Peak Flow Calculator: Predicted PEF, % of Predicted and Asthma Zones
Estimate predicted peak expiratory flow (L/min) from age, sex and height with NHANES III, and see your reading as % of predicted or personal best and its zone.
How it works
NHANES III (Hankinson 1999): PEF (L/s) = b0 + b1 × age + b2 × age² + b3 × height (cm)², with coefficients by sex, ethnic group and age band; L/min = L/s × 60 · Leiner 1963: men PEF (L/s) = 5.48 × height (m) + 1.58 − 0.041 × age; women PEF (L/s) = 3.72 × height (m) + 2.24 − 0.03 × age · Children’s rule: PEF (L/min) = (height cm − 100) × 5 + 100 · % of predicted = reading ÷ predicted × 100 · Zones: green ≥ 80%, yellow 50–79%, red < 50% of personal best (or predicted)
Peak expiratory flow (PEF) is the fastest speed at which you can blow air out after a full breath, measured with a handheld peak flow meter. What counts as normal depends mainly on height, age and sex: taller people and young adults blow higher, and peak flow falls slowly with age after the 20s. The default equation is the US NHANES III reference from Hankinson and colleagues (1999), built from about 7,400 healthy non-smokers aged 8 to 80. It gives a predicted value and a lower limit of normal (the 5th percentile, so 1 healthy person in 20 falls below it). The equations were published separately for White, Black and Mexican American participants; the calculator uses the published coefficients exactly. Current lung-function guidance (the ATS/ERS 2022 interpretation standard and the ATS 2023 statement on race) recommends race-neutral references, but the race-neutral GLI Global equations cover only FEV1 and FVC, and there is no race-neutral peak flow equation yet. That is one more reason to judge your readings against your own personal best rather than a predicted value. You can also choose the older Leiner 1963 adult chart that many printed charts still use, or a simple height rule for children. Enter a reading to see it as a share of predicted, and enter your personal best to set asthma zones the way an action plan does: green is 80–100% of your best, yellow 50–79%, red under 50% (American Lung Association). Your personal best is the better yardstick because predicted values only describe the average healthy person like you. Unlike a general lung-volume or spirometry tool, this page is built around the single number a home peak flow meter gives you.
Worked example
A 35-year-old White man who is 70 in (177.8 cm) tall, NHANES III adult male coefficients: PEF = 1.0523 + 0.08272 × 35 − 0.001301 × 1,225 + 0.00024962 × 31,612.8 = 1.0523 + 2.8952 − 1.5937 + 7.8912 = 10.245 L/s, × 60 = 614.7 L/min, shown as 615 L/min. His lower limit of normal uses 0.00017635 for the height term: 7.929 L/s = 476 L/min. A reading of 500 L/min is 500 ÷ 614.7 = 81.3% of predicted, which falls in the green zone (80% or more). The zone limits from the predicted value are 492 L/min (80%) and 307 L/min (50%).
Sources: Hankinson JL et al. 1999, Spirometric reference values from a sample of the general U.S. population (NHANES III), Tables 4–5 (CDC Stacks) · American Lung Association: Measuring your peak flow rate (zones and technique) · Bowerman C et al. 2023, A race-neutral approach to the interpretation of lung function measurements (GLI Global), Am J Respir Crit Care Med · Moffett AT et al. 2023, Global race-neutral reference equations and PFT interpretation (GLI Global covers FEV1, FVC and FEV1/FVC), JAMA Netw Open · Respiratory Therapy Zone: Predicted PEF calculator (Leiner and children’s formulas, worked examples)
Frequently asked questions
What is a normal peak flow for my age and height?
It depends on sex, age and height. Under NHANES III, a 35-year-old man 5 ft 10 in tall is predicted to blow about 615 L/min and a 35-year-old woman 5 ft 5 in tall about 430 L/min. Readings down to the lower limit of normal (about 475 L/min for that man) are still within the healthy range.
What do the green, yellow and red peak flow zones mean?
Green (80–100% of your personal best) means your asthma is under control. Yellow (50–79%) means your airways are narrowing: follow the yellow-zone steps in your action plan and call your doctor. Red (under 50%) is a medical alert: use your quick-relief medicine and get help right away. Your doctor may set different limits.
How do I find my personal best peak flow?
Measure your peak flow at about the same times each day for 2 to 3 weeks while your asthma is well controlled, taking the highest of three blows each time. The highest reading in that period is your personal best. Recheck it every year or as your doctor advises.
Should I use predicted peak flow or personal best?
Use your personal best once you know it. Predicted values describe the average healthy person of your age, sex and height, and many healthy people sit well above or below that average. Predicted values are useful when you have no personal best yet.
How do I take a peak flow reading correctly?
Move the marker to zero, stand up straight, take the deepest breath you can, seal your lips around the mouthpiece and blow out as hard and fast as you can. Do it three times and record the highest number, not the average. Ignore a blow if you coughed.
Why does the NHANES III equation ask for ethnic group?
The 1999 study published separate equations for White, Black and Mexican American participants, and it remains the main US peak flow reference. Lung-function guidance now recommends race-neutral references, but the race-neutral GLI Global equations (2022) cover FEV1 and FVC, not peak flow, so no race-neutral peak flow equation is available yet. NHANES III also has no equation for other groups. For these reasons, tracking your own personal best is the most reliable approach.
Related calculators
For information only, not a diagnosis. Follow the asthma action plan from your own doctor, and get emergency care for severe breathlessness whatever the reading.