Which reference equation does this use?

It uses the Nunn and Gregg equations, the standard for EU-scale (EN13826) peak flow meters used across the UK and Europe. They are exponential equations in age and height with separate constants for men and women, and they replaced the older Wright-scale norms.

Why does the meter scale matter?

Old Wright-scale meters and newer EU-scale meters give different readings for the same lungs because of how they were calibrated. The Nunn-Gregg equations are matched to EU-scale meters, so your measured value should come from an EU-scale device for the percent-predicted comparison to be valid.

What is a normal percent predicted?

Values within roughly 80 to 100 percent of predicted are generally considered normal. In asthma management, readings of 50 to 80 percent often indicate a moderate exacerbation and below 50 percent a severe one, though personal best is a better individual reference than the population predicted value.

Does this work for children?

The Nunn-Gregg adult equations are validated for adults, broadly from the late teens upward. Children have their own height-based reference equations, so for paediatric patients you should use a dedicated childhood peak flow chart rather than these adult formulas.

Should I rely on predicted or personal best?

For ongoing asthma monitoring, a patient's own personal best, measured when well, is the preferred reference because peak flow varies between individuals of the same age and height. The predicted value is most useful as a first estimate before a personal best is established.

Predicted Peak Expiratory Flow Calculator

Peak expiratory flow rate is the fastest speed a person can blow air out of fully inflated lungs, and it is a simple, cheap marker of airway calibre used heavily in asthma care. To judge whether a reading is good or bad you need to compare it against what is expected for that person’s age, sex, and height.

How it works

This calculator uses the Nunn and Gregg reference equations, matched to EU-scale (EN13826) peak flow meters. They predict PEFR in litres per minute from age in years and height in metres:

Men:   PEFR = exp( 0.544 x ln(age) - 0.0151 x age - 74.7/height + 5.48 )
Women: PEFR = exp( 0.376 x ln(age) - 0.0120 x age - 58.8/height + 5.63 )

If you also enter a measured value, the tool divides it by the predicted to give percent predicted:

percent predicted = measured / predicted x 100

Interpretation and notes

A percent predicted of about 80 to 100 is broadly normal. In asthma, the 50 to 80 percent band commonly signals a moderate flare and below 50 percent a severe one, though thresholds are guidance, not rules. These adult equations apply roughly from the late teens upward; children need their own paediatric reference charts. For day-to-day asthma monitoring, a patient’s personal best, recorded when well, is a more reliable benchmark than the population predicted value. This is an educational tool, not a diagnosis.