Peak Flow Calculator
Use this peak flow calculator to find your predicted peak flow from your sex, age and height, and see your asthma zones from your personal best.
Updated October 22, 2026
Medical Disclaimer: This calculator gives a general estimate for education only and cannot diagnose a condition. Share your result with your doctor or the clinician who ordered your test, and do not start, stop or change any medicine based on it. Read our full Medical Disclaimer.
How Peak Flow Is Calculated
Peak expiratory flow (PEF), or peak flow, is the fastest rate you can blow air out after a full breath in, measured in liters per minute (L/min) with a handheld meter. This calculator estimates the average peak flow for a healthy adult like you and sorts your readings into asthma zones based on your personal best.
What Peak Flow Measures
A peak flow meter shows how well air moves out of your lungs². When the airways narrow, as they do when asthma gets worse, the number drops. It does not diagnose asthma, and it means most when compared with your own usual numbers.
The Formula
Men: predicted PEF = e^(0.544 x ln(age) - 0.0151 x age - 74.7 / height + 5.48)
Women: predicted PEF = e^(0.376 x ln(age) - 0.0120 x age - 58.8 / height + 5.63)
Age in years, height in cm, PEF in L/min. ln is the natural log.
Percent of personal best = reading / personal best x 100
The predicted equations come from a 1989 study of 225 men and 228 women who had never smoked, measured on a standard Wright peak flow meter¹.
How This Calculator Computes Your Result
Enter your sex, age and height in feet and inches or centimeters (1 inch = 2.54 cm). The predicted value is rounded to the nearest 5 L/min, as in the study, and shown only for ages 15 to 85, the range the study covers¹. Add today's reading to see it as a percent of predicted. Add your personal best to see your three zone ranges in L/min and your reading's zone. Percent of personal best is rounded down, so 79.8% shows as 79% and stays yellow. The zones work at any age, including for children, because they only need a personal best.
What Your Result Means
Asthma action plans use three zones based on your personal best. These cutoffs are from the US National Heart, Lung, and Blood Institute (NHLBI)².
Zone | Percent of your personal best | What it signals |
Green | 80 to 100 percent | Good control and no asthma symptoms |
Yellow | 50 to 79 percent | Caution: your asthma is getting worse |
Red | Below 50 percent | Medical alert |
If your reading is in the yellow or red zone, follow your written asthma action plan. In the red zone, NHLBI says to also call your doctor now².
Percent of predicted is not a zone. It compares you with an average, and in the study the lower 90% limit sat roughly 70 to 80 L/min below the predicted value¹.
Predicted Peak Flow by Age and Height
Values in L/min from the same equations, on the older Wright scale¹.
Age | Men 160 cm (5 ft 3 in) | Men 175 cm (5 ft 9 in) | Men 190 cm (6 ft 3 in) | Women 150 cm (4 ft 11 in) | Women 160 cm (5 ft 3 in) | Women 175 cm (5 ft 9 in) |
20 | 565 | 590 | 610 | 455 | 470 | 485 |
30 | 610 | 635 | 655 | 470 | 485 | 500 |
40 | 610 | 635 | 660 | 465 | 480 | 495 |
50 | 595 | 620 | 640 | 450 | 460 | 475 |
60 | 565 | 585 | 605 | 425 | 440 | 450 |
70 | 525 | 550 | 565 | 400 | 410 | 425 |
80 | 485 | 505 | 525 | 375 | 385 | 395 |
How to Measure Correctly
NHLBI gives these steps for a home peak flow meter²:
Stand up. Remove any food or gum from your mouth.
Move the marker to the bottom of the scale.
Breathe in slowly and deeply, then hold that breath.
Close your lips tightly around the mouthpiece. Keep your tongue out of the hole.
Blow out as hard and fast as you can in one blow.
Write down the number. If you cough or make a mistake, do it again.
Do this three times and record the highest number. Do not average the three⁷.
