Peak Flow Calculator NHS
This independent UK peak flow calculator helps you compare a measured peak expiratory flow (PEF) reading with your own personal best and calculate peak-flow variability from a highest and lowest reading. Peak flow is measured in litres per minute (L/min) and is commonly used alongside other tests and a personalised asthma action plan.
Peak flow calculator
What is a peak flow test?
A peak flow test measures how quickly you can blow air out of your lungs. The NHS says it is mainly used to help diagnose and check asthma. Results vary between people and can depend on age, height, sex, time of day, effort and technique.
How to use this calculator
- Use a peak flow meter and perform the test correctly.
- Repeat the blow three times and record the highest reading.
- Enter that measured reading above.
- If you know your personal best, enter it to calculate the percentage of personal best.
- If you are tracking diagnostic variability, you can also enter a highest and lowest PEF from the same monitoring period.
How peak flow is measured
The NHS advises sitting or standing comfortably, resetting the meter, breathing in deeply, sealing your lips around the mouthpiece and blowing as hard and fast as possible. Repeat the test three times and record the highest result. Using the same technique and meter improves comparability.
Formula and methodology
Percentage of personal best = measured PEF ÷ personal best PEF × 100.
For the optional variability calculation this tool uses amplitude percentage mean: (highest PEF − lowest PEF) ÷ ((highest PEF + lowest PEF) ÷ 2) × 100. NICE recommends twice-daily PEF measurement for 2 weeks in certain diagnostic pathways when spirometry is unavailable or delayed, and states that amplitude percentage mean variability of 20% or more supports an asthma diagnosis in that pathway.
Worked example
If today’s best reading is 420 L/min and your established personal best is 500 L/min, the result is 420 ÷ 500 × 100 = 84% of personal best. Your own asthma action plan should tell you what action, if any, to take at a particular percentage.
Personal best versus predicted peak flow
Population-predicted values can be estimated from characteristics such as age, height and sex, but NHS guidance also emphasises what readings are normal for you. For home asthma monitoring, your established personal best and personalised action plan can be more useful than a generic predicted value. This calculator therefore does not invent a predicted-normal PEF from a simplified reference equation.
Important limitations
A single low peak flow reading does not diagnose asthma. NICE recommends objective testing within a clinical pathway, and regular PEF monitoring is not recommended for routine assessment of asthma control unless there is a person-specific reason, such as when it forms part of a personalised asthma action plan. If you are acutely short of breath, unable to speak normally, getting rapidly worse, or your reliever is not helping, follow your asthma action plan and seek urgent medical help.
Authoritative sources
- NHS: Peak flow test
- NICE NG245: Asthma diagnosis, monitoring and chronic asthma management
- NICE: documented personalised asthma action plan
Related calculators
NHS Calculator · NHS Blood Pressure Calculator · NEWS2 Calculator NHS · Anticholinergic Burden Calculator NHS · Heart Rate Calculator NHS · BMI Calculator NHS
FAQs
What unit is peak flow measured in?
Peak expiratory flow is usually recorded in litres per minute (L/min).
Which of three peak flow readings should I record?
The NHS advises repeating the test three times and recording the highest reading.
Can peak flow alone diagnose asthma?
No. Peak flow can form part of objective testing, but asthma diagnosis requires the wider clinical picture and the appropriate diagnostic pathway.
Should I compare my reading with my personal best?
If your clinician has established a personal best and included peak-flow thresholds in your asthma action plan, comparing with that personal best can be useful. Follow the action plan rather than generic internet thresholds.
Medical disclaimer: This independent calculator provides general information only. It is not an NHS diagnostic service and must not be used to change asthma medication without the instructions in your personalised plan or advice from a healthcare professional.