Rate of Perceived Exertion is very useful metric because it is a subjective measure of “rider feeling” of an effort. Measuring RPE involves rating your training from Very Very Light to Very Very Hard.

When considering how hard you felt the session was it is important to consider feelings such as shortness of breath, as well as how tired you feel in your legs and overall.

Dr. Gunnar Borg, who created the scale, set it to run from 6 to 20 as a simple way to estimate heart rate—multiplying the Borg score by 10 gives an approximate heart rate for a particular level of activity.

w you might describe your exertion Borg rating of your exertion Examples 
(for most adults <65 years old)
None  6 Reading a book, watching television
Very, very light  7 to 8 Tying shoes
Very light  9 to 10 Chores like folding clothes that seem to take little effort
Fairly light 11 to 12 Walking through the grocery store or other activities that require some effort but not enough to speed up your breathing
Somewhat hard 13 to 14 Brisk walking or other activities that require moderate effort and speed your heart rate and breathing but don’t make you out of breath
Hard 15 to 16 Bicycling, swimming, or other activities that take vigorous effort and get the heart pounding and make breathing very fast
Very hard 17 to 18 The highest level of activity you can sustain
Very, very hard 19 to 20 A finishing kick in a race or other burst of activity that you can’t maintain for long

There are many similar scales including the Borg CR10 Scale which has values between 1 and 10 and you can use these in combination with a Heart rate monitor to monitor issues such as overtraining

For example if Heart Rate training load is stable and RPE training load is going up this means you feel more fatigue with same internal load (or at the same HR you felt more fatigued).

Here’s a useful link to the Borg Scale