The American Heart Association provides a chart for target heart rates based on age, at heart.org. Alternatively, an individual can find his maximum heart rate by subtracting his age from 220.
In order to calculate target heart rate, first take an individual's pulse at rest, and count the number of heartbeats per minute. This number is the resting heart rate. The average rate for adults is 60-100 beats per minute.
Then take the individual's pulse immediately after exercising, and count the number of pulses for 10 seconds. Multiply that number by 6, and the result is the individual's target heart rate, based on pulses per minute.