The peak expiratory flow rate (PEFR) is a person's maximum speed of expiration, as measured with a peak flow meter. Peak flow meter is a small device used to measure a person's ability to breathe out air. Use this online Peak Expiratory Flow Rate (PEFR) calculator to calculate normal flow rate values using the person's height, age and measured peak flow. In the below calculator enter your age, height, gender, measured peak flow and click calculate to know the Peak Expiratory Flow Rate (PEFR). Peak flow readings will be higher when patients are normal and lower when the airways are constricted.