Working at an appropriate intensity during exercise helps you burn calories and improve cardio-respiratory fitness. The American Heart Association recommends staying within 50 to 85 percent of your maximum heart rate to reap the most benefits from exercise. Understanding where you fall in this range during exercise helps you pace your workout and not tire too soon. Multiple formulas exist to help you determine a working zone for your heart rate.

Use age-adjusted formulas when you have limited information about your fitness level and resting heart rate. Subtract your age in years from 220 to find your maximum heart rate. For example, if you are 40 years old, your maximum heart rate is 180 beats per minute. Multiply by .5 to .8 to find your working zone. For a 40-year-old, this would be a heart rate of 90 to 153 beats per minute.

Determine heart rates zones using the Karvonen formula if you have found your resting heart rate in advance. Upon a natural morning awakening, take a full one-minute pulse count at the wrist or neck before getting out of bed, recommends the University of Rochester Medical Centers. Do this three days in a row and average the number to arrive at an accurate resting heart rate.

Find your maximum heart rate by subtracting your age from 220. From this number subtract your resting heart rate. Multiply the result by .5 to .85--the working range percentage of heart rate maximum. Add back in your resting heart rate to these numbers to find your working zone.

For example, a 40-year-old with a resting heart rate of 60 would take 220 minus 40 to arrive at 180. From 180, subtract the resting heart rate of 60 for a total of 120. Multiply 120 by .5 and .85 to find 60 to 102. Add back in the resting heart rate of 60 to come to a working 50 to 85 percent range of 120 to 162 beats per minute.

Calculate your heart rate during exercise to make sure you are staying within a sustainable working zone. Stop exercising for a moment. Take your pulse at your wrist or neck--carotid artery--for 10 seconds. Multiply the number you get by six to figure out beats per minute.