Peak Flow Calculator - Estimated Peak Expiratory Flow
Enter your sex, age and height to get your predicted peak expiratory flow (PEF) in litres per minute, calculated from the Nunn and Gregg 1989 reference equations for adults or the Godfrey formula for children under 15. Optionally add your measured reading to see what percentage of predicted it represents and which asthma control zone - green, yellow or red - you are in. If you know your personal best, enter that too for a second comparison.
Formula
Worked example
A 35-year-old male, 175 cm tall: height in metres = 1.75. PEF = ((1.75 x 5.48) + 1.58 - (35 x 0.041)) x 60 = (9.59 + 1.58 - 1.435) x 60 = 9.735 x 60 = 584 L/min. If he measures 500 L/min on his meter, his % of predicted = (500 / 584) x 100 = 85.6%, placing him in the green zone.
What is peak expiratory flow?
Peak expiratory flow (PEF), also called peak flow rate, is the fastest speed at which you can blow air out of your lungs after taking a full breath in. It is measured in litres per minute (L/min) using a small, portable device called a peak flow meter. PEF reflects how open or narrowed the large airways are: when the bronchi are inflamed or obstructed - as in asthma or chronic obstructive pulmonary disease - air moves more slowly and PEF falls. It is one of the oldest and most widely used measures in respiratory medicine, used both for diagnosing airway obstruction and for monitoring how well a treatment plan is working.
How predicted PEF is calculated
This calculator uses two evidence-based reference equations. For adults aged 15 and over, the Nunn and Gregg (1989) regression equations are applied, which were derived from a large UK population sample and are the basis of the EU EN 13826 standard scale used on most modern peak flow meters sold in Europe. For males, the formula is: PEF (L/min) = ((height in metres x 5.48) + 1.58 - (age x 0.041)) x 60. For females, the formula is: PEF (L/min) = ((height in metres x 3.72) + 2.24 - (age x 0.03)) x 60. For children aged 5-14, the Godfrey height-based formula is used: PEF (L/min) = ((height in cm - 100) x 5) + 100. This formula is appropriate for children because age and height are closely correlated in growing patients and the simpler equation performs well across the paediatric height range.
Understanding the traffic-light zones
Both the British Thoracic Society (BTS) and the Global Initiative for Asthma (GINA) use a three-zone system to help patients and clinicians interpret PEF readings quickly. The green zone (80-100% of predicted or personal best) indicates that airways are open and asthma is under good control - you should continue your normal management plan. The yellow zone (50-79%) signals caution: airway narrowing is occurring and you should follow your written asthma action plan, which usually means taking extra doses of your reliever inhaler and contacting your doctor if readings do not recover. The red zone (below 50%) is a medical emergency indicating severe airway narrowing; you should take rescue medication, call emergency services, and go to your nearest emergency department without delay.
Predicted vs personal best: which should I use?
Predicted PEF from a reference equation is the best starting point when you do not yet have a personal baseline. Once you have been well for at least two to three weeks, your highest consistent reading becomes your personal best and is a more reliable reference than the predicted value because it accounts for your own lung anatomy and any permanent changes from previous illness. Most asthma action plans are written using your personal best as the reference. If your personal best is significantly lower than predicted, it may indicate some loss of lung function that is worth discussing with your respiratory clinician. If it is higher than predicted, use your personal best as the reference because population equations do not account for above-average lung capacity.
How to use a peak flow meter correctly
Stand or sit upright, attach a clean mouthpiece to the meter, and reset the marker to zero. Take a full breath in, seal your lips tightly around the mouthpiece, and blow out as hard and fast as you can - a short, sharp blast rather than a long slow breath. Note the reading, reset the marker, and repeat twice more. Record the highest of the three readings, not the average. Measure at the same times each day if you are monitoring asthma control - usually on waking before taking any bronchodilator, and in the evening. Rinse the mouthpiece in cold water weekly and replace it every six months or after a respiratory infection.
Asthma peak flow zones
| Zone | % of predicted or personal best | Asthma control | Recommended action |
|---|---|---|---|
| Green | 80-100% | Good | Continue normal treatment plan |
| Yellow | 50-79% | Caution | Use rescue inhaler; contact your doctor if readings persist |
| Red | Below 50% | Medical alert | Seek emergency care immediately |
Traffic-light classification based on percentage of predicted PEF or personal best. Used by the British Thoracic Society and GINA guidelines.
Frequently asked questions
What is a normal peak flow reading?
There is no single universal normal value because PEF depends on sex, age and height. A 35-year-old male standing 175 cm tall has a predicted PEF of around 590 L/min, while a 35-year-old female of the same height would be around 430 L/min. Use this calculator to find the predicted value for your specific characteristics, or use your own personal best as your reference once established.
What does it mean if my PEF is in the yellow zone?
A yellow-zone reading (50-79% of predicted or personal best) means your airways are narrowing, often due to worsening asthma or exposure to a trigger. You should follow your written asthma action plan: this usually involves taking extra doses of your reliever inhaler (such as salbutamol) and monitoring your readings closely. If your PEF does not return to the green zone within a few hours or if it is dropping further, contact your doctor or seek urgent medical advice.
Why does my measured PEF change throughout the day?
Airway tone normally follows a circadian rhythm: PEF is typically lowest in the early morning (around 4-6 am) and highest in the early afternoon. In people with asthma or occupational lung disease, this diurnal variation can be exaggerated, with morning dips of 20% or more being a hallmark of poorly controlled asthma. A variation of more than 15-20% between your lowest and highest readings in a day suggests airway instability and is worth discussing with your doctor.
Can I use this calculator for my child?
Yes. For ages 5-14 the calculator automatically switches to the Godfrey height-based formula, which is appropriate for the paediatric age range. Children under 5 are generally not able to perform a reliable peak flow manoeuvre, so PEF monitoring is not routinely used below that age. Predicted values from reference equations are useful as a rough guide for children; as with adults, a personal best established when the child is well and at their best is the preferred reference for their individual action plan.
Is peak flow the same as FEV1?
No. Peak flow (PEF) is the maximum flow rate achieved in the very first milliseconds of a forced expiration - it reflects the calibre of the large upper airways. FEV1 (forced expiratory volume in one second) is the total volume of air expelled in the first second of a full forced breath, and is measured by spirometry. FEV1 is more sensitive to small-airway disease and is the gold standard for diagnosing and staging COPD and asthma, while PEF is used for day-to-day self-monitoring because a peak flow meter is simple, cheap and portable.
Why is my predicted PEF different from the chart on my peak flow meter?
Different peak flow meters and reference charts may use different scales or equations. Older meters often used the original Wright scale, while most modern meters sold in the UK and Europe use the EU EN 13826 scale, which is based on the Nunn and Gregg equations used in this calculator. The two scales give different numbers for the same individual, so you should always use the predicted values from the same scale as the scale printed on your meter.