The rate at which you burn calories during exercise is primarily dependent on your heart rate. Additional factors such as weight, age and sex are also important in determining your calorie expenditure rate. You can estimate your calorie expenditure from a table of sample values, and you can also calculate it with a formula.
Standard Values
The Mayo Clinic provides standard calorie expenditures by weight and exercise. For example, walking at a speed of 2 mph for one hour burns 183 calories if you weigh 160 lb. It will burn 228 calories if you weigh 200 lb. and 273 calories if you weigh 240 lb.
Limitations
Standard calorie expenditure values by weight and exercise must make a number of simplifying assumptions. For example, they don't take heart rate into account, which provides a much more accurate estimate of your calorie expenditure than the particular exercise. In addition, standard values often don't take age or sex into account.
Heart Rate
Your heart rate correlates fairly well with your calorie expenditure rate when your heart rate is between 90 and 150 beats per minute. This means that it's possible to calculate your calorie expenditure with reasonable accuracy when your heart rate is within this range. You can record your average heart rate during exercise with most modern heart monitors.
Procedure
Wear a heart monitor according to the instructions, and ensure that it's properly receiving your heart rate. Begin exercising but don't turn the heart rate recorder on until your heart rate rises above 90 bpm. Speed up or slow down as needed to keep your heart rate within the range of 90 to 150 bpm for the duration of the exercise. Turn the heart rate recorder off after you complete your exercise, but before your heart rate drops below 90 bpm. This will provide your average heart rate during the exercise and the duration of the exercise.
Formulas
The Journal of Sports Sciences provides calorie expenditure formulas that take heart rate, weight, age, sex and duration of the exercise into account.
The formula for women is: Calories = [(Heart Rate x 0.4472) -- (Weight x 0.05741) + (Age x 0.074) -- 20.4022] x Time / 4.184. Heart rate must be in units of bpm, weight must be in pounds, age must be in years and time must be in minutes.
The corresponding formula for men is: Calories = [(Heart Rate x 0.6309) -- (Weight x 0.09036) + (Age x 0.2017) -- 55.0969] x Time / 4.184.
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