Peak Flow Monitoring Calculator
Personal best percentage and zone classification.
About this calculator
Peak expiratory flow (PEF) monitoring gives patients with asthma a simple, at-home way to track how open their airways are on a given day, using a handheld peak flow meter that measures the fastest speed of air a person can forcefully exhale. This calculator compares a current reading against two reference points: the patient's own personal best (the highest PEF recorded over roughly two weeks of good symptom control) and a predicted value derived from age, height, and sex using the reference equations Nunn and Gregg published in 1989. Personal best is generally considered the more clinically useful comparison for day-to-day monitoring, because it reflects that specific patient's own healthy airway function rather than a population average that may not match their build or lung history, and it is the basis for the standard traffic-light zone system used in asthma action plans. The green zone, 80-100% of personal best, indicates well-controlled asthma where the current treatment plan should continue.
The yellow zone, 50-79%, signals caution and typically triggers a step-up in quick-relief or controller medication per the patient's action plan. The red zone, below 50% of personal best, is a medical alert threshold generally requiring immediate rescue medication and often urgent medical evaluation, since it indicates a substantial and potentially dangerous drop in airflow. Because peak flow can vary meaningfully across the day even in stable asthma, guidelines typically recommend measuring at consistent times (often morning and evening) to build a reliable personal-best baseline and to catch a real downward trend rather than normal daily fluctuation.
Medical Disclaimer
This calculator is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about your health. Never disregard professional medical advice or delay seeking it because of results from this tool.
Inputs
Results
% of Personal Best
80%
Figures current as of 1989. Source: Nunn AJ, Gregg I. New regression equations for predicting peak expiratory flow in adults. BMJ. 1989;298(6680):1068-1070.
How to Use This Calculator
- Enter your current PEF (L/min) and personal best PEF (L/min).
- Enter age (years), height (cm), and sex to calculate the predicted normal PEF for comparison.
- Review your percent of personal best and percent of predicted PEF.
- Check your asthma zone (green >=80%, yellow 50-79%, red <50%) and recommended action.
- Record daily readings at the same time each day for meaningful trend monitoring.
How the result changes with Personal Best PEF (L/min)
| Personal Best PEF (L/min) | % of Personal Best |
|---|---|
| 250 | 160% |
| 375 | 106.7% |
| 750 | 53.3% |
| 900 | 44.4% |
What each input means
- Current PEF (L/min)
- Today's measured peak expiratory flow in liters per minute.
- Personal Best PEF (L/min)
- Highest PEF achieved when well-controlled (over 2 weeks of good control).
- Age (years)
- Patient age in years. The predicted-PEF equations (Nunn & Gregg 1989) were validated for ages 15-85; outside that range, treat the Predicted PEF and % of Predicted outputs as an extrapolation and rely on Personal Best comparison and zone classification instead, which do not depend on age.
- Height
- Patient height in centimeters.
- Sex
- Biological sex for predicted PEF calculation.
What each result means
- % of Personal Best
- Current PEF as percentage of personal best. ≥80% = Green, 50-79% = Yellow, <50% = Red.
- Zone (0=Green, 1=Yellow, 2=Red)
- Traffic light zone: 0 = Green (well controlled), 1 = Yellow (caution), 2 = Red (emergency).
- % of Predicted
- Current PEF as percentage of age/sex/height-predicted value.
- Predicted PEF (L/min)
- Expected PEF for a healthy person of same age, sex, and height (Nunn & Gregg 1989, validated ages 15-85 -- treat as extrapolated outside that range).
- Green Zone Threshold (L/min)
- PEF values above this (80% of personal best) indicate good control.
- Yellow Zone Threshold (L/min)
- PEF values above this (50% of personal best) but below green indicate caution.
- Drop from Personal Best (%)
- How far today's reading sits below personal best, as a percentage (100% minus % of Personal Best). This is a single-reading drop, not the day-to-day/morning-to-evening diurnal variability guidelines reference when citing a >20% variability threshold -- that measure requires comparing multiple readings over time, which this calculator does not track.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersCurrent PEF (L/min) = 400, Personal Best PEF (L/min) = 500, Age (years) = 35, Height (cm) = 170 = 5 input(s) provided
- Calculate % of Personal Best% of Personal Best = (currentPEF / personalBest) * 10080 = 80
- Calculate Zone0 = 0
- Calculate % of Predicted% of Predicted = (currentPEF / predictedPEF) * 10063.5 = 63.5
Figures and sources
- Nunn & Gregg reference equations for predicted peak expiratory flow (1989) — Nunn AJ, Gregg I. New regression equations for predicting peak expiratory flow in adults. BMJ. 1989;298(6680):1068-1070.
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why is personal best used for zone classification instead of the predicted PEF?
Personal best reflects a specific patient's own healthiest achievable airflow, measured during a period of good symptom control, so it accounts for individual airway anatomy and lung history that a population-based predicted value cannot capture. Using predicted PEF instead could misclassify a patient whose healthy baseline naturally sits above or below the population average for their age, height, and sex, which is why established asthma action plans anchor the green/yellow/red zone system to personal best rather than the predicted figure.
What should someone actually do when their PEF falls into the yellow zone?
A yellow-zone reading (50-79% of personal best) signals caution and typically means stepping up quick-relief medication use and, depending on the individual's written asthma action plan, possibly adjusting controller medication as well. It is not itself a medical emergency, but it indicates airway function has meaningfully dropped from baseline and warrants closer symptom monitoring and, if it does not improve with the planned step-up, contacting a healthcare provider.
Why does peak flow need to be measured at consistent times of day?
Airway caliber in asthma naturally fluctuates over the course of a day, often dipping in the early morning hours even in reasonably controlled disease, so a single reading taken at a different time each day can look like a trend when it is really just normal diurnal variation. Measuring at consistent times -- commonly morning and evening -- lets a patient build a reliable personal-best baseline and more confidently identify a genuine downward trend that needs action.
How much does PEF variability by itself indicate about asthma control?
This calculator's "Drop from Personal Best" figure describes a single reading against your personal best, not the guideline concept of diurnal PEF variability, which is computed by comparing multiple readings over time (typically morning versus evening, or day to day) and is a recognized marker of poorly controlled asthma when persistently above roughly 20% -- wide swings between readings suggest the airways are more reactive than they should be on optimal treatment. A single low reading here is meaningful on its own via the zone system, but tracking true variability requires logging multiple readings and comparing them over time, which this calculator does not do for you automatically.
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