About the Peak Flow Calculator — Estimated Peak Expiratory Flow
Peak expiratory flow (PEF) is the fastest speed of air a person can blow out of the lungs after a full breath in, measured in liters per minute with a handheld peak flow meter. This calculator estimates a predicted PEF from age, height, and sex using the Nunn and Gregg (1989) regression equations, then compares an entered meter reading against that predicted value using the standard green, yellow, and red action-plan zones.
How the formula works
The Nunn and Gregg equations are log-linear regressions fitted to peak flow measurements from healthy adults aged 15 to 85. For men, predicted PEF (L/min) equals exp(0.544 × ln(age) − 0.0151 × age − 74.7 ÷ height + 5.48), with height in centimeters. For women, the equation uses exp(0.376 × ln(age) − 0.0120 × age − 58.8 ÷ height + 5.63). Because it is a natural-log model, predicted PEF rises with height, follows an inverted-U shape with age (peaking in early adulthood before declining), and differs by sex due to average differences in airway size and lung volume.
Reading your zone
The calculator divides your entered reading by the predicted value to get a percent of predicted, then reports the green-zone floor (80% of predicted) and yellow-zone floor (50% of predicted). A reading at or above the green floor generally reflects good control; a reading in the yellow band suggests airway narrowing and a prompt to follow an asthma action plan; a reading below the red threshold (under 50%) calls for prompt medical attention. Many clinicians prefer comparing readings to a personal best - your own highest reading recorded over two to three weeks while well - rather than the population-predicted value, since personal best captures individual lung capacity that a formula based only on age, height, and sex cannot.
When to consult a professional
This calculator performs the standard predictive arithmetic; it does not diagnose asthma or COPD, adjust medication, or replace spirometry. If a reading falls in the yellow or red zone, or if breathing symptoms accompany a low reading, follow your written asthma action plan and consult a licensed healthcare provider.