To estimate target heart rate, calculate the maximum heart rate (the heart rate estimated to correspond to the rate of maximum oxygen use), which is 220 minus age. Then, multiply the maximum heart rate by 60 to 85% (or by 0.6 and 0.85, respectively). For example, for a 40-year-old, the maximum heart rate is 180 beats per minute, so the target heart rate is 108 to 153 beats per minute (60 to 85% of 180).