When we exercise, a combination of carbohydrates and fats are utilized as fuel (protein is only utilized in extreme situations). In order to use both carbohydrates and fat, oxygen is needed. Through years of research, it has been established that for every liter of oxygen consumed per minute of activity, there is an estimated amount of Calories utilized. This amount is approximately 5.0 kcal/liter of oxygen. In order to use this constant value to figure out how many Calories are burned during a certain activity, a relative VO2 maintained during that activity must be known (along with other parameters like body weight). Because this information is not readily available (most people don’t know their VO2 as they exercise), standardized MET values have been established for different activities. MET stands for Metabolic Equivalent and is the absolute intensity of a particular activity. The value of 1 METs equates to 3.5 liters of oxygen consumed at rest. To establish a MET value for activities such as walking, running, or swimming 3.5 is used to establish the ratio of energy expended at rest verses with excursion. This is how most fitness machines calculate an estimated Calorie burn when you complete your workout (ie-when the elliptical asks for your weight and age then gives you a total of Calories burned at the end of the workout).
Here is the actual formula using VO2:
[VO2(mL/kg/min) x body weight (kg) / 1000] x 5 kcal/min = Calorie Expenditure per minute
If you do not know the relative VO2, here is the formula using METs:
[(METs x 3.5 x Body weight in kg) / 1000] x 5 kcal/min = Calorie Expenditure per minute
MET values for a variety of activities can be found here:
To make things a bit simpler:
This is a link to the ACE Fit page to find out how many Calories you will burn while performing a certain activity (this calculator uses MET values of the selected activities):
Bryant and Green. (2010) ACE Personal Trainer Manual (4 th Ed) . San Diego, CA.: American Council on Exercise.
Pescatello, Linda. (2014) ACSM’s Guidelines for Exercise Testing and Prescription ( 9 th Ed). Baltimore, MD.: American College of Sports Medicine.