Is 0 RIR the same as failure?

Usually yes, as long as you mean no more clean reps with acceptable technique.

Is RPE 8 always 2 RIR?

It is a useful approximation, not a law. Fatigue and exercise type change how it feels.

Which is more accurate?

Neither is automatically more accurate. The better scale is the one you can apply consistently.

You can, though most people only need one effort scale per set.

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Training

RPE vs RIR

RPE and RIR describe the same thing from opposite sides: effort against reps left.

Updated July 9, 20265 min read

RPE and RIR are two ways of answering the same question: how close was that set to failure? RPE scores effort from 1 to 10. RIR means reps in reserve, the number of good reps you think you had left when you stopped.

Stop a set thinking "I probably had two more clean reps in me" and that is 2 RIR. On the common lifting scale it lines up with roughly RPE 8. Nothing left at all is 0 RIR, or RPE 10.

RIR	Approx. RPE	Plain-English read
4+	6 or lower	Easy to moderate. Plenty left.
3	7	Useful work, still very controlled.
2	8	Hard set with good repeatability.
1	9	Very hard, one clean rep left.
0	10	No clean reps left.

Which one should you use?

Use whichever one makes you more honest. Some lifters prefer RIR because it is concrete: "How many more reps?" Others prefer RPE because it catches the whole feel of the set, bar speed, stability, and confidence included.

RIR earns its keep in hypertrophy work, because most muscle-building sets sit near failure without always reaching it. "Stop at 1 to 3 RIR" gives you a practical effort target without ever testing a true max.

RPE is useful when the load matters. If your squat prescription is 3 sets of 5 at RPE 8, you pick a weight that leaves the fifth rep with about two reps in the tank. That flexes with a great day or a rough one.

For heavy compounds, RPE fits naturally, because the load is the headline.
For accessories, RIR feels easier, because the goal is usually proximity to failure.
For beginners, keep either scale broad at first: easy, moderate, hard, max.

The main mistake is treating the numbers as magic. RPE 8 does not build muscle because the number says 8. It works because the set was hard enough to create a training signal while leaving enough in the tank to keep training well.