Peak Flow Calculator — Predicted PEFR & Asthma Zones
Enter your age, height and sex to get your predicted Peak Expiratory Flow Rate (PEFR) from the Nunn & Gregg (1989) reference equations. Optionally enter your measured reading to see which asthma action-plan zone (Green/Yellow/Red) you fall into.
Sex
years
cm
L/min
Nunn & Gregg (1989) reference value for this age, height and sex
- 1
Age logarithm term
0.544 × ln(35) = 1.9341 - 2
ln(PEFR)
1.9341 − 0.0151 × 35 − 74.7 ÷ 175 + 5.48 = 6.4588 - 3
Predicted PEFR
e^(6.4588) = 638Exponentiate to recover the flow rate in L/min (Nunn & Gregg 1989).
How does this calculator work?
Predicted PEFR (L/min) = exp(0.544 × ln(age) − 0.0151 × age − 74.7/height + 5.48) for men, and exp(0.376 × ln(age) − 0.012 × age − 58.8/height + 5.63) for women (Nunn & Gregg 1989, height in cm). Enter your measured blow to see if you are in the Green (≥ 80%), Yellow (50–79%) or Red (< 50%) asthma zone.
Formula
How this is calculated
Peak Expiratory Flow Rate (PEFR) is the fastest rate at which air can be blown out of the lungs in a short, sharp blow. It is measured with a peak flow meter and reported in litres per minute (L/min). PEFR depends mainly on age, height and sex, and is widely used to monitor asthma — large or rapid drops from a person's normal value suggest airway narrowing.
This calculator uses the Nunn & Gregg (1989) equations, which were derived from spirometry data on healthy non-smoking UK adults and are the basis of the reference values in many peak flow meters and clinical guidelines. The equations predict the natural log of PEFR as a function of age and height; exponentiating the result gives the predicted L/min value. The model applies to adults aged 15–100 and heights 100–250 cm and is less accurate at the extremes.
The three asthma traffic-light zones are based on percentage of predicted (or personal best) PEFR: Green (≥ 80%) means the airways are open and treatment is on track; Yellow (50–79%) is a caution zone indicating worsening; Red (< 50%) is a medical alert. Your doctor may define your personal best differently — always follow the action plan provided by your healthcare team rather than the population-predicted value alone.
Frequently asked questions
Normal predicted PEFR ranges widely by age, height and sex — typically 400–700 L/min for adult men and 300–550 L/min for adult women. The predicted value from this calculator gives your personal reference; what matters clinically is whether your measured reading is close to your own personal best.
The zones compare your measured PEFR to your personal best (or predicted): Green (≥ 80%) = airways open, continue maintenance treatment; Yellow (50–79%) = caution, use reliever and contact your doctor if it persists; Red (< 50%) = medical alert, use reliever immediately and seek medical attention.
PEFR is typically lowest in the early morning and highest in mid-afternoon. Take the best of three blows, standing upright. For trend monitoring, measure at the same time each day and record the result. Diurnal variation greater than 20% is a marker of poorly controlled asthma.
Also known as
TG we-Calculate Editorial Team. (2026). Peak Flow Calculator — Predicted PEFR & Asthma Zones [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/peak-flow-calculator
TG we-Calculate Editorial Team. "Peak Flow Calculator — Predicted PEFR & Asthma Zones." TG we-Calculate. 2026. https://we-calculate.com/calculator/peak-flow-calculator.
TG we-Calculate Editorial Team, "Peak Flow Calculator — Predicted PEFR & Asthma Zones," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/peak-flow-calculator
@misc{wecalculate_peak_flow_calculator, title = {Peak Flow Calculator — Predicted PEFR & Asthma Zones}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/peak-flow-calculator}}, year = {2026}, note = {TG we-Calculate} }
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