Peak Flow Calculator — Estimated Peak Expiratory Flow

This calculator relies on standard pulmonary equations that use your biological sex, age, height, and ethnicity to determine your baseline predicted peak expiratory flow. To assess daily variation, it measures diurnal variability by comparing the difference between your maximum and minimum daily readings against your maximum reading using the formula: Diurnal Variability = ((PEF max - PEF min) / PEF max) * 100. Furthermore, it categorizes your measured readings into color-coded zones (Green, Yellow, Red) by comparing your current score against either your predicted value or your personal best record.

Consider a 40-year-old male who is 175 cm tall, with a measured peak flow of 500 L/min and a personal best of 550 L/min. First, the demographic-based prediction model estimates his normal baseline PEF at 600 L/min. To find his zone percentage relative to his personal best, the calculator uses the formula: Zone Best = (500 / 550) * 100, yielding approximately 90.9%. This places him securely in the green zone, indicating that his asthma or airway function is currently well-controlled and stable.

For the most reliable readings, always take your measurements at the same times each day, typically once in the morning and once in the evening before taking any medication. Ensure the indicator slider on your physical meter is reset to zero, stand up straight, take a deep breath, and blow into the device as hard and fast as possible. Always record the highest value out of three consecutive attempts.