Peak Expiratory Flow Calculator

How this peak expiratory flow calculator works

Peak expiratory flow, usually shortened to PEF or peak flow, is the fastest rate at which you can blow air out after taking a full breath in. It is usually measured in liters per minute with a handheld peak flow meter. People with asthma often use peak flow readings to watch for airway narrowing before symptoms become severe. A lower-than-expected number can be an early warning that inflammation, bronchospasm, or poor control is developing, while a stable number near the expected range can support the idea that treatment is working. This calculator brings that comparison into one place by estimating a predicted PEF from your height, age, and sex, then comparing the estimate with the number you measured yourself.

The comparison matters because the same raw reading can mean very different things for different people. A reading that looks reassuring for one person may be concerning for another if their lung size and expected airflow are much higher. Height matters because taller people generally have larger lungs and airways. Age matters because lung elasticity and muscle performance change over time. Sex matters because average body dimensions and thoracic structure differ between male and female reference groups. A predicted value does not describe your entire health story, but it gives a useful baseline so your reading is not floating without context.

PEF zones at a glance

The zone table above follows the familiar traffic-light language used in many asthma action plans. Green suggests that airflow is close to expected. Yellow means caution: your airways may be narrowing, and it may be time to review symptoms, rescue medication use, or your clinician's action plan. Red signals a major drop in airflow and should be treated seriously. The exact advice tied to each zone depends on your personal plan, your diagnosis, and your clinician's instructions, but the categories remain useful because they translate a number into a practical level of concern.

This page uses simplified reference equations so the calculation can run instantly in your browser. Those equations estimate a predicted PEF in liters per minute. For men, the predicted value is:

$\mathrm{PEF}=5.48\times H-1.58\times A-14.4$.

For women, the relationship is:

$\mathrm{PEF}=3.72\times H-1.03\times A+2.24$.

In both formulas, $H$ is height in centimeters and $A$ is age in years. Once the predicted PEF is calculated, the script compares your measured value with the prediction by converting it into a percentage:

$\mathrm{Percent}=\frac{\text{Measured PEF}}{\text{Predicted PEF}}\times 100$.

That percentage is what determines the green, yellow, or red zone shown in the result box. The calculator does not diagnose asthma, chronic obstructive lung disease, or any other condition on its own. Instead, it answers a narrower question: how does this measured blow compare with a simple predicted baseline? That narrow comparison is surprisingly useful for self-monitoring because it turns a one-off reading into something you can interpret quickly.

To use the calculator, enter your height, age, and sex, then type the peak flow meter reading you measured in liters per minute. When you press the calculate button, the page computes the predicted value, the percent of predicted, and the corresponding zone. The result area shows all three outputs together so you can see not only whether you are in a green, yellow, or red category, but also how far your measured number is from the estimate. That extra context helps when you repeat readings over days or weeks and want to notice patterns rather than isolated events.

It also helps to understand how peak flow should usually be measured. In many home monitoring routines, you stand or sit upright, take as deep a breath as possible, seal your lips around the mouthpiece, and blow out as hard and fast as you can in a single sharp burst. People often repeat the test three times and record the best value. If you cough, hesitate before blowing, or let air leak around the mouthpiece, the number may come out lower than your true peak flow. In other words, technique matters almost as much as the formula when you are trying to make sense of a reading.

A quick worked example makes the process concrete. Imagine a user enters a height of 165 cm, an age of 40 years, selects female, and measured a peak flow of 420 L/min. The calculator first estimates predicted PEF using the female equation. It then divides 420 by that predicted value and multiplies by 100 to get the percent of predicted. If the final percentage is at least 80%, the result is green. If it falls between 50% and 79%, the result is yellow. If it drops below 50%, the result is red. Even without doing the arithmetic by hand, the example shows the logic: the same measured reading can land in different zones depending on the person's body size and age.

The most helpful way to interpret the output is to combine it with your usual baseline and symptoms. If the calculator shows a green-zone result and you feel well, that is generally reassuring. If it shows a yellow-zone result and you also notice chest tightness, cough, wheeze, or extra rescue inhaler use, that combination is more concerning than the number alone. If it shows a red-zone result, especially with marked shortness of breath, difficulty speaking, or worsening symptoms, that situation may need urgent medical attention. Numbers are valuable, but they work best when paired with how you actually feel and with any written action plan from your clinician.

There are also important limits to keep in mind. Reference equations vary by study population, country, and device type. Some clinicians rely more heavily on a person's personal best value than on a population-based predicted number, because personal best can better reflect that individual's normal lung function when their asthma is well controlled. That means a calculator like this should be treated as an educational and monitoring aid, not as the final authority on treatment decisions. The strongest use case is often trend tracking: repeating measurements over time to see whether peak flow is improving, stable, or falling.

Regular monitoring can reveal patterns you might otherwise miss. Some people notice worse morning readings, which can reflect overnight airway narrowing. Others see dips after exercise without proper pre-treatment, during viral infections, or when exposed to pollen, dust, smoke, or cold air. If you log the calculator result along with symptoms and triggers, you create a far richer picture than a single number can provide. Over time, that pattern can help you and your healthcare professional decide whether controller therapy is effective, whether triggers are avoidable, and whether your current plan is giving enough protection.

One more practical point: do not use a calculator result to delay urgent care in a dangerous situation. Severe shortness of breath, blue lips, confusion, faintness, or inability to speak full sentences are emergency warning signs regardless of any predicted value. A red-zone reading can support that concern, but the absence of a low reading does not rule it out if the measurement was performed poorly or if your symptoms are rapidly changing. Treat the number as a helpful guide, not as permission to ignore serious symptoms.

In short, this calculator translates height, age, sex, and a measured peak flow into a clearer interpretation. It tells you what a predicted value would be for the inputs you entered, shows how your measured result compares as a percentage, and assigns that percentage to a simple zone. Used thoughtfully, it can make peak flow monitoring more understandable, more consistent, and easier to discuss with a clinician. Because the calculation happens entirely in your browser, it is also fast and private: your inputs are processed locally on the page rather than being sent away for analysis.