Pulmonary & Lung Volume Calculators

The pulmonary calculators compute the four standard lung volumes used to interpret spirometry and lung-mechanics data: total lung capacity (TLC), vital capacity (VC), functional residual capacity (FRC), and inspiratory capacity (IC). Each is built from sums of the underlying compartment volumes — tidal volume (TV), inspiratory reserve volume (IRV), expiratory reserve volume (ERV), and residual volume (RV).

Use these together to classify restrictive vs obstructive lung-disease patterns, follow disease progression, and confirm spirometer readings against the underlying volume math.

When to use lung-volume calculators

Lung volumes describe how much air the lungs hold during the different phases of breathing. The four standard volumes — TLC, VC, FRC, and IC — are derived from spirometry plus a method that captures residual volume (most commonly helium dilution or body plethysmography). [REVIEW: confirm these are the standard RV-measurement techniques you want to cite].

These calculations come up in pre-operative pulmonary assessment, COPD and restrictive-disease staging, and spirometry interpretation. A reduced TLC alongside reduced VC suggests a restrictive pattern; an elevated FRC and RV with relatively preserved TLC suggest air trapping and an obstructive pattern.

Each calculator solves for any variable in its equation. If you know three of the four sub-volumes that sum to TLC, the calculator gives you the fourth — useful when one component is measured indirectly or being estimated.

Frequently Asked Questions

What's the difference between TLC and VC?

Total lung capacity (TLC) is the total volume of air the lungs can hold at maximum inhalation — every compartment summed. Vital capacity (VC) is the maximum volume a person can voluntarily exhale after maximum inhalation, which excludes the residual volume (RV) that always remains in the lungs. TLC = VC + RV.

How is functional residual capacity (FRC) measured?

FRC cannot be measured by spirometry alone because it includes the residual volume left in the lungs after a normal exhalation. The two standard techniques are helium-dilution (the patient rebreathes a known helium concentration until equilibrium) and body plethysmography (a sealed booth that uses Boyle's law to infer thoracic gas volume from pressure changes). [REVIEW: confirm these are the two techniques you want to highlight].

What does an elevated FRC mean?

An elevated FRC indicates air trapping — the patient is unable to fully exhale to a normal end-expiratory volume. It is a hallmark of obstructive lung diseases such as emphysema and severe asthma, and is often accompanied by an elevated RV and a hyperinflated chest on imaging. [REVIEW: confirm clinical interpretation].

What is the relationship between IC and IRV?

Inspiratory capacity (IC) is the maximum volume a person can inhale after a normal exhalation, so IC = tidal volume (TV) + inspiratory reserve volume (IRV). IRV is the additional volume above a normal tidal breath; IC is the total inhalation capacity from the relaxed end-expiratory position.

Why don't these calculators include residual volume on its own?

Residual volume (RV) cannot be calculated from spirometry — it has to be measured directly via helium dilution or body plethysmography. The calculators on this page use RV as an input where the formula requires it (TLC and FRC), but no calculator solves for RV from the spirometric volumes alone.