Health calculators

Peak Flow Calculator

Updated Jul 13, 2026 By Jehan Wadia
Rate Formulas Medical disclaimer

Required field. Results appear once Age, Sex, Height and Race/Ethnicity are valid.

Patient Information
Accepted range: 5–80 years.
Accepted range: 109–213 cm (43–84 in).
Sex
Race / Ethnicity
Optional Measurement & Formula
Enter a value to compare against the prediction.
Prediction Formula
 

Introduction

A peak flow meter measures how fast you can push air out of your lungs in one big breath. This number is called your peak expiratory flow rate (PEFR). Doctors use it to check how well your lungs are working, especially if you have asthma or another lung condition.

This peak flow calculator finds your predicted PEFR based on your age, sex, height, and race or ethnicity. Your predicted value is the normal flow rate expected for someone with your body size and background. If you also enter your actual measured reading, the calculator compares it to your predicted value and shows your result as a percentage. It then places you in one of three zones — green (normal), yellow (caution), or red (medical alert) — so you can quickly see where you stand.

The tool uses well-known published formulas, including the CDC NHANES III equations (Hankinson 1999), the Knudson adult equations, and the Polgar & Promadhat pediatric table. It picks the best formula for you automatically, or you can choose one yourself. A full step-by-step breakdown is shown with every result so you can see exactly how the number was calculated.

This calculator is meant for educational purposes and does not replace advice from a doctor or other healthcare provider. Always talk to your doctor about your peak flow results and what they mean for your care plan.

How to Use Our Peak Flow Calculator

Enter a few details about the patient below. The calculator will give you a predicted peak expiratory flow rate (PEFR) in both L/min and L/sec, along with a step-by-step breakdown of the math used.

Age: Type the patient's age in years. The accepted range is 5 to 80 years old. Age helps the calculator pick the right formula and directly affects the predicted result.

Height: Type the patient's height. It shows in centimeters by default, but you can click the unit button to switch to inches. The value converts for you automatically. Height is the strongest factor in predicting peak flow. If you need to check where the patient falls relative to population norms, our height percentile calculator can help.

Sex: Select Male or Female. Men and women have different predicted peak flow values, so this choice changes the equation used.

Race / Ethnicity: Choose Caucasian, African-American, Mexican-American, or Other. The CDC NHANES III formula uses race-specific data to improve accuracy. If you pick "Other," the calculator uses Caucasian values as a default.

Measured Peak Flow (optional): If you have the patient's actual peak flow reading, type it here. You can toggle between L/min and L/sec. When you enter this value, the calculator shows the percent of predicted and a color-coded three-zone rating — green (normal), yellow (caution), or red (medical alert).

Prediction Formula: Leave this set to "Auto" and the calculator picks the best formula based on age, sex, and ethnicity. You can also manually choose CDC NHANES III, Knudson (for adults), or Polgar (for children) if you prefer a specific equation.

Click Calculate to see your results. Click Clear to reset all fields and start over.

What Is Peak Expiratory Flow?

Peak expiratory flow (PEF or PEFR) is the fastest speed at which you can push air out of your lungs in one big, quick breath. It is measured in liters per minute (L/min) using a small handheld device called a peak flow meter. Doctors use this number to check how well your lungs are working, especially if you have asthma or another lung condition. Peak flow is one of several values measured during a complete pulmonary function test, which gives a broader picture of lung capacity and airway health.

What Does This Peak Flow Calculator Do?

This calculator estimates your predicted peak flow — the normal value expected for someone your age, sex, height, and race or ethnicity. It uses well-known medical formulas published by researchers, including the CDC NHANES III equations (Hankinson, 1999), the Knudson adult equations, and the Polgar & Promadhat pediatric table. The calculator picks the best formula for you automatically, or you can choose one yourself. Because body size plays a central role in these predictions, tools like the BMI calculator and BSA calculator can provide useful companion measurements when assessing overall patient health.

Why Predicted Peak Flow Matters

Your predicted value acts as a baseline. When you compare your actual measured reading to this predicted number, you get a percentage of predicted. That percentage tells you and your doctor how open or narrow your airways are right now. It is a key part of managing asthma and tracking lung health over time. Athletes and active individuals may also benefit from knowing their VO2 max, which measures overall cardiorespiratory fitness and complements peak flow monitoring.

The Three-Zone System

If you enter a measured peak flow reading, the calculator places you into one of three color-coded zones used in most asthma action plans. This zone approach is similar to the system used in a heart rate zone calculator, where defined ranges guide your response:

  • Green Zone (80–100%): Your airways are open and working well. No changes needed.
  • Yellow Zone (50–79%): Your airways are partly narrowed. You may need to adjust medicine or call your doctor.
  • Red Zone (below 50%): Your airways are very narrow. This is a medical emergency — get help right away.

Important Things to Know

This tool gives an estimate based on population averages. Your personal best peak flow may be higher or lower than the predicted value. Factors such as ideal body weight, fitness level, and overall body composition can all influence lung function. For pediatric patients, checking a child BMI calculator alongside peak flow can provide additional context for growth-related assessments. Always work with your doctor to find your own personal best and build an asthma action plan around it. This calculator is not a substitute for professional medical advice, diagnosis, or treatment.

