Peak flow meters record peak expiratory flow rate (PEFR), the fastest rate at which air can move through the airways during a forced expiration starting with fully inflated lungs. The PEFR correlates well with Forced Expiratory Volume in 1 second (FEV1); the greatest volume of air that can be breathed out in the first second of a large breath.

PEF and FEV1 are used to diagnose bronchial asthma, chronic obstructive pulmonary disease (COPD),   pulmonary tuberculosis, pneumonia, bronchiectasis, pneumoconiosis. Peak flow may also be useful for daily self-monitoring and for checking the effects of new medications.

Peak flow meters are small, portable, convenient and inexpensive devices which can be used with just little bit training.

Peak flow values depend on a number of factors including your age, sex, height and ethnic origin. Your peak flow increases until you are about 40 years old then decreases after that time.

The "personal best" PEFR  is taken by recording PEFR 3-4 times a day for several asymptomatic days (when symptoms of asthma are minimal). The highest reading in this record is termed as the "personal best PEFR" and considered normal for that particular person.