Lung Volumes Calculator

Calculate lung volumes, capacities, and identify obstructive vs restrictive patterns.

FEV1 (L) (L)

FVC (L) (L)

TLC (L, from body plethysmography) (L)

RV (L, residual volume) (L)

Predicted FEV1 (L) (L)

Predicted FVC (L) (L)

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The Two Patterns Spirometry Distinguishes

Nearly all spirometry interpretation begins with one question: is the problem getting air out, or getting enough air in? The FEV₁/FVC ratio separates the two.

Pattern	FEV₁/FVC	FVC	TLC	Typical causes
Obstructive	< 0.70	Normal or low	Normal or high	COPD, asthma, bronchiectasis
Restrictive	Normal or high	Low	Low	Fibrosis, obesity, neuromuscular
Mixed	< 0.70	Low	Low	Both processes present

The counterintuitive part is that obstructive disease often raises total lung capacity. Air trapping behind narrowed airways leaves the lungs hyperinflated, so TLC and residual volume increase even as the patient struggles to exhale. Restriction does the opposite — the lungs simply cannot expand.

Note that a ratio below 0.70 is a fixed threshold, but the ratio falls naturally with age. Using a lower limit of normal derived from reference equations avoids overdiagnosing obstruction in older adults.

Why Some Volumes Cannot Be Spirometered

Spirometry measures air that moves. Residual volume — the air remaining after maximal exhalation — never leaves the lungs, so it cannot be measured by spirometry at all. Any volume including RV must come from body plethysmography or gas dilution.

Volume	Definition	Measurable by spirometry?
Tidal volume	Normal quiet breath, ~500 mL	Yes
Vital capacity	Maximal inhale to maximal exhale	Yes
Residual volume	Remains after full exhalation	No
Total lung capacity	VC + RV	No — includes RV
Functional residual capacity	At end of quiet exhale	No — includes RV

This is why a diagnosis of restriction technically requires lung volume measurement. A low FVC on spirometry suggests restriction but can also result from poor effort or air trapping, and only a reduced TLC confirms it.

Worked Examples

Example 1: COPD: FEV1=1.8, FVC=3.2

FEV1/FVC=0.56 (<0.70)

Result: Obstructive pattern — COPD or asthma

FEV1% predicted determines severity staging

Example 2: Fibrosis: FEV1=2.0, FVC=2.4 (predicted 4.0)

FEV1/FVC=0.83 (normal); FVC=60% predicted

Result: Restrictive pattern

TLC <80% predicted confirms restriction

Example 3: Why TLC rises in COPD

Severe emphysema, FEV₁/FVC = 0.42, TLC 130% predicted

Result: Hyperinflation

Loss of elastic recoil and airway collapse trap air distally. The chest sits at a higher resting volume, which is why the barrel-chest appearance develops.

Example 4: Confirming restriction

FVC 55% predicted, ratio 0.85, TLC 62% predicted

Result: True restriction confirmed

The normal ratio rules out obstruction, and reduced TLC confirms the lungs cannot expand fully rather than the patient underperforming.

Common Mistakes

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Diagnosing restriction from spirometry alone

A low FVC suggests restriction but does not confirm it. Reduced TLC from plethysmography or gas dilution is required, since poor effort and air trapping also lower FVC.

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Using a fixed 0.70 ratio at all ages

The FEV₁/FVC ratio declines naturally with age. A fixed threshold overdiagnoses obstruction in older adults; the lower limit of normal from reference equations is preferred.

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Expecting TLC to be low in obstruction

Air trapping often raises TLC and residual volume in COPD. Hyperinflation is characteristic, not reduced capacity.

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Interpreting values without a bronchodilator test

Reversibility after bronchodilator distinguishes asthma from fixed COPD obstruction. A single pre-bronchodilator measurement cannot.

Frequently Asked Questions

GOLD staging for COPD?

Based on post-bronchodilator FEV1/FVC<0.70 + FEV1% predicted: GOLD 1 (≥80%), GOLD 2 (50-79%), GOLD 3 (30-49%), GOLD 4 (<30%). Also consider symptoms and exacerbations for treatment.

Obstructive vs restrictive?

Obstructive: FEV1/FVC<0.70 (air trapping, slow expiration). Restrictive: FEV1/FVC normal/high, TLC<80% predicted (can't fill lungs). Mixed: both features. Causes: obstructive=COPD, asthma; restrictive=fibrosis, obesity, neuromuscular disease.

What does FEV₁/FVC below 0.70 indicate?

Obstruction — air cannot be exhaled quickly. It is characteristic of COPD and asthma, though age-adjusted thresholds are more accurate than the fixed value.

Why can't spirometry measure total lung capacity?

Because TLC includes residual volume, which never leaves the lungs. Body plethysmography or gas dilution is needed.

Why does COPD increase total lung capacity?

Air trapping behind collapsed airways leaves the lungs hyperinflated, raising both residual volume and TLC even though airflow out is impaired.

How is restriction confirmed?

By a reduced total lung capacity with a normal or high FEV₁/FVC ratio. Low FVC alone is suggestive but not diagnostic.