You can calculate the thermic effect of exercise a number of different ways, and each has its advantages and disadvantages. The most accurate method uses a percentage of your individual VO2 reserve multiplied by 5 calories per minute. However, this method requires a number of mathematic formulas and an accurate exercise test to determine your VO2 reserve. Easier methods use heart rate, metabolic equivalent values and online calculators.
Thermic Effect of Exercise Defined

The thermic effect of exercise is simply the number of calories you burn during exercise that is above and beyond rest. The factors that most effect thermic effect are your weight, exercise intensity and the workload of the activity you're doing. A person who weighs more will burn more calories per minute than a lighter person performing the same workload. Similarly, between two people who weigh the same, the person who is more fit will burn more calories than the less fit individual working at the same intensity, because the fitter individual is able to perform a greater workload at that effort level.
The Most Accurate Method Isn't the Best One

VO2 measures how much oxygen your body consumes per kilogram of body weight per minute. It varies with your level of activity, and the upper boundary is limited by your level of aerobic fitness. VO2 reserve is the difference between that maximum value, which differs for everyone, and your resting VO2, which is a consistent 3.5 milliliters/kg/minute for everyone. If you were able to accurately determine your VO2 reserve for a given activity, you could multiply that number by 5 kcal/min and get the exact number of calories burned per minute. This makes using VO2 the most accurate method for calculating thermic effect. Unfortunately, finding your VO2 reserve is a highly complicated process that involves a clinical exercise test and a number of mathematic equations.
Using Heart Rate to Estimate thermic effect

Using heart rate can be an easy and fairly accurate way to calculate thermic effect. While it will be more accurate if you know your VO2 max, it is still possible to get a good estimate without it. The variables you need to know are your average exercise heart rate, gender, age, weight and minutes of exercise. You then plug this information into an online calculator, which will estimate calories burned per minute. The calorie calculator built into your heart rate monitor can be equally accurate, provided long as you've programmed your personal information into it. Limiting factors for heart ratebased calculations include stress, medications or drugs and dehydration, which can all affect your heart rate.
Using METs and Online Activity Calculators

Metabolic equivalent, or MET represents a multiple of the amount of energy a person uses at rest. One MET is equal to 1 calorie per kilogram of body weight per hour. The thermic effect for specific activities is calculated by multiplying this formula by predetermined factors that are based on activity intensity. Those multiplication factors can be found in the Compendium of Physical Activities, but they are also the basis of many online calorie calculators. One of the easiest ways to calculate thermic effect, then, is by plugging your weight and time of exercise into an online calculator for the specific activity you performed.
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