One reason to calculate heart rate during exercise is to know if a person is getting beneficial exercise to keep the heart and body healthy. In order to determine the safe and effective range of exercise to get cardiovascular benefits, two measurements are often taken. The first is the maximal heart rate, a number related to a person’s age (the heart beats slower with age). To estimate the maximal heart rate, subtract a person’s age from the number 220. For example, if someone is 40 years old, his or her maximal heart rate is 180.

The next measurement is the target heart-rate zone. This number uses the maximal heart rate and represents the number of times per minute at which a heart should be beating during aerobic exercise. For most healthy people, the range is 50 percent at the lower limit to 80 (some charts say 75) percent at the upper limit of their maximal heart rate. When a person’s heart rate reaches a value within this zone during exercise, it means he or she has achieved a level of activity that contributes to his or her cardiovascular fitness. For instance, from the above example of a maximal heart rate of 180, the beats per minute for the lower range would be 180 multiplied by 50 percent (0.50), or 90 beats per minute; the upper range would be 180 multiplied by 80 percent (0.80), or 144 beats per minute.

If you work out and maintain a lower-than-50- or higher-than-80-percent limit, there are few beneficial effects from the exercise. In terms of the lower limit, the heart is not working hard enough for any cardiovascular benefit; in terms of the upper limit (besides the strain and injuries that can result), the heart is working too fast for any benefit and the body can’t replenish oxygen that quickly.