Rate of Perceived Exertion

If you don't own a heart rate monitor and don't want to stop in the middle of a workout to take your pulse you can opt to use the Rate of Perceived Exertion (RPE) scale to monitor your intensity.  An RPE scale measures intensity based how hard you feel you are working during exercise or physical activity.

Swedish psychologist Gunnar Borg developed the first RPE scale with a scale of 6 to 20:

6   No exertion at all

7   Extremely light
   
8

9   Very light

10

11  Light

12

13  Somewhat hard

14

15  Hard

16

17  Very hard

18

19  Extremely hard

20  Maximum exertion

The Borg scale is good to follow but it's a little clumsy using the numbers 6 thru 20.  There are other RPE scales based on the Borg scale, one common version uses a simplified scale of 0 to 10:

0   No exertion at all

1   Extremely light    

2   Very light

3   Moderate

4   Somewhat hard

5   Hard

6

7   Very Hard

8

9   Extremely hard

10  Maximum exertion

The RPE scale can be simplified even further with a scale of 1 to 4, which is easier to remember and use:

1  Easy
2  Moderate
3  Hard
4  Very hard

Any RPE scale can be correlated with the Target Heart Rate Zone, this chart uses an RPE scale of 0 to 10: