Both advanced and elite level heart rate monitors come with a feature that allows you to calculate the number of calories you burn during physical activity. The exact formulas these monitors use are proprietary and vary from brand to brand. They may use any combination of a range of factors to estimate calorie expenditure: weight, height, gender and age, in addition to heart rate. Elite monitors may even factor in information on training conditions such as temperature and altitude. How accurate these monitors are depends on many variables.
Oxygen and Calorie Burn
The most accurate method of measuring calorie burn is through VO2 testing, which measures your body’s maximum capacity to take in oxygen. The test is done in a lab using a breathing apparatus that can measure your oxygen intake and carbon dioxide discharge. Under very controlled conditions, where your heart rate is monitored for safety reasons and as a part of the overall test analysis, you perform an aerobic exercise to test your oxygen capacity. These tests verify that the more demanding a physical activity is, the faster you breathe and the harder your heart works to deliver incoming oxygen to where it’s needed. The harder you work, the more energy you require, and therefore the more oxygen you need. Combustion, which turns the fuel you receive from food into energy, needs oxygen. Because there is a direct relationship between how much oxygen you use and how much energy you burn, by measuring the oxygen to carbon dioxide exchange through VO2 testing, you will get a fairly precise gauge of how many calories you're burning.
VO2 tests also show that there's a linear correlation between how intense your heart rate is and how much oxygen you use (and consequently, how many calories you burn as well). Formulas that calculate energy burn based on heart rate work backwards: Using the heart rate, you can estimate how much oxygen you use, which allows you to approximate how many calories you burn. And while it is a perfectly legitimate method of estimating energy burn, it’s not without fault and limitations.
Heart Rate and Calorie Burn
You may calculate how many calories you burn based on your heart rate; however, this method is not entirely accurate. Even though there is a linear correlation between how intense your heart rate is and how much oxygen you use as it relates to physical activity, your heart rate may also be influenced by other factors that are not related to physical activity and therefore do not impact how much oxygen you use, such as stress and other emotions, temperature, caffeine and illness. Heart rate can also vary considerably from person to person.
Heart Rate Monitors
The manufacturers of heart rate monitors don't make public the various formulas they use to calculate energy burn, so these formulas haven't been reviewed or studied thoroughly. Heart rate monitor formulas, however, presumably factor in the relationship between oxygen consumption and heart rate discovered during VO2 testing. According to a report by researchers from BodyMedia (Recent Advances in Free-Living Physical Activity Monitoring: A Review), in testing the performance heart rate monitors, rather than their formulas, they’ve been shown to be most accurate in calculating calories burned during moderate to vigorous activity level. Calculations are more prone to error during low intensity activities.