A spirometer measures how much air your lungs can hold and how quickly you can push that air out. These two core measurements, reported in liters, reveal whether your airways are narrowed, your lung capacity is reduced, or both. The test is the most common way to evaluate lung function and plays a central role in diagnosing conditions like asthma and COPD.
The Two Main Measurements
Every spirometry test produces two key numbers. The first is forced vital capacity (FVC): the total volume of air you can blow out after taking the deepest breath possible. The second is forced expiratory volume in one second (FEV1): how much of that air you can force out in just the first second of blowing. Both are measured in liters.
On their own, these numbers don’t mean much. What matters is how they compare to predicted values for someone of your age, height, and sex. A result at or above 80% of your predicted value is generally considered normal. Between 70% and 79% is mild impairment, 60% to 69% is moderate, and anything below 50% is severe.
The Ratio That Reveals Airway Problems
The most diagnostically useful number from spirometry isn’t FVC or FEV1 alone. It’s the ratio between them: FEV1 divided by FVC. This ratio tells your doctor what proportion of your total air you can push out in that critical first second.
In healthy middle-aged adults, the ratio should be above 70%. A ratio below that threshold is the hallmark of obstructive lung disease, meaning something is narrowing your airways and slowing airflow. This is the primary way COPD is diagnosed. If, on the other hand, your FVC itself is low but the ratio stays normal, it suggests a restrictive pattern, where the lungs can’t fully expand. When both the ratio and FVC are low, doctors call it a mixed defect.
Additional Measurements on the Spirometer
Beyond FVC and FEV1, spirometry captures several other data points. Peak expiratory flow (PEF) is the maximum speed of airflow during the forced breath out, essentially the fastest moment of your exhale. Forced expiratory flow rates at 25%, 50%, and 75% of your breath show how air moves through your smaller airways in the middle portion of exhalation. These mid-expiratory flow rates can sometimes detect early small-airway disease before FEV1 drops noticeably.
Modern spirometers also produce a flow-volume loop, a graph that plots airflow speed against the volume of air being exhaled and inhaled. The shape of this loop gives clinicians a visual fingerprint. A scooped-out curve suggests obstruction. A symmetrically shrunken loop points toward restriction. Flattening on specific parts of the loop can flag problems like vocal cord dysfunction or upper airway narrowing.
How Predicted Values Are Calculated
Your spirometry results are always compared to what’s expected for someone with your characteristics. The Global Lung Function Initiative, which provides the reference equations used worldwide, factors in age, height, and sex. Older adults naturally have lower lung function than younger adults, and taller people move more air than shorter people.
Ethnicity has traditionally been included in these calculations because average lung volumes differ across populations. However, the GLI has recently developed race-neutral equations in response to growing evidence questioning whether race-specific references are appropriate. Some labs now use these universal equations instead.
Obstructive vs. Restrictive Patterns
The pattern your results fall into shapes everything that comes next. An obstructive pattern (low FEV1/FVC ratio, below 70%) means air has trouble getting out. Your airways may be inflamed, filled with mucus, or structurally narrowed. This pattern is characteristic of asthma, COPD, chronic bronchitis, and emphysema. In asthma, a follow-up test with a bronchodilator often shows improvement, which helps distinguish it from COPD.
A restrictive pattern (low FVC with a normal ratio) means your lungs can’t fill up as much as they should. This happens in conditions like pulmonary fibrosis, where scarring stiffens the lung tissue, or in neuromuscular diseases that weaken the muscles of breathing. Obesity and chest wall deformities can also produce restrictive numbers. Confirming a true restrictive defect usually requires additional testing beyond spirometry, such as measuring total lung capacity in a body plethysmograph (an airtight booth).
What Happens During the Test
Spirometry is straightforward. You sit upright, place your lips tightly around a mouthpiece, breathe in as deeply as you can, then blast all the air out as hard and fast as possible until your lungs feel completely empty. That forced exhale needs to last at least six seconds for the results to be valid. Most people repeat the maneuver at least three times so the technician can confirm the readings are consistent.
Some people feel lightheaded or dizzy from the effort, especially during repeat attempts. This is normal and passes quickly. The entire session typically takes 15 to 30 minutes.
How to Prepare
A few things can skew your results if you don’t plan ahead. You should avoid heavy exercise for at least 30 minutes before the test and not smoke at all on the day of testing. If you use inhalers, your doctor will likely ask you to hold them beforehand. Short-acting rescue inhalers should be stopped 6 hours before the test, while longer-acting inhalers need to be paused 24 to 36 hours in advance, depending on the medication. Your ordering physician will give you specific instructions.
Certain medical situations can make the forceful breathing maneuver unsafe. Because spirometry temporarily spikes pressure inside your chest, abdomen, and head, it’s typically postponed for four weeks after brain, chest, or abdominal surgery. Other situations that warrant caution include a recent heart attack (within one week), uncontrolled high blood pressure, a known cerebral aneurysm, a collapsed lung, or late-term pregnancy. Active respiratory infections like tuberculosis are also a reason to delay testing, both for your safety and to prevent transmission.
What Your Results Mean in Practice
Spirometry results land in one of a few categories. Normal results (FVC and FEV1 at or above 80% predicted, ratio above 70%) mean your lungs are moving air as expected. Mildly abnormal results often produce no noticeable symptoms, which is why spirometry sometimes catches problems before you feel them. Moderate or severe abnormalities almost always correspond to symptoms you’re already experiencing: shortness of breath, chronic cough, wheezing, or reduced exercise tolerance.
One test gives a snapshot, but repeated spirometry over time tracks whether your lung function is stable, declining, or improving with treatment. For people with asthma, it helps gauge how well their condition is controlled. For people with COPD, it helps stage the disease and monitor progression. For smokers without symptoms, it can reveal hidden damage that may motivate quitting before irreversible loss sets in.