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. 

He based it on the principle that a young, reasonably fit individual will have a resting heart rate of 60 and a maximum of about 200. By referring the scale to the way your breathing feels you can use it to gauge the effort or heart rate zone you are working in without the need for a heart rate monitor.

If you do have a heart rate monitor and you are not a 20-year-old you will need to adjust the heart rates you actually use. The traditional approach to working out what your maximum heart rate is to deduct your age from 220. Thus a 20-year-old has an HR Max of 200, a 40-year-old – 180 and a 60-year-old – 160. So, the 20-year aiming for a heart rate of 80% would be operating at 160 bpm, while the 60-year-old would be at 128 bpm.

Actually, the calculation is a little more complicated, but there is an excellent calculator available by clicking the link below.

Target Heart Rate Calculator