What peak flow measures
Peak expiratory flow (PEF) is the fastest speed at which you can blow air out of your lungs after a full breath, measured in liters per minute with a small hand-held meter. It falls when the airways narrow, so people with asthma use it to track their control and spot attacks early.
Predicted values
Predicted peak flow depends on sex, age and height. This calculator uses the equations of Nunn and Gregg (1989), from adults aged 15 to 85:
- Men: ln(PEF) = 0.544 × ln(age) − 0.0151 × age − 74.7 ÷ height (cm) + 5.48
- Women: ln(PEF) = 0.376 × ln(age) − 0.0120 × age − 58.8 ÷ height (cm) + 5.63
Peak flow rises in early adulthood, peaks around age 30 to 40 and then slowly declines.
Worked example
A 40-year-old man, 5 ft 10 in (177.8 cm) tall, blows 520 L/min and does not know his personal best yet.
- Predicted value: about 641 L/min.
- His reading is 520 ÷ 641 = 81% of predicted: green zone.
- Green zone starts at 513 L/min and red zone is below 320 L/min.
The asthma zones
| Zone | Share of personal best | What to do |
|---|---|---|
| Green | 80 to 100% | asthma is controlled, keep your usual treatment |
| Yellow | 50 to 79% | follow the yellow part of your asthma action plan |
| Red | below 50% | take your rescue medicine and get medical help now |
Zones should be based on your personal best, not the predicted value, as soon as you know it. Your best can be higher or lower than the tables.
How to measure
- Stand up and move the marker to zero.
- Breathe in as deeply as you can.
- Seal your lips around the mouthpiece and blow out as hard and fast as possible, in one short blast.
- Repeat 3 times and record the highest number, not the average.
Measure at the same times each day, often morning and evening. Meters vary between brands, and older Wright-scale meters read differently from modern EU-scale ones, so always use the same meter to compare readings.
When to call for help
Call 911 if your peak flow is in the red zone and does not improve quickly with your rescue inhaler, or if you have trouble walking or talking, blue lips or fingernails, or your breathing gets worse despite treatment.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.