The Perceived Rate of Exertion (PRE) scale was first proposed by Borg and is used to measure the physical activity intensity level. The scale goes from 6 to 20, with 6 being very light exercise and 20 being the maximum intensity.

the PRE scale can be used to deduct your heart rate during training.  There is a correlation between a person’s perceived exertion rate and his heart rate. The correlation factor is 10. For example, if your rating of perceived exertion (RPE) is 14, then 14 x 10 = 140, which is your estimated heart rate.