It’s not 100% accurate, only proper testing will do that for you, but it’s a very good guide that many coaches and personal trainers use to indicate how hard workouts should be, and a good guide for yourself to monitor effort level.
Here is the full rate of perceived exertion rankings with an explanation and how to use them in training.
1) Nothing at all
2) Very very light
3) Very light
4) Fairly light
6) Somewhat hard
8) Can talk, but chooses not to
9) Very hard
10) Very very hard (maximal)
Of course this is only effective in relation to you own personal past experiences. The closer you have been to your maximal heart rate in the past, the more accurate the RPE will be.
How to use RPE in training
Warm ups and warm downs should be at RPE 4-5/10: fairly light to moderate, doing exactly what it says on the tin, preparing your body for the main event.
Cardiovascular training is usually at 5-8/10 RPE. This is long slow training miles for the purpose of conditioning and endurance. It’s benefit is that it helps to reduce blood pressure, reduced resting heart rate, increases VO2 Max and decreases fat mass.
Interval training another form of cardiovascular activity which includes harder higher bursts of effort. This is usually at 6-8/10 RPE and usually involves bouts of activity at an intensity that cannot be maintained continuously, followed by rest period of lower intensity activity. This utilises the anaerobic system. It is an efficient way to use limited time, and increases aerobic capacity.
Resistance training involves working the muscles to gain strength, endurance, increased muscle size and / or power. The benefits are increased size/strength /endurance of a muscle, increased bone density and decreased fat mass. This type of workout is usually done at 8-10/10 RPE for a high intensity hard workout with heavy weight and low reps.