Peak Flow Calculator (predicted PEF and asthma zones)

Understanding the peak flow calculator — concept and principle

This calculator gives the predicted peak expiratory flow (PEF) for an adult from sex, age and height, and places your own reading in the green, yellow or red zone of an asthma action plan. People with asthma use it to check how their lungs are doing day to day; using a personal best makes the zones more accurate.

Reading vs predicted: with the default inputs the result is 81% (green zone (80–100%)).

How it is calculated

Men: PEF = e^(0.544 × ln age − 0.0151 × age − 74.7 ÷ height + 5.48)

Women: PEF = e^(0.376 × ln age − 0.0120 × age − 58.8 ÷ height + 5.63)

Age in years (15–85), height in cm, PEF in L/min (Nunn and Gregg, 1989).
Reference = your personal best if entered, otherwise the predicted value.
Zones: green 80–100% of reference, yellow 50–79%, red below 50%.

The principle

Peak flow is the fastest speed of air you can blow out after a full breath. It depends on the size of the lungs and airways, which grow with height and are larger in men, and on age: it rises into early adulthood and then slowly falls. The equations capture these patterns in healthy people. In asthma, narrowed airways lower the peak flow, so comparing a reading with your usual best shows how much narrowing there is.

Worked example

Default inputs: male, 35 years, 175 cm, reading 520 L/min, no personal best.

Exponent = 0.544 × 3.555 − 0.0151 × 35 − 74.7 ÷ 175 + 5.48 = 6.459.
Predicted PEF = e^6.459 = 638 L/min.
Reading = 520 ÷ 638.3 = 81% → Green (80–100%).
Green from 0.8 × 638.3 = 511; yellow from 0.5 × 638.3 = 319; red below 319 L/min.

How to read the result

Where does your result fall?

Green means your asthma is under control. Yellow means it is getting worse; follow the steps in your action plan. Red is a medical alert: use your reliever and get help now. Predicted values vary a lot between individuals, so find your personal best over two to three weeks of good control and enter it.

Common mistakes

Not taking a full breath first or not blowing as hard and fast as possible.
Recording one blow instead of the best of three.
Using meters with different scales; EU-scale meters read lower in the mid range than older Wright meters.

Keep in mind: This is not a diagnosis. Severe breathlessness, difficulty speaking or a reading in the red zone needs emergency care.

Frequently asked questions

What is a normal peak flow?

It depends on sex, age and height; the calculator gives the predicted value, but your personal best is the better reference.

When should I measure peak flow?

Often in the morning and evening, before using a reliever inhaler, or as your action plan says.

Does peak flow diagnose asthma?

Variation over time can support a diagnosis, but spirometry and a clinical assessment are needed.

Sources: Nunn AJ, Gregg I, "New regression equations for predicting peak expiratory flow in adults", BMJ 1989;298(6680):1068–1070 · NHLBI National Asthma Education and Prevention Program, Expert Panel Report 3, Guidelines for the Diagnosis and Management of Asthma (2007)

Checked 9 Nov 2026. Nunn AJ, Gregg I, “New regression equations for predicting peak expiratory flow in adults”, BMJ 1989;298(6680):1068–1070 (equations as reproduced in the MSD Manual “Peak Expiratory Flow Prediction” calculator); zones: NHLBI National Asthma Education and Prevention Program, Expert Panel Report 3, “Guidelines for the Diagnosis and Management of Asthma” (2007).

Your numbers