Forced Vital Capacity (FVC) is a measurement of lung size, which represents the volume of air in the lungs that can be exhaled following a deep inhalation. It is one of the key spirometry measurements used in pulmonary function tests (PFTs) to evaluate lung function. During the FVC test, the patient is asked to take a deep breath and then exhale as forcefully and quickly as possible. The amount of air exhaled during the first, second, and third seconds of the FVC test is called Forced Expiratory Volume (FEV) . The FEV1/FVC ratio is also measured and is used to determine whether there is airflow obstruction. A normal FEV1/FVC ratio is greater than 0.70, and both FEV1 and FVC should be above 80% of the predicted value. The FVC test is useful in diagnosing and monitoring lung diseases such as asthma, chronic obstructive pulmonary disease (COPD), and restrictive lung disease.