Indirect calorimetry measures O consumption and CO production. On the assumption that all the oxygen is used to oxidize degradable fuels and all the CO thereby evolved is recovered, it is possible to estimate the total amount of energy produced from the chemical energy of nutrients and converted into the chemical energy of ATP, with some loss of energy during the oxidation process.
Respiratory indirect calorimetry (IC) is a noninvasive and highly accurate method of metabolic rate, which has an error of less than 1%. It has high reproducibility and has been considered a gold standard method. This method allows estimating BEE and REE as well as identification of energy substrates that are predominantly metabolized by the body at a specific moment. It is based on the indirect measurement of the heat produced by oxidation of macronutrients, which is estimated by monitoring O consumption and CO production for a certain period of time. 
The calorimeter has a gas collector that adapts to the subject and through a unidirectional valve minute by minute collects and quantifies the volume and concentration of O inspired and CO expired by the subject. After a volume is met, Resting Energy Expenditure is calculated by the Weir formula and results are displayed in software attached to the system. Another formula used is:
where RQ is the respiratory quotient (ratio of volume CO produced to volume of O consumed),  is , the heat released per litre of oxygen by the oxidation of carbohydrate, and  is , the value for fat. This gives the same result as the Weir formula at RQ = 1 (burning only carbohydrates), and almost the same value at RQ = 0.7 (burning only fat).