Measuring peak flow

Peak flow is a simple measurement of how quickly you can blow air out of your lungs. It is used to diagnose and monitor asthma.

You measure peak flow by blowing as hard and fast as possible into a small hand-held device called a peak flow meter. It records airflow in litres per minute (L/min).

Why is peak flow measured?

Measuring your peak flow regularly can be useful for monitoring your asthma.

Changes in your score can show:

• If your condition is getting worse (sometimes before you have any symptoms)
• If a trigger has led to certain symptoms
• If your medicines are working

Some people don’t notice changes in their peak flow while experiencing symptoms. Speak to your healthcare professional to discuss monitoring your asthma safely.

How to measure your peak flow

It's important to take your peak flow correctly and consistently. This helps avoid incorrect readings.

If you have been advised to monitor peak flow at home, follow these steps:

1. pull the counter (the red arrow) back as far as it will go to the top near the mouthpiece
2. stand or sit upright (choose what’s most comfortable for you and always do it that way)
3. take the deepest breath you can
4. make sure your mouth makes a tight seal around the mouthpiece
5. blow as hard and as fast as you possibly can into the meter
6. write down your score (the number next to the pointer)

Repeat this three times in a row so you get three scores.

All three scores should be roughly the same. If they aren't, it might be that you haven't yet found the best technique.

Make sure your lips form a seal around the mouthpiece. Then blow as hard as you can.

It is common for people to be asked to monitor their peak flow for two weeks at a time. This can be very useful when recovering from an asthma attack or after a medication change.‍


You will get the most valuable results by checking your peak flow twice per day. Do this at the same time in the morning and evening. Give it your best effort every time.

What is a normal peak flow reading?

Normal peak flow score (also known as peak expiratory flow or PEF) will vary by age, sex and height.

It can also vary at different times of the day. It’s normal for your score to be slightly lower in the morning.

The most important thing is what’s normal for you. To understand what that might mean, you need to see a pattern over time. If you are using your medicines as prescribed, you should stay close to your best score most of the time.

If you get a peak flow score that's lower than normal, it can be an early warning sign. Some variation in score will be normal.

If your score is 20-50% lower than normal, you need to take action. Follow your asthma action plan or speak to your healthcare professional.