The ONLY reliable way to count calories out is to wear a mask that measures carbon dioxide exhaled - this is what you see them doing in labs with a volunteer on

The ONLY reliable way to count calories out is to wear a mask that measures carbon dioxide exhaled - this is what you see them doing in labs with a volunteer on a treadmill or stationary bike hooked up to what looks like breathing apparatus. This obviously isn't practical for every day use but the data has been used to approximate calories out correlated with heart rate if body weight is known - several sports watches such as Suunto use this approach.