Formulas used

CDC NHANES III Predicted PEF (L/s)
PEF_{L/s} = b_0 + b_1 \times \text{age} + b_2 \times \text{age}^2 + b_3 \times H_{cm}^2
Convert PEF to L/min
PEFR_{L/min} = PEF_{L/s} \times 60
Knudson Adult Male PEFR
PEFR = \left( (H_m \times 5.48 + 1.58) - (\text{age} \times 0.041) \right) \times 60
Knudson Adult Female PEFR
PEFR = \left( (H_m \times 3.72 + 2.24) - (\text{age} \times 0.03) \right) \times 60
Pediatric Height Approximation
PEFR = (H_{cm} - 100) \times 5 + 100
Percent of Predicted
\%\;\text{predicted} = \frac{\text{Measured PEFR}}{\text{Predicted PEFR}} \times 100

Frequently asked questions

What is a normal peak flow reading?

A normal peak flow reading is 80% or more of your predicted value. This puts you in the green zone. For most healthy adults, readings fall between 400 and 700 L/min, but the exact number depends on your age, sex, and height. A tall young man will have a higher normal value than a shorter older woman. Use this calculator to find the predicted value for your specific body size.

How do I measure my peak flow at home?

Follow these steps:
  1. Stand up straight.
  2. Set the marker on your peak flow meter to zero.
  3. Take a deep breath in — fill your lungs all the way.
  4. Put the mouthpiece in your mouth and close your lips tight around it.
  5. Blow out as hard and as fast as you can in one quick burst.
  6. Write down the number.
  7. Repeat two more times.
  8. Record the highest of the three numbers.
Use that highest number as your measured peak flow in this calculator.

What is the difference between predicted peak flow and personal best?

Your predicted peak flow is the average expected value for someone your age, sex, height, and ethnicity. It comes from a math formula based on population data. Your personal best is the highest peak flow number you can reach when your lungs are feeling good, usually measured over a two-week period. Doctors often prefer to use your personal best as the baseline for your asthma action plan because it is specific to you.

Can children use this peak flow calculator?

Yes. This calculator works for ages 5 to 80. For children under 20, it automatically uses the Polgar & Promadhat pediatric table if the child's height falls between 43 and 67 inches (109–170 cm). You can also manually select the Polgar formula. Keep in mind that very young children may have trouble blowing hard enough to get an accurate reading on a peak flow meter.

Why does the calculator ask for race or ethnicity?

The CDC NHANES III equations were built using separate data sets for Caucasian, African-American, and Mexican-American populations. Studies found that average lung size and airway dimensions differ between these groups, even after adjusting for height and age. Using race-specific coefficients makes the predicted value more accurate. If you choose "Other," the calculator uses Caucasian coefficients as a default.

What formula should I pick?

Leave it on Auto. The calculator will choose the best formula for the patient's age and data:
  • Polgar — for children under 20 with height in the pediatric table range.
  • CDC NHANES III — for ages 8–80 when race or ethnicity is provided.
  • Knudson — for adults when you want a simpler height-and-age-only equation.
You only need to override this if your doctor has asked you to use a specific formula.

What do L/min and L/sec mean?

L/min stands for liters per minute. L/sec stands for liters per second. Both measure airflow speed. Most home peak flow meters show results in L/min. To convert, divide L/min by 60 to get L/sec, or multiply L/sec by 60 to get L/min. This calculator shows both units so you can use whichever your device or doctor prefers.

Why is height the most important factor?

Taller people have larger lungs and wider airways. Bigger airways can move more air in a single breath. That is why height has the strongest effect on predicted peak flow. A person who is 6 feet tall will have a much higher predicted PEFR than someone who is 5 feet tall, even if their age and sex are the same.

What should I do if my result is in the yellow zone?

The yellow zone (50–79%) means your airways are partly narrowed. You should:
  • Use your quick-relief inhaler as directed in your asthma action plan.
  • Avoid triggers like smoke, dust, or cold air.
  • Check your peak flow again after 20–30 minutes.
  • Contact your doctor if you do not move back into the green zone.
Do not ignore a yellow zone reading. It is a warning that your asthma may be getting worse.

What should I do if my result is in the red zone?

The red zone (below 50%) is a medical emergency. Take your rescue inhaler right away and call your doctor or go to the emergency room. Do not wait to see if you feel better. Severe airway narrowing can be life-threatening. Follow the emergency steps in your asthma action plan.

How often should I check my peak flow?

It depends on your condition. Many doctors recommend checking twice a day — once in the morning and once in the evening — especially if your asthma is not well controlled. When your asthma is stable, your doctor may say once a day or a few times a week is enough. Always check your peak flow when you feel symptoms like coughing, wheezing, or shortness of breath.

Can peak flow readings change throughout the day?

Yes. Peak flow is usually lowest in the early morning and highest in the afternoon. A difference of up to 20% between morning and evening is common in people with asthma. If the difference is more than 20%, it may be a sign that your asthma is not well controlled. Track your numbers at the same times each day so comparisons are fair.

Does this calculator replace a doctor's advice?

No. This calculator is an educational tool that uses published medical formulas to estimate your predicted peak flow. It does not diagnose any condition and does not replace a visit to your doctor. Always share your peak flow numbers with your healthcare provider so they can help you build or adjust your treatment plan.

Why is my predicted value different from a chart my doctor gave me?

Different charts and formulas use different study data. Your doctor's chart may come from a different reference equation, a different population study, or may round values differently. Small differences of 10–30 L/min between sources are normal. If you want to match your doctor's chart exactly, ask which formula they use and select it manually in the Prediction Formula option.

Can I use this calculator if I do not have asthma?

Yes. Peak flow is useful for anyone who wants to check their lung function. Doctors also use it for people with COPD, chronic bronchitis, and other lung conditions. However, the three-zone system (green, yellow, red) was designed mainly for asthma action plans. If you have a different lung condition, talk to your doctor about what your peak flow numbers mean for you.