A mains electricity (AC-powered) device intended to be used clinically to measure oxygen consumption (VO2) to estimate the resting metabolic rate (RMR) of a patient. It typically includes a mouthpiece with a unidirectional breathing valve into which the patient breathes, displays for direct read out, and connections for transfer of data to a computer for further analysis. It may be used in a wide variety of healthcare or research settings to obtain accurate measurements used in weight management, nutritional, or other medically-supervised programs.