How Heart Rate Monitors Calculate Calories Burned

Heart rate monitors use three factors to determine how many calories you burn during exercise. The monitor takes the information it receives about your pulse, or heart rate, and, using a mathematical formula, it estimates how many calories per minute a person of your age, weight and/or other factors burns at that heart rate.

Data Collection

The first step in getting calorie information from a heart rate monitor is to put your personal information into the device. This can include your age, weight, gender, resting heart rate and maximum heart rate. The more information -- and the more accurate the information -- the better results you will get. Using a manual formula to calculate your maximum heart rate, such as subtracting your age from 220, will give you general approximation of your maximum heart rate. If you are 35 years old, for example, you will enter your maximum heart rate number into the monitor as 185. To find your resting heart rate, place two fingers against your wrist or neck, count the number of pulses, or beats, you feel in 30 seconds and then multiply by two. Calculate your resting heart rate after you wake in the morning and recalculate it on different days to get the most accurate result. These methods give you a general estimate of your maximum and resting heart rates. A professionally-administered stress test will give you the most accurate numbers to enter into a heart rate monitor.

Pulse

Heart rate monitors measure your heart rate during exercise to estimate your calories burned. They do this by counting your heart beat via an electronic receiver. Personal heart rate monitors often come with a chest strap and wrist receiver, while exercise machines often have you grasp one or two of the machine’s handles with a sensor or sensors built in. Some heart rate monitors are one-piece devices, such as a ring you wear. If you exercise wearing a heart rate monitor while someone near you is using one, you might pick up the other person’s pulse signals. Other electronic devices might also interfere.

Formulas

Different manufacturers use different formulas in their heart rate monitors to calculate calorie estimations. If you perform the same exercise at the same speed on the same machine wearing two different brands of heart rate monitors, you might get different results. The results from the heart rate monitor on a cardio machine you are using might also show different results from the personal monitor on your wrist. The Consumers Union product-testing organization found that heart rate monitors that use a chest-strap and wrist-monitor configuration provide the most accurate results.

Misinterpretation

Your heart rate can rise and fall based on your emotions, leading a heart rate monitor to interpret a spike in your heartbeats per minute as increased muscle use. If you are wearing a heart rate monitor during a tennis match, for example, your heart rate might rise rapidly if you start losing, get angry or get excited about coming back from a deficit. Your heart rate monitor will credit you with more exercise and more calories burned than you are actually achieving.

Accurate Comparisons

Even if your heart rate monitor does not provide accurate results, you can still benefit from training with one if you use it to compare workouts to each other. For example, if your monitor undercounts your calories burned by 10 percent, it will undercount every workout by 10 percent, letting you compare how a treadmill burns calories compared to an elliptical, or how your Wednesday workout compares to your Monday workout.