Peak Flow Meter

A peak flow meter is a device that measures how well your lungs are working. For people with asthma, it is a simple but important tool to help with daily asthma management. Peak flow meters are available over the counter.

The readings from the meter will help you and your health care provider:

  • Determine the severity of your asthma.  

  • Evaluate the effectiveness of your current treatment.  

  • Determine when to add or stop certain medicines.  

  • Recognize an asthma attack before signs or symptoms appear.  

  • Decide when to seek emergency care.  


When using the peak flow meter, breathing too quickly may cause dizziness. At an extreme, this could cause you to pass out. Take your time so you do not get dizzy or light-headed.


Your "personal best" is the highest peak flow rate you can reach when you feel good and have no asthma symptoms. Your flow rate serves as a benchmark in your daily self-management plan. Because everyone's asthma is different, your personal best will be unique to you.

Your health care provider will help you figure out your personal best. Typically, you will take readings once or twice a day for 2 weeks when you are not having symptoms. The highest reading during the trial period is your personal best. Because your lung function can change over time, your personal best should be remeasured each year.


  1. Move the upper marker to the number that is your personal best.  

  2. Move the lower marker to the bottom of the numbered scale.  

  3. Connect the mouthpiece to the peak flow meter.  

  4. Stand up.  

  5. Take a deep breath. Fill your lungs completely.  

  6. Place your lips tightly around the mouthpiece. Blow as hard and as fast as you can with a single breath, as if you are blowing out candles. If your lips are not placed tightly around the peak flow meter mouthpiece, you will get incorrect low readings.  

  7. Note the final position of the lower marker. This is your peak flow rate.  

  8. Move the lower marker back to the bottom of the numbered scale.  

  9. Repeat these steps 2 more times. Record the highest reading of the 3 tries in your asthma diary. Do not calculate the average for your 3 tries, just record the highest.  

Always write down the results in your asthma diary. After using your peak flow meter, rest and breathe slowly and easily. Keep a record of your progress. Your health care provider can provide you with a simple table to help with this. For the most accurate readings, it is important to keep your peak flow meter clean. Follow the manufacturer's instructions on how to take care of your peak flow meter.

Your health care provider will give you instructions on when to do regular monitoring. You may also need to check your peak flow when:

  • Asthma symptoms wake you up at night.  

  • You have increased symptoms during the day.  

  • You have a cold, flu, or other illness that affects your breathing.  

  • You need quick-relief "rescue medicine." If possible, check your peak flow before you use rescue medicine. Check it again 20–30 minutes after taking the medicine.  


You and your health care provider can use color-coded zones on the meter to see how your peak flow rate compares to your personal best. The color code for each zone reflects progressively more severe symptoms:

Green Zone = Stable

  • Your peak flow rates are 80–100% of your personal best. This means that your asthma is under control.  

  • You probably have no asthma signs or symptoms.  

  • Take your preventive medicine as usual.

Yellow Zone = Caution

  • Your peak flow rates are 50–80% of your personal best. This means that your asthma is getting worse and could be improved.  

  • You may have signs and symptoms such as coughing, wheezing, or chest tightness. However, your peak flow rates may decrease before symptoms appear.  

  • You may need to increase or change your asthma medicine. If you have an asthma action plan, follow all of the steps listed for the yellow zone, including medicine changes.

Red Zone = Danger

  • Your peak flow rates are less than 50% of your personal best. This means that you may be in danger of a medical emergency.  

  • You may have severe coughing, wheezing, and shortness of breath. Stop whatever you are doing. Use your rescue inhaler or other medicines to open your airways.  

  • Your asthma action plan will help you decide whether to call your health care provider or seek emergency care.

If your flow readings fall too far below your personal best into the yellow or red zone, you will need to take action to prevent or minimize an asthma attack.


You are in the yellow zone. If you have an asthma action plan, follow all of the steps listed under the yellow zone section of the plan. Let your health care provider know that you are in the yellow zone.


You are in the red zone. If you have an asthma action plan, follow all of the steps listed under the red zone section of the plan while you are seeking immediate medical care.