Your personal best is the highest number you reach over 2 to 3 weeks of readings, taken at least twice a day while your asthma is under good control². Children as young as five can usually use a peak flow meter⁷.
Why Personal Best Beats Predicted Values
The NHLBI asthma guideline advises using changes in percent of personal best, rather than percent of predicted, to guide asthma care³. Normal varies widely: in the study, age and height explained only about 30% of the differences in peak flow between people¹. The meter maker also notes that the values come from white (Caucasian) populations⁶.
Why Older Peak Flow Charts Differ
The study used the Wright meter scale. In a lab test, Wright-type meters overread true flow by up to 80 L/min between 300 and 500 L/min⁴. In September 2004, Wright and Mini-Wright meters moved to the EU scale (EN 13826), which reads closer to true flow⁵, and the maker issued an EU-scale version of this chart⁶. So a healthy reading on a newer meter may sit below a Wright-scale prediction like ours. A personal best from an old Wright-scale meter is also not on the same scale as a newer meter.
Predicted Peak Flow by Age and Height
Average peak flow for healthy adults who never smoked. It peaks around age 30 to 40 and is higher in taller people.
- Men 160 cm (5 ft 3 in)
- Men 175 cm (5 ft 9 in)
- Men 190 cm (6 ft 3 in)
Show data table
| Age | Men 160 cm (5 ft 3 in) (L/min) | Men 175 cm (5 ft 9 in) (L/min) | Men 190 cm (6 ft 3 in) (L/min) |
|---|---|---|---|
| 20 | 565 | 590 | 610 |
| 30 | 610 | 635 | 655 |
| 40 | 610 | 635 | 660 |
| 50 | 595 | 620 | 640 |
| 60 | 565 | 585 | 605 |
| 70 | 525 | 550 | 565 |
| 80 | 485 | 505 | 525 |
Asthma Zones by Percent of Personal Best
The three action plan zones. Enter your reading as a percent of your personal best to mark it.
- Red: medical alert
- Yellow: caution
- Green: good control
Show data table
| Scale | Red: medical alert | Yellow: caution | Green: good control |
|---|---|---|---|
| Percent of personal best | Below 50 percent | 50 to 79 percent | 80 to 100 percent |
A Note on Accuracy
The predicted value is an average for healthy adults who had never smoked, from a 1989 English study on the older Wright scale¹. Every reading depends on effort, technique and the meter. This tool gives no children's predicted values, makes no ethnicity adjustment and does not diagnose asthma. Your zones are only as good as the personal best you enter.
Frequently Asked Questions
What is a normal peak flow for my age and height?
A 40-year-old man who is 175 cm (5 ft 9 in) has a predicted peak flow of about 635 L/min, and a 40-year-old woman who is 160 cm (5 ft 3 in) about 480 L/min¹. These are averages, and healthy readings can sit 70 to 80 L/min below them¹.
What are the peak flow zones?
Green is 80 to 100 percent of your personal best, yellow is 50 to 79 percent, and red is below 50 percent². A red zone reading is a medical alert: follow your written asthma action plan and call your doctor now².
Should I use my predicted peak flow or my personal best?
Use your personal best for your zones. NHLBI advises tracking percent of personal best rather than percent of predicted³.
How accurate is a peak flow reading?
It depends on effort, technique and the meter; older Wright-scale meters overread by up to 80 L/min in the middle of the range⁴. Your own personal best is a better yardstick than a predicted value³.
Sources
New Regression Equations for Predicting Peak Expiratory Flow in Adults, Nunn and Gregg, 1989
Monitoring Your Asthma, NHLBI, 2020
Expert Panel Report 3: Guidelines for the Diagnosis and Management of Asthma, NHLBI, 2007
The Accuracy of Portable Peak Flow Meters, Miller et al., 1992
Effects of Equipment and Technique on Peak Flow Measurements, Bongers and O'Driscoll, 2006
Peak Expiratory Flow Rate Normal Values (EU Scale), Clement Clarke International, 2004
Measuring Your Peak Flow Rate, American Lung Association