Predicted Forced Expiratory Volume in 1 second (FEV1)

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 in your lungs:

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

You can calculate your predicted FEV₁ using this website or you can ask your doctor. Remember that you have to enter the value in units of Litres in 1 second. The number should be in the range of: 1 – 6.

If in doubt about your peak flow readings, contact your medical team for advice.