Enter your age, gender, and height to calculate your predicted peak expiratory flow rate (PEFR). You can also input your measured peak flow reading to see what percentage of your predicted value you're achieving. Results are based on standard clinical formulas for children, adult males, and adult females.
Predicted PEFR
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% of Predicted (Your Reading)
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Peak Flow Zone
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Green Zone (≥80% Predicted)
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Yellow Zone (50–79% Predicted)
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Red Zone (<50% Predicted)
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A peak flow meter is a small, portable hand-held device that measures how fast you can blow air out of your lungs in a single forced breath. It is commonly used by people with asthma to monitor their lung function over time and to detect early signs of an asthma attack.
PEFR stands for Peak Expiratory Flow Rate — the maximum speed of air expelled from the lungs during a forced exhalation, measured in litres per minute (L/min). It is a key indicator of airway obstruction and is used to diagnose, monitor, and manage conditions like asthma and COPD.
Predicted PEFR is based on age, gender, and height using established clinical formulas. For adult men: PEFR = (((Height in m × 5.48) + 1.58) − (Age × 0.041)) × 60. For adult women: PEFR = (((Height in m × 3.72) + 2.24) − (Age × 0.03)) × 60. For children: PEFR = ((Height in cm − 100) × 5) + 100.
The three zones mirror traffic light colours. Green (≥80% of predicted) means your airways are open and well-controlled. Yellow (50–79%) signals caution — airways are narrowing and medication may be needed. Red (<50%) is a medical alert indicating serious airway obstruction requiring immediate action.
A reading at or above 80% of your personal predicted value is generally considered within the normal green zone. Readings between 50–79% suggest partial airway obstruction, while anything below 50% is a serious warning sign requiring prompt medical attention.
Stand upright, reset the marker to zero, take a deep breath, seal your lips tightly around the mouthpiece, and blow as hard and fast as possible in a single blast. Repeat three times and record the highest reading. Technique matters — always follow your healthcare provider's instructions.
Diurnal peak flow variability of 20% or more (measured twice daily over 2–4 weeks) can support an asthma diagnosis when combined with clinical assessment. A single peak flow reading alone is not sufficient to diagnose asthma, but it is a valuable monitoring tool.
Yes. Predicted PEFR increases as height increases and typically peaks in early adulthood (around age 30–35 for adults), then gradually declines with age. Children's predicted values are based on height alone. This is why the calculator requires all three inputs — age, gender, and height — for an accurate prediction.