An electrically-powered device primarily intended to measure oxygen consumption (VO2) and/or carbon dioxide output (VCO2) to estimate resting energy expenditure (REE) and/or resting metabolic rate (RMR), typically for weight loss/maintenance and nutritional assessments. It is intended to be used with a face mask, mouthpiece and/or head hood into which the patient breathes, and includes a display, controls, and connection ports. It is used in healthcare or research settings.