A spirometer is a device for measuring oxygen consumption and, more broadly, lung function in a living organism. The subject breathes through a mouthpiece while the instrument records the volumes of air moved, the flows achieved, and the gases exchanged. In human medicine, spirometry diagnoses asthma and chronic obstructive pulmonary disease by measuring how much air can be forced out and how fast. In nutrition and exercise science, more elaborate spirometry underpins indirect calorimetry: measuring oxygen consumed and carbon dioxide produced reveals exactly how much energy the body is burning and which fuels it uses. From the clinic to the sports lab, it turns breathing into numbers.