What is peak flow and how does a peak flow meter work?

A device to help diagnose asthma


A peak flow test measures how fast you can breathe out of your lungs. It is commonly used to help diagnose asthma alongside monitoring your condition to determine how well your asthma is being managed and whether or not it is getting worse. This is measured by blowing into a small, handheld device that measures the quality of your breath. A high score indicates that your airways are open and the lungs are healthy. A low score is a sign that the airways are tight and your asthma is not well controlled. 

What is peak flow? 

Your peak flow score is how many litres of air you can breathe out per minute. When you blow into the peak flow meter, the arrow will slide along the meter to give you your reading. Your doctor will measure this reading against the normal score for your age, gender and height. 

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How is peak flow measured? 

Peak flow is measured by using a peak flow meter. This is a small device that you blow into and it records your peak flow score. It may be necessary to do this on a daily basis to get an accurate picture of your lung functioning and asthma condition. In this case, you would be given a peak flow meter to take home and record your results each day. It's advised to measure your peak flow twice a day to get an accurate diagnosis or to find out what may be triggering your asthma. 

How do I use a peak flow meter? 

Using a peak flow meter is a straightforward process. To measure your peak flow at home begin either sitting or standing in an upright position. 

  • Reset the meter so that the arrow is set at the beginning of the scale 
  • Hold the meter in a horizontal position, making sure that your fingers are not covering the measurement scale 
  • Take a deep breath before placing the mouthpiece of the meter between your lips 
  • Breathe out as fast and hard as you are able to
  • Write down the number which the arrow lands on 
  • Repeat this process 3 times
  • The highest measurement that you have recorded is your peak flow score 

Your GP or nurse will show you how to use a peak flow meter and how to calculate your peak flow score. 

How is asthma diagnosed? 

A peak flow test is one of the factors used to diagnose asthma. Your doctor will also get you to do a spirometry test which measures how fast you can breathe out and how much air you can hold in your lungs. They will also ask you about your symptoms and whether there are triggers that make them worse and whether or not you have a family history of asthma or other respiratory conditions. 

Can a peak flow meter be used for COPD? 

A peak flow meter can be used to monitor your condition if you have COPD. This helps to determine whether or not your condition is getting worse, which can happen with COPD over time. 


NHS> Peak Flow Test
Asthma UK> Peak Flow Test

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