
| Record | Question it answers | What it does not tell you alone |
|---|---|---|
| RIR | How many more repetitions did I think were possible? | The exact number that would have been completed |
| RIR-based set RPE | How close did this set seem to its endpoint? | The demands of the entire workout |
| Session RPE | How demanding did the whole session feel? | The effort of each individual set |
How accurate are repetitions-in-reserve estimates?
Halperin and colleagues found imperfect predictions and substantial variation across the studies in their review. Accuracy tended to improve when estimates were made closer to task failure, but experience did not make every prediction exact. Differences in tasks and methods complicate simple conclusions. Read the review.
That is a reason to label the number as an estimate, not a reason to force every set to failure to prove it. The reviewed measurement protocols are not instructions for how every reader should train.
If you are unsure, a note such as “rough estimate; unfamiliar exercise” can be more informative than inventing extra precision. The log should preserve uncertainty when uncertainty was part of the session.
A worked log entry
Consider two fictional entries for the same exercise:
Entry A: Row, 8, felt hard.
Entry B: Same row machine and grip as last time; eight repetitions at the recorded load; approximately two RIR; shorter rest than usual.
The second entry is useful because it explains the task, the estimate, and a change in context. It does not prove that the trainee’s estimate was correct. A future comparison can now ask whether the machine, setup, rest, and effort description are reasonably similar.
What should you keep in the log?
Start with the observations you actually plan to review: exercise, load, sets and repetitions, a clearly named effort scale, and relevant context. If a metric never informs a question, collecting more of it may simply create a longer record.
Keep observations separate from explanations. “Higher session RPE than usual” describes an experience. “My programme is failing” is a conclusion that the number alone cannot support.
For a broader structure, use the five planning questions. The point of tracking is to make training easier to interpret, not to turn a day of training into a score you have to defend.
COMMON QUESTIONS
A little more clarity.
Does RPE 8 always mean two repetitions in reserve?
That relationship belongs to a RIR-based set RPE convention. It does not apply to every RPE scale or to an overall session rating. Record the scale alongside the number.
Can beginners use RIR?
RIR can be recorded as an estimate, but confidence and familiarity matter. Research does not support treating any trainee's prediction as an exact count simply because they have more experience.
RESEARCH & REFERENCES
Follow the evidence.
The studies below support the research statements. Planning frameworks and fictional examples are editorial explanations.
- Applied research reviewHelms et al. (2016). Application of the repetitions in reserve-based rating of perceived exertion scale.
Explains the RIR-based rating convention for resistance-training sets.
- Scoping review and exploratory meta-analysisHalperin et al. (2022). Accuracy in predicting repetitions to task failure in resistance exercise.
Predictions were imperfect and results varied substantially across study protocols.
- Exercise monitoring studyFoster et al. (2001). A new approach to monitoring exercise training.
Studied session RPE during cycling and basketball; a session-wide observation, not a clinical assessment.
General education for adult recreational trainees. Individual training or health decisions may require qualified guidance. Read our editorial approach.