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.

What is the Predicted Peak Expiratory Flow Calculator?

Compute predicted peak expiratory flow rate from age, sex, and height using the Nunn-Gregg EU/EN13826 reference equations, then express a measured PEFR as percent predicted. For respiratory nurses and asthma management. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Predicted Peak Expiratory Flow Calculator

Name: Predicted Peak Expiratory Flow Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Peak expiratory flow rate (PEFR) is the fastest speed a person can blow air out of fully inflated lungs, measured in litres per minute. It is a simple, inexpensive, and widely available marker of airway calibre used heavily in asthma diagnosis and monitoring. 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 × ln(age) − 0.0151 × age − 74.7/height + 5.48 )
Women: PEFR = exp( 0.376 × ln(age) − 0.0120 × 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 × 100

EU-scale vs Wright-scale meters: why it matters

Two different meter calibrations have been used in clinical practice: the older Wright scale and the newer EU scale (EN 13826), introduced in the UK and Europe from the early 2000s. The two scales give different numerical readings for the same breath because of how the meters were engineered and calibrated — an EU-scale meter typically reads higher than a Wright-scale meter for the same patient.

The Nunn-Gregg equations were validated against EU-scale meters. If a measured reading comes from an old Wright-scale meter, the percent-predicted comparison will be inaccurate. Confirm your meter type before using the output clinically. Most modern meters sold in the UK and Europe since around 2004 comply with EN 13826 and use the EU scale.

Interpretation guide

Percent predicted	Broad interpretation
80–100%	Normal range
50–79%	Moderate airflow limitation — review and treat
33–49%	Severe airflow limitation — urgent review
Below 33%	Life-threatening — emergency treatment

These bands are guidance, not diagnostic thresholds. They are widely used in UK asthma management protocols but should always be interpreted alongside symptoms, oxygen saturation, and clinical history. Individual patients can have a personal best that sits above or below the population predicted value.

Personal best vs predicted value

For ongoing asthma monitoring, a patient’s own personal best — measured when well and at their most stable — is the preferred reference. The population predicted value from the Nunn-Gregg equations is most useful when:

A personal best has not yet been established (newly diagnosed patient)
A patient cannot recall or report their previous best readings
Screening or occupational health contexts where a group reference is needed

Once a personal best is established, express the current reading as a percentage of that personal best rather than the predicted value. Personal best reflects the individual’s physiology, not just the average for their age and height group.

Paediatric patients

The Nunn-Gregg equations are validated for adults — broadly from the mid to late teens upward. Children have different reference ranges that scale with height alone (not age) using separate paediatric charts. Do not apply these adult equations to children and adolescents.

This is an educational and clinical support tool. It does not constitute medical advice or replace clinical assessment by a qualified healthcare professional.