Peak Flow Calculator
Peak expiratory flow rate (PEFR) measures how fast you can blow air out of your lungs in one breath. It is one of the simplest and most widely used tests for monitoring lung function, particularly in asthma management. Your personal predicted PEFR depends on your age, height, and sex. By comparing your actual measured peak flow to your predicted value, you can determine which zone you are in: green (well-controlled), yellow (caution), or red (alert). This calculator uses the Nunn and Gregg (1989) reference equations, the standard used by the National Asthma Education and Prevention Program and endorsed by the National Heart, Lung, and Blood Institute. Enter your age, height, and sex to find your predicted PEFR, and optionally enter your measured value to see your percentage of predicted.
PEFR reference equations (Nunn and Gregg, 1989)
Male PEFR = ((Height cm - 80) x 6.22) - (Age x 0.029) + 288
Female PEFR = ((Height cm - 80) x 5.50) - (Age x 0.023) + 202
Green zone = Predicted x 0.80; Red zone = Predicted x 0.50
Peak flow: frequently asked questions
What is peak flow (PEFR)?
Peak expiratory flow rate (PEFR) is the maximum speed at which you can exhale air after a full breath in. It is measured in litres per minute (L/min) using a peak flow meter. PEFR is primarily used to monitor asthma and assess airway obstruction.
What is a normal peak flow rate?
Normal PEFR depends on age, height, and sex. For a 35-year-old male at 175 cm, the predicted PEFR is about 580-620 L/min. For a 35-year-old female at 165 cm, it is about 420-470 L/min. Individual predicted values are used to calculate personal best and interpret results.
How do I use my peak flow reading?
Compare your measured peak flow to your personal predicted best. Green zone: 80-100% of predicted (good control). Yellow zone: 50-79% (caution, may need medication adjustment). Red zone: below 50% (medical alert, seek care immediately). These zones apply specifically to asthma management.
Which formula does this calculator use?
This calculator uses the Nunn and Gregg (1989) equations, which are widely recommended for UK and US clinical practice. Males: PEFR = ((Height cm - 80) x 6.22) - (Age x 0.029) + 288. Females use different coefficients. These match the values printed on standard peak flow charts.
When should I see a doctor about my peak flow?
Seek medical advice if your peak flow consistently falls in the yellow or red zone, if you notice a pattern of morning dipping (morning readings significantly lower than evening), or if your personal best has declined over time without explanation. Regular monitoring is recommended for people with diagnosed asthma or COPD.
Official sources
- National Heart, Lung, and Blood Institute: Peak Flow Monitoring.
- National Asthma Education and Prevention Program: Expert Panel Report 3.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.