Peak Flow Meters

A peak flow meter is a handheld device that measures how well air moves out of your lungs. During an asthma episode, your airways often narrow. A peak flow meter can measure this narrowing hours, even days, before you have any asthma symptoms.

Peak flow testing for asthma

In fact, a peak flow meter is so sensitive to changes in your airways that it can alert you to breathing issues even better than a doctor can when listening to your lungs with a stethoscope.

By taking your asthma medicine when your peak flow meter indicates changes – before you notice symptoms – you may be able to avoid a serious asthma episode.

A peak flow meter can also help you and your doctor:

  • Asthma Action Plan
    Decide if your Asthma Action Plan is working well
  • Adjusting Medicine
    Decide when to add, increase or even stop a medicine
  • seek emergency care
    Decide when to seek emergency care
  • identify asthma triggers
    Identify triggers (things that cause asthma symptoms)
  • Understand your asthma
    Better understand and talk about your asthma

Measure your peak flow at least once a day, usually in the morning before you take your asthma medicines. You might have to check it more often if you are having more episodes than usual, or if your reading is below 80 percent of your personal best. Check your Asthma Action Plan. Remember to always use the same peak flow meter.

How Do I Use a Peak Flow Meter?

  1. Place the indicator at the base of the numbered scale.
  2. Stand up.
  3. Take a deep breath.
  4. Place the meter in your mouth between your teeth and close your lips around the mouthpiece. Do not let your tongue block the hole in the mouthpiece.
  5. Blow out as hard and fast as you can in a single blow.
  6. Write down the number you get. But if you cough as you use the meter or make a mistake, don’t use that number. Blow into the meter again and get a new reading.
  7. Immediately repeat steps 1 through 6 two more times.
  8. Write down the highest of the three numbers in your asthma diary.

How Do I Find My Personal Best Number?

Your personal best peak flow number is the highest peak flow number you have over a two- to three-week period when your asthma is under good control. Good control is when you feel good and do not have any asthma symptoms.

Everyone’s asthma is different, so your best peak flow may be different from the average number for someone of your height, weight and gender. Your Asthma Action Plan needs to be based on your personal best peak flow number. 

To find out your personal best peak flow number, take peak flow readings:

  • At least two times a day for two to three weeks
  • When you wake up and between noon and 2 p.m.
  • Before and after taking quick-relief medicine
  • When you get a new peak flow meter, even if it is the same kind as you have used in the past
  • As instructed by your doctor

What Are the Peak Flow Zones?

Once you know your personal best peak flow number, your doctor will give you a range of numbers that tell you what to do. The peak flow numbers are put into a chart with zones that are set up like a traffic light. It is easiest if these target zones are added to your Asthma Action Plan. This will help you know what to do when your peak flow number changes. For example: 

80 to 100 percent of your personal best numberGreen Zone (80 to 100 percent of your personal best number) signals all clear. No asthma symptoms are present. Take your daily controller medicines (if prescribed), as usual. 


50 to 80 percent of your personal best numberYellow Zone (50 to 80 percent of your personal best number) signals caution. You should take a quick-relief medicine and check to see if your peak flow returns to the green zone. You may be having an asthma episode that requires an increase in your medicines or the addition of new medicines. Or your overall asthma may not be under control. Your doctor may need to change your Asthma Action Plan. Follow your Asthma Action Plan and seek additional medical care as indicated. 


below 50 percent of your personal best numberRed Zone (below 50 percent of your personal best number) signals a medical emergency. You must take your quick-relief medicine right away. Call your doctor immediately if your peak flow number does not return to the yellow or green zone and stay in that zone. If you cannot reach your doctor and your peak flow stays in the red zone, go directly to the emergency room. Your Asthma Action Plan should provide clear instructions.

What Are Some Important Things to Remember?

  • Write down your peak flow number in your asthma diary every day.
  • A decrease in peak flow of 20 to 30 percent of your personal best may mean the start of an asthma episode.
  • Your Asthma Action Plan may tell you to take your peak flow reading more often and to adjust your medicines.
  • Use only one meter. Be sure to bring it to your asthma check-ups.

A peak flow reading will only be as good as your effort. Numbers can be artificially high with tongue thrusts or spitting. Or they may be artificially low due to not enough effort or poor technique.

Medical Review December 2017.