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To manually calculate your target heart rate zone, first determine your maximum heart rate, which is 220 minus your age. (This calculation represents a general guideline only.) For example, if you are 35, your maximum heart rate is 220 - 35 = 185. Next, calculate your target heart rate zone. This is generally 50% to 75% of the maximum heart rate for most people during the first six months of regular exercise. For example, 50%-75% of your maximum heart rate of 185 is (185 x 50) ÷ 100 = 93; (185 x 75) ÷ 100 = 139. So your target heart rate zone for exercise, in this example, would be 93-139 heartbeats per minute.

If you haven't been exercising, the American Heart Association (AHA) recommends that you then start at 50%, with the goal of gradually building up to 75% during this six-month period, but only after checking with your physician. People who have not been exercising or who intend to change their exercise program significantly need to get their physician's approval. After exercising regularly for six months, some people might be able to exercise comfortably at up to 85% of their maximum heart rate, according to the AHA. However, the AHA notes that you don't have to exercise that hard (at 85%) to stay in condition.