Peak expiratory flow (PEF) is one of the simplest ways to track how well air moves through the lungs, and it's central to most asthma action plans. This calculator predicts a population-average PEF from age, height, and sex using the Nunn & Gregg (1989) regression, then lets you compare a measured reading to a baseline using the standard green/yellow/red zone system. This guide explains where the numbers come from, how to read your zone, and where the tool's limits are.
Where the predicted-PEF formula comes from
The predicted values use Nunn AJ, Gregg I, "New regression equations for predicting peak expiratory flow in adults," BMJ 1989;298(6680):1068-70. The study fit separate log-linear regressions for men and women, on the EU/EN13826 peak-flow-meter scale, from healthy lifelong non-smokers aged 15-85. Both equations combine a slowing-growth term in age (via a logarithm), a linear decline with age, and a height term, reflecting the way PEF tends to rise through early adulthood, peak around age 30-35, then gradually decline.
Several calculators circulating online cite a simplified linear formula for this study that does not reproduce the paper's own reference chart — this calculator implements the coefficients read directly from the original results section, and its output has been checked against the paper's own published reference curves.
How the zone check works
Once you enter a measured PEF reading, the calculator computes what percentage of a baseline it represents. The baseline is your personal best if you've entered one, or the predicted PEF otherwise. That percentage is classified into the standard three-zone system used by most asthma action plans: green (80% or more) for good control, yellow (50-79%) as a caution signal, and red (under 50%) as a medical alert.
Personal best is preferred over predicted PEF as a baseline whenever it's available, because it reflects an individual's own lung capacity rather than a population average — two healthy people of the same age, height, and sex can still have meaningfully different PEF.
Limits and when to seek expert review
The Nunn & Gregg equations were derived and validated for adults aged 15-85; predictions outside that range are an extrapolation, not a validated result. The equations also describe averages for a study population of non-smoking Caucasian adults — an individual's normal PEF can reasonably fall up to 100 L/min below predicted for men, or 85 L/min below predicted for women, and still be within normal limits.
This calculator is a clinical decision-support and education tool. It does not diagnose asthma or any other condition, does not replace a peak flow meter reading or spirometry performed and interpreted by a clinician, and should not be the sole basis for starting, stopping, or adjusting medication. Follow your own written asthma action plan and consult a qualified clinician for any medical decision.