Peak Flow Calculator

How to Use This Calculator

1. Enter age, height (cm), and sex to get predicted PEFR.
2. Enter your measured PEFR (best of 3 attempts) to calculate % predicted and action zone.
3. Use Pediatric tab for ages 5–18 using Polgar reference equation.
4. Use Asthma Action Zones tab to identify Green/Yellow/Red boundaries for your personal best.
5. Professional tier adds ethnic adjustments and diurnal variation calculation.

Formula

Example

Frequently Asked Questions

• What is peak flow and why measure it? ▼
  Peak Expiratory Flow Rate (PEFR) is the maximum speed at which air can be expelled from the lungs during a forceful exhalation following a full inhalation. Measured in litres per minute (L/min) using an inexpensive hand-held peak flow meter, PEFR reflects the degree of airflow obstruction in the large airways. It is a cornerstone of asthma monitoring because it provides an objective, reproducible, and near-instantaneous measure of airway calibre that correlates with spirometric measurements of FEV1. Regular home monitoring allows patients to detect early deterioration in asthma control — often before symptoms become severe — enabling timely intervention. In occupational settings, serial PEFR measurements over working and non-working days are the standard method for diagnosing occupational asthma. PEFR is also useful for monitoring response to bronchodilator therapy and for titrating step-up or step-down treatment according to asthma action plans. Because each person has their own "personal best" as a reference point, PEFR monitoring is inherently personalised. The technique requires a good seal around the mouthpiece and three maximal effort attempts, recording the highest value.
• How do I use peak flow zones for asthma? ▼
  Asthma action plans divide PEFR readings into three colour-coded zones based on either predicted values or personal best. The Green Zone (≥80% of predicted or personal best) indicates good asthma control — continue current medications. The Yellow Zone (50–79%) signals caution: airway obstruction is significant. The patient should use a short-acting bronchodilator (SABA, such as salbutamol), reassess after 20–30 minutes, and contact their clinician if improvement is insufficient. Yellow zone readings that persist or recur frequently indicate the need for step-up therapy. The Red Zone (<50%) is a medical emergency indicating severe obstruction. The patient should use a SABA immediately and, if the PEFR does not recover to the Yellow or Green Zone promptly, call emergency services or present to the emergency department. Prednisolone (oral corticosteroid) is often included in the Red Zone action plan. GINA guidelines recommend that asthma action plans be individualised, written, and regularly reviewed. For children, the Childhood Asthma Control Test (C-ACT) supplements peak flow in assessing control. Always establish personal best during a period of maximum control, not during an exacerbation.
• Why are pediatric and adult PEFR predictions different? ▼
  Adult and paediatric PEFR reference equations are derived from different population samples and use different predictor variables because the determinants of lung function change with development. The Nunn & Gregg (1989) equations for adults are based on height, age, and sex in a large British adult population and reflect the decline in PEFR with ageing — particularly after age 35–40 — alongside the influence of lung size (height) and sex differences in airway calibre and lung volumes. Adult PEFR peaks in the mid-twenties and declines progressively thereafter. In children, lung function grows in parallel with somatic growth: height is the dominant predictor, while age adds little independent explanatory power once height is accounted for. The Polgar equations use height-based logarithmic regression derived from paediatric populations. Sex differences emerge at puberty — boys develop larger airways and greater lung volumes than girls of equivalent height — so paediatric equations sometimes apply sex-specific predictions for adolescents. Because children are constantly growing, a single reference value cannot serve as a "personal best" for extended periods without reassessment. For this reason, GINA recommends reassessing predicted PEFR in children every 6–12 months.
• What is a normal peak flow for my age and height? ▼
  Normal PEFR varies considerably with age, height, and sex. As a guide using Nunn & Gregg adult predictions: a 25-year-old male, 175 cm tall has a predicted PEFR of approximately 620–650 L/min; a 25-year-old female, 165 cm tall approximately 490–510 L/min. By age 50, predicted values decline to around 550–580 L/min for men of the same height and 430–450 L/min for women. Taller individuals have larger lungs and proportionally higher predicted values. Ethnic adjustments apply: African and Caribbean populations have predicted values approximately 13% lower than Caucasian predictions, and South Asian populations approximately 10% lower, due to structural differences in thoracic dimensions. These corrections are now incorporated into modern spirometry reference equations (GLI-2012) and should be applied when interpreting absolute PEFR values. Your "personal best" — the highest PEFR you achieve when asthma is optimally controlled — is actually a more clinically useful reference than population-predicted values for ongoing monitoring, because it accounts for your individual airway characteristics. Mini-Wright and standard ATS-compliant meters are in agreement at 30–800 L/min; EU scale meters read slightly differently from US scale — ensure consistent device use.
• How is peak flow used in occupational asthma diagnosis? ▼
  Serial peak flow monitoring is the recommended first-line investigation for suspected occupational asthma according to the British Occupational Health Research Foundation (BOHRF) and ACCP guidelines. The standard protocol involves the patient measuring PEFR at least 4 times per day (on waking, at midday, at end of shift, and at bedtime) for a minimum of 4 consecutive working weeks, recording readings on both working and non-working days. The key diagnostic pattern in occupational asthma is a systematic fall in PEFR during working days with recovery on rest days or holidays. Specific patterns include: a consistent fall of ≥20% from personal best on workdays, progressive deterioration across the working week with weekend recovery, or a late-asthmatic response (PEFR fall 4–8 hours after exposure). OASYS (Occupational Asthma Expert System) software is commonly used to analyse serial PEFR records and calculate a diagnostic score based on work-relatedness. PEFR monitoring has sensitivity of approximately 70–80% and specificity of 85–90% for occupational asthma when performed correctly. Specific inhalation challenge testing is the gold standard where PEFR is equivocal. Diurnal variation >20% (amplitude/mean × 100%) supports asthma diagnosis in any setting.

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