The RPE Scale is a common method for determining exercise intensity levels. It is commonly used in some research studies for that purpose, but may also be used in training programs to describe the intensity of training sessions. The scale of perceived exertion is how hard you feel your body is working, and so is a subjective measure.
The scale is based on the research of Borg, and is sometimes called the Borg Scale. You can download a copy of this RPE Scale for printing.
There are two versions of this scale. Below is shown the category-ratio version with the ratings between 0 (nothing at all) and 10 (very very hard). This is also called the Borg CR10 Scale. There is another very similar RPE scale also developed by Borg, with the ratings between 6-20.