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Predicted Peak Flow (PEF) Calculator

Enter your age, height in centimetres, and sex.

How it is calculated

The tool uses a fixed linear equation with height in metres (your centimetres divided by 100) and age in years. It first finds a flow in litres per second, then multiplies by 60 for litres per minute:

Male: L/s = height(m) × 5.48 + 1.58 − age × 0.041
Female: L/s = height(m) × 3.72 + 2.24 − age × 0.030
PEF = L/s × 60

The result is a reference-style prediction for a healthy person of that sex, age and height, rounded to a whole L/min (the per-second figure is shown to two decimals). Accepted ranges are age 3 to 120 and height 50 to 272 cm; sex must be male or female. Anything outside these limits, including a height typed in metres, shows no result. One straight-line equation is applied across the whole age range, so it is rougher for children and the very old. It is a value to compare a peak flow meter reading against, not a diagnosis.

Worked example

A 35-year-old man who is 175 cm tall: height is 1.75 m. The inner expression is 1.75 × 5.48 + 1.58 − 35 × 0.041 = 9.59 + 1.58 − 1.435 = 9.735 L/s, shown as 9.74 L/s. Multiplying by 60 gives 584.1, so the predicted peak flow is 584 L/min. A woman of the same age at 165 cm comes out lower, at 440 L/min, because the female equation has smaller coefficients.

Try this example →

Common mistakes

  • Entering height in metres or feet. The field is in centimetres; 1.75 is rejected as below the 50 cm minimum and gives no result, and a height in feet such as 5.8 is rejected the same way.
  • Treating the predicted value as your personal best. Meter readings are compared with predicted figures and, for asthma plans, usually with your own best reading, which can differ a lot.
  • Choosing the wrong sex. At 35 years and 165 cm the male equation gives 551 L/min and the female one 440 L/min, a difference of more than 100.
  • Using the number for a child or a very old person without caution. The same straight line is applied from age 3 to 120, so the figure is only a rough guide at the two ends of that range.
  • Confusing PEF with FEV1. PEF is the peak speed in L/min from a hand meter; FEV1 is a volume in litres from spirometry.

Sources

About this calculator

Peak expiratory flow (PEF) is the fastest speed of air a person can force out of their lungs after a full breath in, measured in litres per minute with a handheld peak flow meter. Because healthy lung capacity varies predictably with age, height, and sex, simple reference equations provide a predicted PEF value that a person's own measured reading can be compared against. This calculator uses a fixed linear approximation (men: ((height in m × 5.48) + 1.58 − age × 0.041) × 60; women: ((height in m × 3.72) + 2.24 − age × 0.030) × 60) and shows the predicted flow in L/min. It does not take your reading or give a percentage of predicted.

This informational reference is widely used by people managing asthma or COPD, and by clinicians monitoring respiratory function, since a reading that drops well below the predicted value can be a signal worth discussing with a healthcare provider. The calculator provides the general predicted figure for educational purposes only — it doesn't diagnose or interpret an individual's results, which should be reviewed with a doctor.

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