Predicted normal values
Predicted PEF varies with height, age, and sex. Taller individuals have larger airways and higher predicted values; older adults have lower values due to normal age-related lung function decline. The equations used here are simplified linear approximations based on European Respiratory Society reference data.
If you have a known personal best — recorded during a period when your asthma was well-controlled — use that instead of the predicted value. Personal best gives more accurate zone thresholds.