Peak flow meters measure how fast you can blow air out. Comparing readings with your personal best (or the predicted value) helps detect asthma attacks early and guide treatment in an asthma action plan.
How to use the Peak Flow Calculator
- Person
- Sex
- Age
- Height
- Your peak flow reading
- Personal best (optional)
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter your details and your best of three peak flow blows. Use your personal best instead of the predicted value if you know it.
Inputs explained
| Field | What to enter |
|---|---|
| Person | Choose one: Adult, Child |
| Sex | Choose one: Male, Female |
| Age | In years |
| Height | Units: cm, in, m, ft, mm |
| Your peak flow reading | In L/min |
| Personal best (optional) | In L/min |
Example values
How the main result changes when Age changes and everything else stays at the example values above:
| Age (years) | Reading as % of reference (%) |
|---|---|
| 17.5 | 72 |
| 26.25 | 74 |
| 35 | 77 |
| 43.75 | 80 |
| 52.5 | 83 |
| 70 | 90 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Men: PEF (L/min) = ((height m × 5.48) + 1.58 − age × 0.041) × 60
Women: PEF = ((height m × 3.72) + 2.24 − age × 0.03) × 60
Children: PEF ≈ (height cm − 100) × 5 + 100
Zones: green ≥ 80% · yellow 50–79% · red < 50% of predicted or personal best
Worked example
Inputs — Person: Adult; Sex: Male; Age: 35 years; Height: 175 cm; Your peak flow reading: 450 L/min; Personal best (optional): 0 L/min.
Results — Predicted peak flow = 584 L/min; Reading as % of reference = 77 %; Yellow zone starts below = 467 L/min; Red zone starts below = 292 L/min.
Key terms
- PEF
- Peak expiratory flow.
Frequently asked questions
What is a normal peak flow?
It depends on age, sex and height – often 400–650 L/min for adult men and 300–470 L/min for women.
Should I use predicted or personal best?
Personal best, measured when your asthma is well controlled, is more useful.
What should I do in the red zone?
Take your reliever and seek urgent medical help.