Peak Expiratory Flow Rate (PEFR) Calculator
Male
Female
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Predicted PEFR (L/min)
0%
Percentage of Predicted
What is Peak Expiratory Flow Rate?
Peak Expiratory Flow Rate (PEFR) is a measurement of how fast a person can exhale air. It is a critical metric for individuals with asthma or chronic obstructive pulmonary disease (COPD) to monitor their lung function and the effectiveness of their treatment plan. PEFR is measured using a small, handheld device called a peak flow meter.
How the Predicted Value is Calculated
Your "normal" or predicted peak flow is not a single number for everyone. It depends on your height, age, and biological sex. As height increases, lung volume generally increases, leading to a higher PEFR. Conversely, as we age, the elasticity of lung tissue and the strength of respiratory muscles can decrease, causing a gradual decline in PEFR values.
This calculator uses the ERS (European Respiratory Society) standard linear regression models to estimate your expected lung capacity:
- For Men: Predicted PEFR = (((Height in m × 5.48) + 1.58) – (Age × 0.041)) × 60
- For Women: Predicted PEFR = (((Height in m × 3.72) + 2.25) – (Age × 0.031)) × 60
Understanding the Traffic Light Zones
Clinicians often use a "traffic light" system to help patients manage their respiratory health based on their percentage of predicted PEFR:
| Zone | Percentage | Meaning |
|---|---|---|
| Green | 80% – 100% | Your breathing is stable. Continue normal maintenance medication. |
| Yellow | 50% – 80% | Caution. Your airways are narrowing. You may need a rescue inhaler or dosage adjustment. |
| Red | Below 50% | Medical Emergency. Severe airway narrowing. Seek immediate medical attention. |
Example Calculation
Let's look at a realistic example: A 40-year-old male who is 180 cm tall.
- Height: 1.80 meters.
- Calculation: ((1.80 × 5.48) + 1.58 – (40 × 0.041)) = 9.864 + 1.58 – 1.64 = 9.804 L/s.
- Convert to L/min: 9.804 × 60 = 588 L/min (Predicted).
- If he measures 450 L/min on his meter, his percentage is (450 / 588) × 100 = 76.5%.
- This falls in the Yellow Zone, suggesting he should consult his asthma action plan.