Whilst direct calorimeters measure the rate of heat dissipation of a subject, indirect calorimeters measure the rate of heat generation; averaged over a long period of time the two rates will be equal or very nearly so. Thus it is a mistake to believe that because of its name, indirect calorimetry is a second-rate means of measuring heat production.

Indirect calorimetry estimates heat production from quantitative measurements of materials consumed and produced during metabolism. Most methods involve estimation of respiratory gas exchange and these may be classified according to their operating principles as confinement, closed-circuit, total collection, and open-circuit systems.

In confinement systems the subject is held in a sealed chamber and the rates of change of gas concentration in the chamber are recorded.

In closed-circuit systems the subject is again held within or breathes into a sealed apparatus; the carbon dioxide and water vapour produced by the subject are measured as the weight gain of appropriate absorbers, and the amount of oxygen consumed is measured by metering the amount required to replenish the system. In total collection systems all the air expired by the subject is accumulated in order to measure subsequently its volume and chemical composition.

There are two major forms of open-circuit calorimetry. In one the subject breathes directly from atmosphere and by means of a non-return valve system expires into a separate outlet line. In the second form, the subject inspires from, and expires to, a stream of air passing, by means of a pump or fan, across the face.