The formula to calculate target heart rate range is: heart rate = [(220 - age) x 0.50] to [(220 - age) x 0.85]. This target range is 50 to 85 percent of the average maximum heart rate for age, according to the American Heart Association.
Maximum heart rate for exercise is calculated by subtracting a person's age from 220. During the initial weeks of a new exercise regimen, the target heart rate should be near 50 percent of the computed maximum rate. The target rate should be gradually increased with consistent exercise, and comfortable exercise at 85 percent of maximum heart rate may be possible after six months or more, states the American Heart Association.