What is Peak Expiratory Flow?
Peak Expiratory Flow (PEF) is the maximum speed at which air can be forcefully exhaled from the lungs after a full inhalation. Measured in liters per minute (L/min), PEF is a key metric in monitoring respiratory health, particularly for patients with asthma and chronic obstructive pulmonary disease (COPD).
PEF is measured using a peak flow meter, a simple, inexpensive, and portable device. Regular monitoring helps patients and clinicians detect early signs of airway narrowing, assess treatment response, and determine the severity of asthma exacerbations before symptoms become severe.
Predicted PEF values depend on age, sex, and height. By comparing your actual reading to the predicted value, you can determine what percentage of expected airflow you are achieving and respond accordingly.
PEF Prediction Formulas
Several validated equations exist for predicting normal PEF. This calculator uses the Hankinson et al. (1999) reference equations, which are among the most widely accepted:
Where age is in years and height is in centimeters. The result is in L/min.
If an actual peak flow reading is provided, the percentage of predicted is calculated as:
Traffic Light Zones
The traffic light system was developed to help asthma patients quickly assess their respiratory status and take appropriate action:
| Zone | % of Predicted | Meaning | Action |
|---|---|---|---|
| Green Zone | 80–100% | Well controlled | Continue current treatment plan |
| Yellow Zone | 50–80% | Caution — airway narrowing | Use rescue inhaler; follow action plan |
| Red Zone | <50% | Medical emergency | Use rescue inhaler immediately; seek emergency care |
Peak Flow Zone Diagram
Reference Values by Age and Height
The following table shows approximate predicted PEF values for adults at selected ages and heights:
| Age | Male 160cm | Male 175cm | Male 190cm | Female 155cm | Female 165cm | Female 175cm |
|---|---|---|---|---|---|---|
| 20 | 554 | 618 | 689 | 410 | 447 | 487 |
| 30 | 528 | 592 | 663 | 393 | 430 | 470 |
| 40 | 495 | 559 | 630 | 372 | 409 | 449 |
| 50 | 455 | 519 | 590 | 347 | 384 | 424 |
| 60 | 409 | 473 | 544 | 318 | 355 | 395 |
| 70 | 356 | 420 | 491 | 285 | 322 | 362 |
Using a Peak Flow Meter
- Stand up straight or sit upright for the most accurate reading
- Reset the pointer to zero before each attempt
- Take a deep breath — fill your lungs completely
- Place the mouthpiece between your lips, creating a tight seal
- Blow as hard and fast as you can in one short burst (like blowing out a candle forcefully)
- Record the reading and repeat two more times
- Use the highest of the three readings as your PEF
- Measure at the same time each day for consistent monitoring
Asthma Action Plans
An asthma action plan is a written, personalized plan developed with your doctor that tells you what medications to take daily, how to handle worsening symptoms, and when to seek emergency care. Peak flow monitoring is a cornerstone of effective action plans.
Green Zone Actions (80-100%)
You are doing well. Continue your regular controller medications (if prescribed). No rescue inhaler needed unless before exercise. This is your target zone.
Yellow Zone Actions (50-80%)
Your asthma is getting worse. Take your quick-relief (rescue) inhaler as prescribed. You may need to add or increase controller medications. Contact your doctor if you stay in the yellow zone for more than 24 hours or if you need your rescue inhaler more than every 4 hours.
Red Zone Actions (<50%)
This is a medical emergency. Take your rescue inhaler immediately (4-6 puffs). Take oral corticosteroids if prescribed. Call your doctor or go to the emergency department immediately. If your peak flow does not improve after using your rescue inhaler, call emergency services (911).
Worked Example
A 35-year-old male, 175 cm tall, with an actual peak flow reading of 450 L/min:
This reading falls in the Yellow Zone, indicating caution. The patient should use their rescue inhaler and follow their asthma action plan.
Frequently Asked Questions
How often should I check my peak flow?
For well-controlled asthma, once daily (usually in the morning before taking medications) is sufficient. During flare-ups or when adjusting medications, check twice daily (morning and evening). Always check when you feel symptoms worsening.
What is "personal best" peak flow?
Your personal best PEF is the highest reading you achieve over a 2-3 week period when your asthma is well controlled. Many doctors prefer using personal best rather than predicted values for zone calculations, as it is more individualized.
Can children use peak flow meters?
Yes, children aged 5 and older can generally use peak flow meters effectively. Pediatric reference values differ from adult values. Consistent technique is important for reliable readings in children.
Why are my readings different each time?
PEF naturally varies throughout the day (typically lowest in the early morning). Variation greater than 20% between morning and evening readings, or day-to-day variation exceeding 10%, may indicate poorly controlled asthma and should be discussed with your doctor.
Does peak flow replace spirometry?
No. Spirometry (performed in a clinic) provides more detailed information including FEV1 and FVC. Peak flow is a simpler home monitoring tool. Both are valuable: spirometry for diagnosis and detailed assessment, peak flow for daily monitoring and early warning.