About the Peak Flow Calculator
Peak expiratory flow (PEF) is the fastest speed you can blow air out after a full breath in, measured in litres per minute with a simple peak flow meter. People with asthma use it to spot narrowing airways early, often before symptoms become obvious. This calculator estimates the normal (predicted) peak flow for someone of your age, height and sex, and — if you enter a reading — tells you what percentage of that value you achieved and which asthma action-plan zone you are in.
For adults it uses the Nunn and Gregg (1989) reference equations, which underpin widely used UK predicted peak flow charts. The equations were derived on the older Wright meter scale, so readings from EU-scale (EN 13826) meters may differ somewhat from the predicted value. For children and teenagers under 18 it uses the common height-based approximation (height in cm − 100) × 5 + 100. If you know your personal best — the highest reading you reach when your asthma is well controlled — enter it; zones are then based on your personal best, which clinicians prefer over predicted values.
Zones follow the traffic-light system used in asthma action plans: green at 80% or more, yellow from 50% to 79%, and red below 50%. A red-zone reading, or severe breathlessness at any reading, needs urgent medical help. Predicted values vary between populations and meters, so treat them as a guide.
With the default inputs, the predicted peak flow is 643 L/min. Change any value above to recalculate instantly.
How to use the peak flow calculator
- 1Select units and sex, then enter age and height.
- 2Stand up, breathe in fully and blow as hard and fast as you can into the meter — repeat three times.
- 3Enter the highest of the three readings.
- 4Enter your personal best if you know it.
- 5Check your predicted value, percentage and asthma zone.
Formula and method
For adults, Nunn and Gregg fitted log-linear equations to peak flow readings from healthy adults; A is age in years and H is height in centimetres, and the result is in litres per minute. Predicted peak flow rises into early adulthood and then declines slowly with age, and is higher in taller people and in men.
For under-18s the calculator uses the simple height rule often printed on paediatric charts. The zone thresholds are 80% and 50% of the reference value — your personal best if you enter one, otherwise the predicted value. Readings depend on technique, so take the best of three blows.
- PEF
- Peak expiratory flow (L/min)
- A
- Age (years)
- H
- Height (cm)
Worked examples
Man, 35, 5 ft 10 in, reading 500 L/min
Height is 177.8 cm. The adult male equation gives ln PEF ≈ 6.465, so predicted peak flow is about 643 L/min. A 500 L/min reading is 78% of predicted, just inside the yellow zone (green starts at 514 L/min).
Woman, 45, 162 cm, personal best 450
Predicted peak flow is about 473 L/min, but zones use her personal best of 450. A reading of 400 is 89% of that — green zone. Yellow starts below 360 and red below 225 L/min.
Child, 10, 140 cm, reading 200 L/min
Using the paediatric rule, (140 − 100) × 5 + 100 = 300 L/min predicted. A 200 L/min reading is 67% of that, in the yellow zone.
Frequently asked questions
What is a normal peak flow?+
It depends on age, height and sex. A healthy 35-year-old man of average height typically blows around 600–650 L/min, and a woman of similar age around 450–500 L/min. Readings at or above about 80% of predicted (or of your personal best) are usually considered normal.
What are the peak flow zones?+
Asthma action plans use traffic-light zones: green is 80–100% of your personal best (good control), yellow is 50–79% (caution — follow your plan), and red is below 50% (medical alert — get help now).
Should I use predicted or personal best?+
Personal best is preferred. Find it by recording readings twice daily for two to three weeks while your asthma is well controlled; the highest value becomes your personal best for zone calculations.
How do I use a peak flow meter correctly?+
Reset the marker, stand up, take the deepest breath you can, seal your lips around the mouthpiece and blow out as hard and fast as possible. Do it three times and record the highest reading.
Why do different charts give different predicted values?+
Reference equations come from different populations and meter scales (older Wright scale versus the EU scale introduced in 2004). Differences of 10% or more between charts are common, so use one chart consistently.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.