Reviewed by David Tinkelman, MD

A peak flow meter is a small, easy-to-use instrument that reveals how well your lungs are working. It does this by measuring your peak expiratory flow, which tells you how fast you can blow out air after a maximum inhalation. You use the peak flow meter to help you identify lung performance patterns, which give you information to prevent asthma episodes and develop your asthma management plan.


How it Works

First, you establish your "personal best," or the highest number you regularly blow. This helps you see when you have changes with your asthma, because it gives you something to measure against. Once you know you personal best, you and your doctor establish treatment rules or "zones.". You establish your personal best by recording the peak flow values for two weeks first thing in the morning before taking any medications and late afternoon when your asthma is under control. See an example peak flow zone chart.

  • If your peak flow is less than 80% of your personal best, you take your rescue medication, then wait 20 to 30 minutes and check your peak flow again.

    • If your peak flow is not back above 80%, report this to your doctor.

    • If your peak flow is back above 80%, recheck your peak flow about every 4 hours for a day or so. Call your doctor if you continue to need rescue medicine

  • If your peak flow is less than 60% consider this an emergency: Take your rescue medicine, and call your doctor or go to the emergency room right away.

How to Measure Your Expiratory Flow

Whenever you measure your flow, it is a good idea to write your peak flow numbers in a place where you can track them. Establish one central place to do this so you can more easily keep track of your numbers, such as in a peak flow sheet or Asthma Health Diary. Here's how to regularly measure your expiratory flow:

  1. Grab your peak flow recording sheet or health diary and a pen and record the date and time, along with any changes in how you feel, changes in your medicines, and/or anything you think may be making your asthma worse.

  2. Stand up or sit up straight.

  3. Slide the indicator to the base of the meter.

  4. Take in a deep breath.

  5. Place the mouthpiece in your mouth and seal your lips around it.

  6. Blow out as hard and fast as you can (one quick blow).

  7. Repeat that process 2 more times.

  8. Select the highest number of the 3 efforts.

  9. Record this number on your peak flow diary or on a graph.

Learn how to use a Piko peak flow meter.


When to Check Your Expiratory Flow

  • When the numbers do not change much from time to time: The peak flow number should be checked once a day (ideally in the morning when you wake up).

  • When you are doing well: Use the peak flow meter two times during the week and once on the weekend.

  • When you:

    • Begin to wake at night with asthma symptoms

    • Experience more daytime asthma symptoms

    • Have a respiratory infection (a cold)

    • You are sick or have asthma symptoms

    Check your peak flow number at least twice a day (once in the morning and once in the evening)

  • When you need to use "rescue medicine": This is medicine prescribed by your doctor to be used for quick relief of asthma symptoms. Check your peak flow before taking the rescue medicine, then check it again 20-30 minutes later.

Reporting Peak Flow Numbers to Your Doctor

Take your peak flow meter and your asthma health diary with you each time you visit with your doctor or nurse. If you have an Asthma Action Plan from your doctor, follow the plan for each peak flow zone, and compare your peak flow numbers to your personal best.

Signs Your Asthma is Getting Worse

Your peak flow meter is only an aide to you, so do not rely on your peak flow numbers alone when deciding whether to take your rescue medicine or call your doctor. In addition to measuring your peak flow on a daily basis, you should always look out for early warning signs of an asthma attack, which are:

  • Runny, stuffy nose
  • Fatigue
  • Chin or throat itches
  • Headache
  • Moodiness
  • Cough with activity or laughing
  • Wheezing with activity
  • Waking up at night or early morning with a cough or wheeze
  • Faster breathing rate
  • Irritability

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