π¬οΈ Peak Flow Calculator
Your predicted PEF and asthma traffic-light zone.
For informational purposes only, not medical advice. Consult a healthcare professional.
Peak flow measures how fast you can blow air out of your lungs, and it is the home monitoring tool for asthma. The predicted value comes from reference equations fitted to healthy non-smokers: Nunn and Gregg (1989) for adults 15 and up, and the Godfrey height-based formula for children 5 to 14. Comparing your best blow to the predicted value gives percent of predicted, which asthma action plans translate into green, yellow, and red zones. This calculator runs the reference math and the zone logic together.
How to Use This Calculator
Enter age, sex, and height to get the predicted value, then enter your best of three peak flow blows. The calculator gives percent of predicted and the green, yellow, or red zone.
Measure standing up, take a deep breath, and blow as hard and fast as you can. Repeat three times and keep the highest number.
Enter age, sex, and height. The reference equations differ by sex and by child versus adult, so all three matter.
Read percent of predicted and the zone. Green is 80 to 100 percent, yellow is 50 to 79, and red is under 50.
How the Math Works
For adults, the Nunn and Gregg equations predict the natural log of PEF in liters per minute. For men it is 0.544 times ln(age) minus 0.0151 times age minus 74.7 divided by height in cm, plus 5.48; for women it is 0.376 times ln(age) minus 0.0120 times age minus 58.8 divided by height, plus 5.63. Exponentiating gives the predicted L/min. For children 5 to 14, the Godfrey formula is simpler: height in cm minus 100, times 5, plus 100. Percent of predicted is measured divided by predicted times 100. The zone cutoffs follow the BTS and GINA convention: 80 to 100 percent green, 50 to 79 yellow, below 50 red.
Worked Example
A 30-year-old man, 178 cm tall, blows a best of 450 L/min. The Nunn and Gregg equation predicts about 638 L/min for him, so his percent of predicted is 70.5, landing in the Yellow zone the calculator reports. The gap between 450 and the predicted value is the actionable information: his written asthma plan would say what extra reliever steps the yellow zone triggers.
Reading Your Result
Green zone readings mean airways are open and the usual plan continues. Yellow zone means narrowing is underway: follow the written asthma action plan, which typically adds reliever doses and may add controller steps, and contact your clinician if readings do not recover. Red zone under 50 percent is an emergency: take rescue medication and seek urgent care. One more nuance experienced patients know: once you establish your personal best during a good period, many plans use percent of personal best instead of percent of predicted, since it fits the individual better.
| Percent of predicted | Zone | Action |
|---|---|---|
| 80 to 100 | Green | Continue usual plan |
| 50 to 79 | Yellow | Follow action plan, add reliever |
| Under 50 | Red | Rescue medication, urgent care |
Limitations
Reference equations describe populations, not individuals: healthy people vary widely around predicted values, and the equations were derived in specific study populations. Technique dominates accuracy; a poor seal or half-hearted blow understates the true value. This tool supports asthma self-management education and does not replace a written action plan from your clinician or spirometry in a clinic.
Related Calculators
Related tools: oxygenation index calculator PaO2/FiO2 ratio calculator target heart rate zones.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is a good peak flow number?
It depends on your age, sex, and height, which is why predicted values exist. A 30-year-old man of 178 cm predicts around 640 L/min while a shorter older woman predicts far less. The meaningful number is percent of predicted or percent of your personal best, not the raw liters per minute.
How do I use a peak flow meter correctly?
Stand up, set the marker to zero, breathe in as deeply as you can, seal your lips around the mouthpiece, and blast the air out as hard and fast as possible in one go. Do it three times and record the highest. The common errors are a poor lip seal, not inhaling fully first, and coughing into the device.
What do the green, yellow, and red zones mean?
Green, 80 to 100 percent of predicted or personal best, means asthma is controlled and you continue the usual plan. Yellow, 50 to 79 percent, signals airway narrowing and triggers the extra steps in your written action plan. Red, under 50 percent, is a medical emergency needing rescue medication and urgent care.
Should I use predicted or personal best?
Clinicians often prefer personal best once it is established during a stable period, because it reflects your individual lungs rather than a population equation. Predicted values are the starting point, especially at diagnosis. Many action plans are written against personal best.
Why is my peak flow lower in the morning?
Morning dipping is classic in asthma: airways narrow overnight and the morning reading runs lower than the evening one. A large day-to-night swing, often over 20 percent variability, actually supports the asthma diagnosis. Record readings at consistent times to make the diary interpretable.
Can a cold affect my peak flow?
Yes, viral infections inflame airways and can drop peak flow for a week or more even in well-controlled asthma. That is expected and usually recovers. If the drop is deep, prolonged, or comes with fever and colored sputum, check in with your clinician rather than waiting it out.
Do I need a peak flow meter if I feel fine?
For persistent asthma, guidelines favor regular monitoring because peak flow often falls before symptoms appear. It catches the quiet deterioration that leads to flare-ups. For mild intermittent asthma, symptom-based monitoring may be enough; your clinician decides.
How often should I check peak flow?
Common advice is twice daily, morning and evening before reliever use, during periods of poor control or after plan changes, plus any time symptoms appear. Once stable for months, many people step down to symptom-triggered checks. Consistency of timing matters more than frequency.
Can children use peak flow meters?
Yes, from about age 5 with a suitable meter and coaching. The Godfrey height-based equation gives their predicted values. Technique is harder for small children, so the readings need a clinician to interpret alongside symptoms rather than standing alone.
Does peak flow replace spirometry?
No. Spirometry in a clinic measures far more, including the FEV1/FVC ratio that defines obstruction, with calibrated equipment. Peak flow is the home monitoring tool: simpler, cheaper, and repeatable daily. They complement each other; one does not substitute for the other.