Calculate peak expiratory flow (PEF) as a percentage of predicted or personal-best values and place the result into the usual Green/Yellow/Red monitoring zones.
The Peak Flow Percentage Calculator compares your current peak expiratory flow (PEF) with either your personal best or a predicted reference value based on age, sex, and height. PEF is commonly used as a home-monitoring number for asthma and other obstructive airway conditions because changes in the value can show airflow narrowing before the trend is obvious from symptoms alone.
The result is grouped into the familiar Green / Yellow / Red zone structure used in many asthma action plans. On this page those zones are presented as interpretation bands, not as a stand-alone treatment protocol.
Use the calculator to track percentage changes over time, compare a current reading with a known baseline, and document the number clearly. The result should still be interpreted against the asthma action plan or monitoring plan already given by the treating clinician.
Peak-flow numbers are most useful when they are compared with a known baseline rather than read in isolation. This page keeps the percentage calculation and zone label together so that a current value can be placed into a familiar monitoring framework without turning the calculator into an asthma treatment order set.
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) Monitoring zones used on this page: • Green Zone: 80–100% • Yellow Zone: 50–80% • Red Zone: <50% These zone labels are for interpretation and should be paired with the user's existing action plan rather than treated as direct treatment instructions.
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 reading in the Yellow Zone used on this page, meaning the value is below baseline and should be reviewed with the user's usual asthma monitoring plan.
PEF changes during the day, so one reading is less useful than a trend. Comparing morning and evening values or repeated daily readings usually gives a clearer picture than reacting to a single isolated number.
Peak-flow monitoring is most useful when the child can perform the maneuver consistently. In growing children, a personal-best reference is often more helpful than an older predicted value because height changes quickly over time.
Peak flow is a simple airflow-monitoring tool. Spirometry provides a broader pulmonary-function assessment and is generally used for fuller diagnostic evaluation, while peak flow is more practical for repeated home tracking.
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This page calculates peak expiratory flow as a percentage of either a user-entered personal best or a predicted adult reference value. When predicted mode is used, the calculator applies the Nunn and Gregg adult peak-flow equations that are already implemented in the page logic. The green/yellow/red bands are the familiar action-plan zones used for monitoring context; they do not replace the user's own asthma action plan or clinician guidance.
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 means the reading is below the usual baseline and deserves review using the person's existing asthma action plan. This page labels the zone, but the treatment step still depends on the clinician-provided plan and the symptom picture.
The Red Zone means the reading is far below the baseline used for comparison. That level should be treated seriously and interpreted with symptoms and the person's emergency asthma plan rather than with the calculator alone.
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 airflow and is not a stand-alone infection test. A falling trend can be one clue that breathing is worsening, but it still has to be interpreted with the rest of the clinical picture.