Peak flow

Checking your peak flow every day can help you keep an eye on your asthma and could cut your risk of an asthma attack

Health advice > Managing your asthma 

Find out how to test your peak flow, what your scores mean, and how you can make the most of using peak flow to help you manage your asthma.

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What is peak flow?

Peak flow is a measure of how quickly you can blow air out of your lungs. You measure peak flow by blowing as hard and as fast as you can into a small hand-held device called a peak flow meter. 

Peak flow can pick up changes in your airways, sometimes before you have any symptoms. If your airways are tight and inflamed, your peak flow score will be lower than normal. This could be a sign your asthma is getting worse.

See your GP if:

you're having symptoms, or need to use your reliever inhaler three or more times a week, even if your peak flow scores are normal.

When to check your peak flow

Talk to your GP or asthma nurse about whether you need to monitor your peak flow, how often, and what to do when your scores change.

Monitoring peak flow is most useful at key times. For example, if you’re recovering from an asthma attack, or your treatment plan has changed.

You may be asked to keep a peak flow diary for a couple of weeks to help your GP confirm an asthma diagnosis. If you get a pattern of scores showing up and down variations which are not normal, it could indicate asthma including occupational asthma

“Some people with asthma don't notice changes in their peak flow scores when they have symptoms,” says Dr Andy Whittamore, Asthma UK's in-house GP. “Speak to your healthcare professional about why this might be, and how you can monitor your asthma safely.”

Understanding your peak flow scores

People get different peak flow scores depending on their age, height, and gender.

Peak flow can vary at different times of the day too. For example, it’s normal for your score to be slightly lower in the morning. The most important thing is whether your score is normal for you.

Keep a peak flow diary

To understand your own peak flow, you need to see a pattern of scores over time. So it’s important to note down your scores every day, whether you’re using peak flow all the time, or have been asked by your GP to use it for two or more weeks to see how your asthma is.

You can do this using our peak flow diary. Or you might want to use a calendar on your phone to note your scores.

You and your GP or asthma nurse can then look at the scores you’ve been getting and identify what score to expect when you’re well.

Your best score

The score that’s usual for you when you’re well is sometimes called your ‘best score’.

If you’re using your medicines as planned and following your action plan, you should stay close to your personal best score most of the time.

If you’ve had a change of medicines recently, a good peak flow score can reassure you that your new medicines are working well.

You need to record your peak flow for at least two weeks to get a personal best score. Ask your GP to re-assess your best score if it’s been a few years since it was checked.

Lower than your best score 

If you get a peak flow score that’s lower than your best score, it can act as an early warning sign and help you prevent an asthma attack.

It can also help you identify triggers, allergies, or infections that could be making your asthma worse. For example, you might notice lower peak flow readings when pollen’s high, or you've got a cold, or when you’re at work.

In your peak flow diary, there's a space to make a note if you've had symptoms or if you think there's a reason why your readings could be lower than normal.

Talk to your GP or asthma nurse about what low scores you need to look out for and what action you need to take. You can write this down on your asthma action plan. 

How much daily change is normal?

The scores you get, and any changes from them, are very personal to you. You and your GP can agree what changes you need to look out for and what action you need to take. 

It is usual to see slight variations, either between morning and evening scores or from one day to the next. Your GP or asthma nurse will be looking at percentages of your personal best score.

For example, a score which is within 20% lower than your usual personal best score is usually considered normal and nothing to worry about.

But a score which is 20-50% lower means you need to take action - see your GP or follow the instructions agreed in your action plan. 

If you do not have your action plan to hand take your reliever inhaler right away. Call your GP or 111 for an urgent assessment. If the reliever inhaler does not completely relieve your symptoms call 999.

How to use a peak flow meter

When you first get a peak flow meter, ask your GP, asthma nurse, or pharmacist to show you how to use it and how to take a reading.

Take these steps to make sure you use your peak flow meter correctly:

  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).
  7. Do this three times in a row so you get three scores (all three scores should be roughly the same).
  8. Use the highest of these scores to fill in your peak flow diary.

How to take your peak flow reading

Asthma + Lung UK nurse, Suzanne, demonstrates how to use your peak flow meter to get a reading.

Video: How to take your peak flow reading

Asthma + Lung UK nurse, Suzanne, demonstrates how to use your peak flow meter to get a reading.
Transcript for 'How to take your peak flow reading'

0:05 To take your peak flow reading make sure you’re either sitting in a chair or standing up

0:10 but always do one or the other.

0:13 Your peak flow meter has a small arrow and you need to push this right to the top near the mouthpiece.

0:21 Then hold your peak flow meter like so, so your fingers are not going over the arrow.

0:26 Take a deep breath in and then you blow hard and fast into the mouthpiece like this.

0:35 Wherever the arrow goes, next to which number, that would be your peak flow

0:38 score which you can then plot in your peak flow diary.

Related resources
Peak Flow Diary

Peak Flow Diary

Download a diary to track your peak flow.

Download (1 MB)
Adult Written Asthma Action Plan

Adult action plan

Download your written asthma action plan here.

Download (351 KB)

You’ll get the most useful results if you:

  • Check your peak flow at least twice a day, every day to get a useful pattern of results. Ask your GP how often and for how long, they would like you to take your peak flow readings.
  • Check it at the same time every morning and every evening. Remember, the earlier you do your peak flow test after waking up, the lower the score will be. This is because scores are naturally lower early in the morning.
  • Use your peak flow meter before you take your asthma medicine otherwise it will change the score.
  • Use your best effort every time you blow into the meter so you’re comparing like with like.
  • Use the same peak flow meter every time.

Top tips for using peak flow to monitor your asthma

  1. Write down your symptoms and what you’ve been doing too. For example, ‘I exercised on Tuesday’ or ‘I came into contact with a pet.’ There's space for you to do this in your peak flow diary. It helps you and your GP understand how different triggers affect your asthma and may help you prevent future dips.
  2. See your GP or asthma nurse if you’re having symptoms or you’re using your reliever inhaler three or more times a week, even if your peak flow score is good.
  3. Take your peak flow diary to your asthma review and any other asthma appointments to show your GP, asthma nurse, or consultant how your asthma has been
  4. Always use an asthma action plan alongside your peak flow diary so you know what to do if your asthma symptoms get worse. There's space on the plan for you to write down actions you and your GP have agreed if your peak flow score is lower than usual.
  5. If you’re finding it difficult taking your peak flow, or if you feel breathless or tight-chested afterwards, speak to your GP or asthma nurse.
  6. Keep your peak flow meter clean.

Where to get a peak flow meter

You can get a peak flow meter on prescription from your GP or asthma nurse. If you pay for your prescriptions, you'll need to pay the usual cost for one peak flow meter.

You can also buy a peak flow meter yourself from a pharmacy or online. You need to make sure it has an EU standard scale. And if you're buying one for a child, it should have a smaller mouthpiece and a lower scale.

Most peak flow meters come with a peak flow calendar to note your scores or your GP will give you one. You can also download our peak flow diary. 

For more advice about peak flow, call our Helpline on 0300 222 5800 (9am-5pm, Monday -Friday) to talk to a respiratory nurse specialist. Or you can WhatsApp them on 07378 606728.


Last updated November 2020

Next review due November 2023


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