Calorimetry is the act of calculating the heat of chemical reactions or physical changes, or the science of making such measurements. Calorimetry is performed with a calorimeter. The word calorimetry is derived from the Latin word calor, meaning heat.
Indirect calorimetry calculates heat that living organisms produce from their production of carbon dioxide and nitrogen waste (frequently ammonia in aquatic organisms, or urea in terrestrial ones), OR from their consumption of oxygen. Lavoisier noted in 1780 that heat production can be predicted from oxygen consumption this way, using multiple regression. The Dynamic Energy Budget theory defines why this procedure is correct. Of course, heat generated by living organisms may also be calculated by direct calorimetry, in which the entire organism is placed inside the calorimeter for the measurement.