Calculate the maximum heart rate. For men, start with 220 beats per minute and subtract the age from the number. For example, a 30-year-old man would take 220 and subtract 30 to get 190 beats per minute. This is the maximum heart rate. According to The Walking Site, a woman does a similar calculation, but starts at 226 rather than 220.

Calculate the optimal range. The optimal range for fat burning is between 60 and 70 percent of the maximum beats per minute. The optimal range when looking at burning fat while building endurance is between 70 and 80 percent, but the endurance building is slightly less effective for fat burning. Take the number found previously and multiply it by 0.6; repeat the process with 0.7. For example, take the 190 beats per minute for a 30-year-old man and multiply 190 by 0.6 and again by 0.7. The range ends up 114 beats per minute to 133 beats per minute. This is the optimal range for a 30-year-old man to burn fat.

Perform an exercise and check the heart beat after the heart rate starts to increase. Put the pointer and middle finger lightly against the side of the neck. Do not apply pressure. Count the beats for 30 seconds and then multiply by two to determine the current heart rate. If the heart rate is within the optimal range, fat burning is at its best. Exercise more vigorously until it gets to the appropriate range if it is low. Decrease vigorous exercise until it lowers to the appropriate range if it is high.