Updated November 22, 2014.

Definition: FEV1 is the maximal amount of air you can forcefully exhale in one second. It is then converted to a percentage of normal. For example, your FEV1 may be 80% of predicted based on your height, weight, and race. FEV1 is a marker for the degree of obstruction with your asthma:

• FEV1 greater 80% of predicted= normal
• FEV1 60% to 79% of predicted = Mild obstruction
• FEV1 40% to 59% of predicted = Moderate obstruction

• FEV1 less than 40% of predicted = Severe obstruction

FEV1 is one of the most common indices used to assess airway obstruction. It is automatically calculated during spirometry or pulmonary function testing. It is calculated using a spirometer.
_Sources:

National Heart, Lung, and Blood Institute. Accessed: January 1, 2009. Expert Panel Report 3 (EPR3): Guidelines for the Diagnosis and Management of Asthma

Clinical Pulmonary Function Testing, Exercise Testing, and Disability Evaluation. In Chest Medicine: Essentials Of Pulmonary And Critical Care Medicine. Editors: Ronald B. George, Richard W. Light, Richard A. Matthay, Michael A. Matthay. May 2005, 5th edition.