Peak Flow Calculator

Frequently asked questions

What is peak expiratory flow (PEF)?

Peak expiratory flow (PEF) is the fastest speed at which you can blow air out of your lungs after a full breath in, measured in litres per minute (L/min) with a small handheld device called a peak flow meter. It reflects how open the large airways are: when the bronchi are narrowed, as in asthma, PEF falls. Take the best of three blows as your recorded reading.

How is my predicted PEF calculated?

This calculator uses the Nunn & Gregg (1989) reference equations for adults (Br Med J 1989;298:1068-70), derived for handheld peak flow meters: men: ln(PEF) = 0.544·ln(age) − 0.0151·age − 74.7/height + 5.48; women: ln(PEF) = 0.374·ln(age) − 0.0120·age − 67.6/height + 5.66, with age in years and height in cm, PEF in L/min. Example: a 35-year-old man, 175 cm has a predicted PEF of 638 L/min. The equations were verified against the published Nunn & Gregg normal-value tables (see the sources note below).

What is the difference between “predicted” and “personal best”?

Predicted PEF is a population average for your sex, age and height — a reference point, not a target. Your personal best is the highest reading YOU achieve when your asthma is stable and under control, measured on your own meter. Asthma action-plan zones (green/yellow/red) are always computed from your personal best, because guidelines recommend comparing each reading with your own previous best, not with a population table.

What are the green, yellow and red asthma zones?

Zones are percentages of your personal best: green is 80–100% (asthma appears under control — continue your prescribed plan), yellow is 50–80% (caution — asthma may be worsening; follow your personal action plan and contact your healthcare provider if readings stay yellow), red is below 50% (danger — can be a medical emergency). Exactly 80% counts as green; exactly 50% counts as yellow.

What should I do in the red zone?

Treat a red-zone reading (below 50% of your personal best) as a possible medical emergency: follow the emergency steps in the asthma action plan written with your doctor and get medical help right away (emergency department or your local emergency number). This calculator is educational only and cannot tell you what medicine to take.

How do I find my personal best?

Measure your peak flow twice daily for one to two weeks while your symptoms are controlled, taking the best of three blows each time. Your personal best is the highest number you record. Ask your doctor to confirm it and to write the zone limits into your personal asthma action plan. Recheck it periodically, since it can change as you grow or as control improves.

What is diurnal variability and how is it calculated?

Diurnal variability describes how much your PEF swings within a day (the typical pattern is lowest in the morning, highest in the evening). This calculator uses the GINA method: (highest − lowest) ÷ mean × 100. Example: morning 400 and evening 520 L/min gives (520 − 400) ÷ 460 × 100 = 26.1%. A variation above 20% (above 10% with twice-daily readings) is characteristic of asthma — discuss persistent variability with your doctor.

Why do different calculators give different predicted values?

Predicted PEF depends on the reference population and equation used — studies have shown the choice of formula alone can shift a classification (Radeos & Camargo, Am J Emerg Med 2004). This tool uses Nunn & Gregg 1989, the classic reference for handheld meters. Also note the scale: the UK switched from the original Wright scale to the EU (EN 13826) scale in 2004, and readings differ between scales — always compare readings taken on the same meter, and use your personal best for action plans.

Can children use this calculator?

For ages under 18 this calculator uses an approximate ready-reckoner (PEF = (height in cm − 100) × 5 + 100 L/min), which is a rough guide only — children's reference values (e.g. Polgar 1971) are tabulated by height rather than given by a single formula, and a child's personal best should be established with their doctor. Example: a child 137 cm tall gives roughly 285 L/min.

Is this calculator medical advice?

No. It is an educational tool: it computes reference numbers and zone arithmetic, but it cannot diagnose asthma, judge your control, or tell you what medicine to take. Your personal asthma action plan — including what to do in each zone — must come from your own doctor. If you are in the red zone or struggling to breathe, get medical help right away.