Peak flow (or peak expiratory flow) is a measure of how fast you can blow air out of your lungs, taken with a small handheld device called a peak flow meter. It's commonly used by people with asthma to help monitor how well their lungs are working.
For someone with asthma, peak flow readings can give a useful picture of how well-controlled their condition is. Readings that drop below a person's usual 'best' can be an early sign that their asthma is worsening — sometimes before they notice symptoms — which can prompt them to follow their action plan. Tracking readings over time also helps reveal patterns and triggers, and helps healthcare professionals see whether treatment is working.
Peak flow is usually measured regularly (often morning and evening) and compared against your personal best and usual range, rather than a one-off number. Many people use it alongside a personal asthma action plan agreed with their GP or asthma nurse, which sets out what to do at different reading levels.
Peak flow monitoring is a tool used with medical care, not a substitute for it. If you have asthma, work with your GP or asthma nurse on an action plan, follow it, and seek help if your symptoms or readings worsen — in an asthma emergency, call 999. Our free peak flow diary helps you record readings. This is general information, not medical advice.