The short distinction

RPE rates how hard a set felt, commonly on a 1–10 scale. In resistance training, an RPE of 10 often means no additional repetition was available with acceptable technique.

RIR estimates how many repetitions remained. An RIR of 2 means you believe roughly two more repetitions were possible.

The scales often map approximately: RPE 10 to 0 RIR, RPE 9 to 1 RIR, and RPE 8 to 2 RIR. That mapping is a convention, not a measurement instrument.

Why use either scale

Fixed load and repetition targets do not describe every day equally. Sleep, accumulated fatigue, equipment differences, and technique can change the difficulty of the same prescription. RPE or RIR adds a compact description of the set as experienced.

This can help when reviewing whether a load increase was genuinely manageable or whether a planned easy set repeatedly became near-maximal.

Where estimation gets difficult

People often become better at estimating proximity to failure with practice, but the estimate can still be wrong. New exercises, high repetitions, unfamiliar machines, pain, motivation, and fear of failure can all distort it.

Do not treat RPE 8.5 as laboratory precision. The value is most useful when the scale definition stays consistent and the surrounding facts—load, repetitions, exercise, and technique—are also recorded.

Choose one primary language

If your program is written in RIR, record RIR. If it uses RPE, record RPE. Recording both is reasonable when deliberately learning the relationship, but it can create unnecessary friction for routine logging.

A simple approach is:

• record only completed work sets;
• use whole-number RIR or half-point RPE when genuinely meaningful;
• add a note when technique, range, or equipment changed;
• review trends across several sessions rather than reacting to one rating.

RPE and RIR are subjective training tools, not medical measures and not substitutes for qualified guidance about pain, injury, or safe participation.