Calculate your peak expiratory flow (PEF) percentage of predicted or personal best. Get asthma zone classification (Green/Yellow/Red) and action plan guidance.
The Peak Flow Percentage Calculator evaluates your peak expiratory flow (PEF) reading against either your personal best or your predicted PEF based on age, sex, and height. PEF measures the fastest rate at which you can blow air out of your lungs and is a key monitoring tool for asthma management. Regular peak flow monitoring helps detect worsening airway obstruction before symptoms become severe.
The results are classified into the three-zone Asthma Action Plan system developed by the National Asthma Education and Prevention Program (NAEPP): Green Zone (80–100% of best/predicted) means well-controlled asthma; Yellow Zone (50–80%) indicates caution with early signs of restriction; Red Zone (below 50%) is a medical alert requiring immediate action.
Using a peak flow meter at home allows patients to track lung function trends over time, identify triggers, assess medication effectiveness, and make early decisions about adjusting treatment before an emergency develops. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation.
Asthma affects over 25 million Americans. Symptoms can be unreliable indicators of lung function — many patients have significantly reduced airflow before feeling symptoms. Peak flow monitoring provides an objective, quantitative measure of airway function. Studies show that patients who use asthma action plans based on PEF monitoring have fewer ER visits, hospitalizations, and missed work/school days.
PEF % = (Current PEF / Reference PEF) × 100 Predicted PEF (Nunn & Gregg equations): Males: PEF = (((height_cm × 0.5536) + (age × -0.0579) - 0.1766) × 60) Females: PEF = (((height_cm × 0.3608) + (age × -0.0410) + 0.5118) × 60) Asthma Zones (NAEPP): • Green Zone: 80–100% — Well controlled • Yellow Zone: 50–80% — Caution • Red Zone: <50% — Medical alert
Result: 70% of personal best — Yellow Zone (Caution)
A current PEF of 350 L/min against a personal best of 500 L/min gives 70% (350/500 × 100 = 70%). This places the patient in the Yellow Zone, indicating early airway narrowing. The action plan recommends using a quick-relief inhaler (e.g., albuterol) and monitoring. If the value doesn't improve to Green Zone within 20–60 minutes, contact your healthcare provider.
PEF naturally varies throughout the day (diurnal variation). Healthy individuals show less than 10% variation between morning and evening readings. In poorly controlled asthma, diurnal variation can exceed 20%. High PEF variability is an independent risk factor for severe asthma exacerbations. Tracking both the absolute value and the variability provides a more complete picture of asthma control.
Peak flow monitoring is recommended for children aged 5 and older who can perform the technique correctly. Predicted values for children are based on height (since age and height are highly correlated in children). Establishing a personal best is especially important in children because they outgrow predicted values rapidly during growth spurts.
Peak flow measures only the maximum flow rate during forced exhalation, while spirometry provides detailed flow-volume curves including FEV1, FVC, and FEV1/FVC ratio. Spirometry is the gold standard for diagnosing and classifying asthma severity. Peak flow is the practical home monitoring tool. They complement each other: spirometry for periodic clinical assessment, peak flow for daily self-monitoring.
Normal PEF depends on age, sex, and height. For adult males, typical values range from 400–700 L/min; for adult females, 300–500 L/min. However, what matters most is YOUR personal best — the highest PEF you can achieve when your asthma is well controlled. Tracking percentage of personal best is more clinically useful than comparing to population averages.
For stable, well-controlled asthma: once daily in the morning or as your doctor recommends. During illness, after trigger exposure, or when symptoms change: measure 2–4 times daily. During an exacerbation: measure before and after quick-relief inhaler to assess response. New patients or those with poorly controlled asthma may benefit from twice-daily monitoring until stability is achieved.
The Yellow Zone (50–80%) means take action: Use your quick-relief inhaler (usually albuterol) as prescribed. Wait 20–60 minutes and recheck your PEF. If it returns to Green Zone, continue monitoring closely. If it stays in Yellow Zone, follow your asthma action plan (which may include increasing controller medication). Contact your doctor if you spend more than 24 hours in the Yellow Zone.
The Red Zone (below 50%) is a medical emergency: Take your quick-relief inhaler immediately (usually 4–6 puffs albuterol). If PEF doesn't improve to above 50% within 15 minutes, call 911 or go to the emergency room. Do NOT wait to see if it gets better on its own. Red Zone readings indicate severe airway obstruction that can be life-threatening.
Yes. Your personal best is the most accurate reference for your individual lung function. Predicted values are population averages that don't account for individual variation. A healthy person might have a personal best well above or below the predicted value. Always establish and use your personal best when available. Predicted values are useful when personal best hasn't been established.
Peak flow measures large airway obstruction (asthma, COPD) but is not a reliable tool for detecting infections that primarily affect the small airways and lung tissue (like COVID-19 or pneumonia). A normal PEF does not rule out respiratory infection. However, a declining PEF trend in someone with asthma during a respiratory illness can indicate worsening airway inflammation requiring treatment adjustment.