The main difference is that spirometry measures how much air you can exhale, while body plethysmography measures the total amount of air in your lungs. Spirometry only tracks moving air. In contrast, the plethysmograph uses Boyle’s Law to calculate your Total Lung Capacity and Functional Residual Capacity. This provides a more complete picture of your respiratory health than a standard flow-volume loop alone by accounting for air that stays in the chest.