RPE Calculator

Rating of perceived exertion turns how hard something feels into a number that can be prescribed and recorded. Two different scales share the name and are routinely confused: the original Borg 6 to 20 scale used in cardiac rehabilitation and aerobic testing, and the 1 to 10 repetitions-in-reserve scale used in strength training. They are not interchangeable, and a programme calling for RPE 8 means something entirely different in each.

The formula

Borg 6-20: approximate heart rate = RPE x 10
RIR-based RPE (strength): RPE = 10 - repetitions left in reserve
Estimated 1RM from RPE: load / percentage corresponding to (reps, RIR)

• The 6 to 20 range was designed so that multiplying by 10 approximates heart rate in a healthy adult
• RIR = repetitions in reserve, how many more you could have completed before failure

Borg G, psychophysical scaling of perceived exertion. The RIR-based 1 to 10 scale is the resistance-training adaptation in common use.

The two scales side by side

The Borg 6 to 20 scale is anchored to heart rate: 6 corresponds to complete rest at roughly 60 beats per minute and 20 to maximal effort at roughly 200. It is the scale used in cardiac rehabilitation and exercise testing, where a target might be to work at an RPE of 11 to 13.

The strength-training scale runs 1 to 10 and is anchored to proximity to failure, not to heart rate. RPE 10 means no further repetitions were possible; RPE 8 means two were left. This anchoring is what makes it useful for autoregulating load day to day.

• The talk test aligns well with the Borg scale: able to hold a conversation is about 12 to 14, able to speak only in short phrases is about 15 to 16.
• When a strength programme says RPE, it almost always means the 1 to 10 RIR scale. When a cardiac rehabilitation programme says RPE, it means Borg 6 to 20.

Why autoregulation uses RPE

Percentage-based programming assumes a fixed one-rep maximum, but actual capacity fluctuates day to day with sleep, stress, nutrition and accumulated fatigue, commonly by 5 to 10 percent. A prescription of 80 percent is therefore a different stimulus on a good day than on a bad one, and following it rigidly produces sessions that are too easy or too hard.

Prescribing by RPE inverts this: instead of fixing the load and letting the effort vary, it fixes the effort and lets the load vary. Someone told to work up to a set of five at RPE 8 lifts what that feels like today. That is the practical advantage, and it is why RPE-based programming became standard in strength sport.

Accuracy and how it develops

Novices are consistently poor at estimating repetitions in reserve, typically underestimating how many they had left, which means they train further from failure than intended. Accuracy improves with experience and improves faster when lifters occasionally take a set to actual failure to calibrate what the end genuinely feels like.

Accuracy is also better closer to failure and better on compound lifts done for moderate repetitions. Estimating RIR on a set of fifteen is much harder than on a set of three, because the final repetitions of a long set slow down in a way that is easier to feel than to predict.

• Expect RPE estimates to be unreliable for the first few months of training and to improve substantially with practice.
• Occasional calibration sets to failure improve accuracy, but taking every set to failure impairs recovery without improving results.

Limitations

• The two scales share a name and are not interchangeable. Applying a 1 to 10 interpretation to a Borg 6 to 20 prescription, or the reverse, produces the wrong intensity.
• RPE is subjective and correlates imperfectly with physiological load. Two people reporting the same RPE can be working at different relative intensities.
• Novices systematically misjudge repetitions in reserve, so RPE-based prescriptions are less reliable in beginners than in experienced lifters.
• The Borg scale relationship to heart rate breaks down for anyone on beta blockers or other rate-limiting medication.
• Environmental conditions, particularly heat and altitude, raise perceived exertion at the same workload.
• RPE is a training tool. It does not assess cardiac safety, and anyone with cardiac symptoms should be assessed rather than relying on how exertion feels.

Frequently asked questions

What is the difference between the two RPE scales?

The Borg scale runs 6 to 20 and is anchored to heart rate, used in cardiac rehabilitation and aerobic testing. The strength-training scale runs 1 to 10 and is anchored to repetitions in reserve. RPE 8 means very different things in each.

What does RPE 8 mean in lifting?

Two repetitions left in reserve: you could have completed two more before failure. RPE 9 is one left, and RPE 10 is a maximal set with nothing left.

Why does the Borg scale start at 6?

Because it was designed so that multiplying the rating by 10 approximates heart rate in a healthy adult. A rating of 6 corresponds to about 60 beats per minute at rest and 20 to about 200 at maximum.

Why train by RPE instead of percentages?

Because actual capacity varies day to day by roughly 5 to 10 percent with sleep, stress and fatigue. Fixing the effort and letting the load float matches the stimulus to what your body can do that day, rather than to a number recorded weeks ago.

How accurate are RPE estimates?

Poor in novices, who typically underestimate how many repetitions they had left, and considerably better with experience. Accuracy is highest close to failure and on compound lifts for moderate repetitions, and lowest on long sets.

References

1. Borg G. Psychophysical scaling with applications in physical work and the perception of exertion. Scandinavian Journal of Work, Environment and Health, 1990.
2. US Department of Health and Human Services. Physical Activity Guidelines for Americans, 2nd edition.
3. MedlinePlus. Exercise and physical fitness.
4. American College of Sports Medicine. Physical activity guidelines resources.