Lung capacity is calculated by adding together four smaller air volumes that your lungs handle during breathing: tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume. In a healthy adult, these add up to roughly 6 liters of total lung capacity. You can estimate your expected values using prediction equations based on your height, age, and sex, or get precise measurements through clinical tests like spirometry and body plethysmography.
The Four Volumes That Make Up Lung Capacity
Your lungs don’t work as one big balloon. Pulmonologists break the air inside them into four distinct volumes, each representing a different phase of breathing:
- Tidal volume (TV): The amount of air you move in and out during a normal, relaxed breath. For most adults, this is about 0.5 liters.
- Inspiratory reserve volume (IRV): The extra air you can force in after a normal inhale, like when you take a deep breath on purpose. This is typically around 3 liters.
- Expiratory reserve volume (ERV): The extra air you can push out after a normal exhale, by actively squeezing your abdominal muscles. Usually about 1.1 liters.
- Residual volume (RV): The air that stays trapped in your lungs even after you blow out as hard as you can. Your lungs never fully empty. This is roughly 1.2 liters.
These four volumes account for all the air your lungs can hold. Every “capacity” measurement is just a combination of two or more of them.
The Key Formulas
Total lung capacity (TLC) is the simplest calculation. Add all four volumes together:
TLC = TV + IRV + ERV + RV
There’s a shortcut version too. Vital capacity (VC) is everything except the residual volume, meaning all the air you can voluntarily move. So the formula can also be written as:
TLC = VC + RV
A few other combinations come up frequently in lung testing:
- Vital capacity (VC): TV + IRV + ERV. This is the maximum amount of air you can exhale after the deepest breath possible. It’s the single most useful number from a breathing test.
- Functional residual capacity (FRC): ERV + RV. This is how much air sits in your lungs at the end of a calm, passive exhale, before you take your next breath.
- Inspiratory capacity (IC): TV + IRV. This is the maximum air you can inhale from a resting breathing position.
Clinicians often calculate TLC using the formula: TLC = mean FRC + mean inspiratory capacity. This approach is common in body plethysmography labs because it uses values the equipment measures directly.
Predicting Your Expected Lung Capacity
Your actual lung capacity depends heavily on your height, age, and sex. Taller people have larger rib cages and therefore bigger lungs. Lung elasticity decreases with age, which lowers several of the volumes. Men typically have larger lung volumes than women of the same height and age.
Prediction equations were developed by testing large groups of healthy, non-smoking adults and running regression analyses using height, age, and weight as variables. These equations produce a “predicted” value for each lung measurement. When you get a pulmonary function test, your results are reported as a percentage of your predicted value. Scoring 80% or higher is generally considered normal for most measurements.
You won’t typically calculate these equations by hand. Spirometry software does it automatically based on the demographic information entered before your test. But knowing the concept helps you understand your results: if your report says “FVC 92% predicted,” that means your forced vital capacity is 92% of what’s expected for someone your size, age, and sex.
How Spirometry Measures Lung Volumes
Spirometry is the most common lung function test and the one you’re most likely to encounter at a doctor’s office. You breathe into a mouthpiece connected to a device that measures airflow, then software calculates volume from those flow readings. Some older spirometers, like wedge bellows models, measure volume directly and calculate flow from it.
During the test, you’ll be asked to inhale as deeply as possible, then blow out as fast and as long as you can. This produces several key numbers:
- Forced vital capacity (FVC): The total air expelled during that forceful exhale.
- FEV1: How much air you blew out in the first second alone.
- FEV1/FVC ratio: The proportion of your total air that came out in that first second. This ratio is critical for distinguishing between types of lung problems.
- Peak expiratory flow (PEF): The fastest rate of airflow during the exhale.
Spirometry has one important limitation: it can only measure air that moves in and out. It cannot measure residual volume, the air permanently trapped in your lungs. That means spirometry alone cannot calculate total lung capacity.
Measuring What Spirometry Misses
To get a true total lung capacity reading, you need a test that can measure residual volume. Body plethysmography does this. You sit inside a sealed, transparent booth that looks like a phone box. The booth measures pressure changes as you breathe against a closed shutter, and from those pressure shifts, the equipment calculates the total gas volume inside your chest, including the air you can never voluntarily exhale.
Unlike spirometry, plethysmography doesn’t require forced breathing maneuvers. It captures total lung volume, residual volume, airway resistance, and the gas volume in your chest at rest. Once the equipment knows your functional residual capacity and inspiratory capacity, it calculates TLC by adding them together. Residual volume is then derived by subtracting your largest vital capacity from TLC: RV = TLC minus VC.
What the Patterns Tell You
Lung capacity numbers aren’t useful in isolation. Clinicians look at the pattern across multiple measurements to identify what type of problem exists.
An obstructive pattern, seen in conditions like asthma and COPD, shows a reduced FEV1 with a normal or only slightly reduced vital capacity. The FEV1/FVC ratio drops because air gets trapped behind narrowed airways and can’t come out quickly. The flow-volume curve has a characteristic scooped-out, concave shape.
A restrictive pattern, seen in conditions like pulmonary fibrosis or severe chest wall deformities, shows a reduced FVC with a normal or even elevated FEV1/FVC ratio. The lungs are smaller or stiffer, but the airways themselves are open. The spirometry trace looks normal in shape, just smaller overall.
How Body Weight Affects the Numbers
Excess weight compresses the lungs, particularly when you’re lying down or seated. The effect is most pronounced on functional residual capacity, which drops in a dose-dependent way with increasing body mass. Overweight individuals see FRC reductions of up to 10%. Mild obesity cuts it by about 22%, and severe obesity can reduce FRC by as much as 33%.
This happens because abdominal and chest wall fat pushes the diaphragm upward, reducing the space available for lung expansion. FRC, lung compliance, and the compliance of the entire respiratory system all decrease exponentially as BMI rises. Interestingly, chest wall compliance itself barely changes. The restriction comes almost entirely from the extra mass compressing the lungs from below and around the sides.
Standard spirometry measures like FEV1 and FVC are only mildly reduced in most people with obesity, and the FEV1/FVC ratio usually stays normal unless BMI exceeds roughly 62 kg/m². Where fat is distributed matters as much as how much there is. Someone carrying weight primarily in the abdomen will experience more mechanical compression on the lungs than someone with the same BMI but a different fat distribution pattern. This is one reason BMI alone is a limited tool for predicting how obesity will affect an individual’s lung function.