Lung Volumes & Breathing

How much air fits in your lungs, and how much of each breath actually reaches the air sacs? From the four lung volumes get the capacities (vital capacity, total lung capacity and more) on a spirogram, the minute ventilation and the alveolar ventilation after the dead space. Add a spirometry result for the FEV₁/FVC ratio that separates obstructive from restrictive lung disease.

PhysiologyRespirationSpirometry

Inputs

Input examples

The tidal volume is one normal breath. From the top of a normal breath you can still breathe in the inspiratory reserve; from the bottom you can still breathe out the expiratory reserve. The residual volume always stays behind (it is measured with gas dilution or body plethysmography, not with a spirometer). The dead space is the air in the airways that never reaches the air sacs: about 150 mL in an adult, roughly 2 mL per kg. FEV₁ is the air blown out in the first second of a forced breath out, FVC the total; leave FVC empty to use the vital capacity. For education only — not medical advice.

Tidal volume V_T — mL

Inspiratory reserve IRV — mL

Expiratory reserve ERV — mL

Residual volume RV — mL

Breathing rate f — breaths per minute

Dead space V_D — mL

FEV₁ — L, optional

FVC — L, optional

Volumes, Capacities and Ventilation

The lungs hold four volumes that do not overlap: the tidal volume, the inspiratory and expiratory reserve volumes, and the residual volume. Adding two or more gives a capacity. The vital capacity is the biggest breath you can take out after the deepest breath in; with the residual volume it makes the total lung capacity. The functional residual capacity is the air left after a normal breath out: it keeps the oxygen level in the blood steady between breaths.

Breathing rate times tidal volume is the minute ventilation. But the first 150 mL or so of each breath only fills the windpipe and bronchi, the dead space, so the air that refreshes the air sacs is the alveolar ventilation. That is why fast, shallow breathing is inefficient: the same 6 L per minute brings far less fresh air to the air sacs.

In obstructive disease (asthma, COPD) the airways are narrowed: air comes out slowly, so FEV₁/FVC falls below 0.70 and air stays trapped (high residual volume). In restrictive disease (fibrosis, a stiff chest wall) the lungs are small but the airways are open: all volumes shrink and FEV₁/FVC is normal or even high.

adult TLCabout 6 L (less in women and children)

FEV₁/FVC < 0.70points to obstruction

dead space≈ 150 mL per breath

at rest12–20 breaths, 5–8 L/min

Spirogram

TLC

normal breaths, then the deepest breath in and out

residual volume: always stays in the lungs

Result

Enter the values above

For education only, not medical advice. These are simplified estimates compared with average adult values. Do not use them to diagnose or treat anyone; ask a doctor about real measurements.

Compared With a Healthy Adult at Rest

lung size depends on height, age and sex

Quantity	Your values	Typical adult

Worked Techniques

Capacities Are Sums

Every capacity contains at least two volumes. Any capacity that includes the residual volume (FRC, TLC) cannot be measured with a spirometer alone.

Example: 500 + 3000 + 1200 = 4700 mL vital capacity; with 1200 mL residual volume TLC = 5900 mL.

Subtract the Dead Space

Take the dead space off each breath before multiplying by the rate. Halving the tidal volume and doubling the rate keeps the minute ventilation, but the alveolar ventilation drops.

Example: 24 breaths of 250 mL: still 6 L/min, but only 24 × 100 = 2.4 L/min reaches the air sacs.

The Tiffeneau Ratio

A healthy person blows out most of the vital capacity in the first second. A ratio below 0.70 after a bronchodilator is the GOLD criterion for COPD.

Example: 3.8 / 4.7 = 0.81: normal.

Sources & Verification

References

J. E. Hall and M. E. Hall, Guyton and Hall Textbook of Medical Physiology, 14th ed., Elsevier, 2021 — the chapter on pulmonary ventilation: lung volumes and capacities, minute and alveolar ventilation, dead space.
J. B. West and A. M. Luks, West’s Respiratory Physiology: The Essentials, 11th ed., Wolters Kluwer, 2021 — ventilation and dead space; obstructive and restrictive patterns.
Global Initiative for Chronic Obstructive Lung Disease (GOLD), Global Strategy for the Diagnosis, Management and Prevention of COPD, 2024 report — post-bronchodilator FEV₁/FVC < 0.70.