Tidal volumeAir breathed in or out in one quiet breath. It is the height of each small wave on the trace.
Inspiratory reserve volumeExtra air a deepest breath in adds. It lies above the top of a quiet breath.
Expiratory reserve volumeExtra air a deepest breath out pushes out. It lies below the bottom of a quiet breath.
Residual volumeAir left in the lungs after the deepest breath out. A simple spirometer cannot measure it.
Capacities 4
Vital capacityTidal volume plus both reserve volumes. It runs from the deepest breath in to the deepest breath out.
Total lung capacityAll four volumes added together. It is vital capacity plus residual volume.
Inspiratory capacityTidal volume plus inspiratory reserve volume. It starts at the bottom of a quiet breath.
Functional residual capacityExpiratory reserve volume plus residual volume. It is the air left after a quiet breath out.
How this list was compiled: Every name and every sum read on 2026-10-03 and kept only where two independent sources give it. (1) Wikipedia, Lung volumes and capacities, text and tables only, https://en.wikipedia.org/wiki/Lung_volumes_and_capacities : the four volumes, the four capacities and which volumes each capacity adds up; what a spirometer can and cannot measure. (2) Lutfi, The physiological basis and clinical significance of lung volume measurements, Multidisciplinary Respiratory Medicine 12:3 (2017), open access, https://europepmc.org/article/PMC/PMC5299792 : the same eight names on a spirometer trace, and that residual volume, functional residual capacity and total lung capacity cannot be measured with a simple spirometer. (3) Printed physiology texts read by full-text search on archive.org for the sums: https://archive.org/details/essentialsofphys0000mcla (1977), https://archive.org/details/lungphysiologicb0000unse (1986) and https://archive.org/details/isbn_9780073390567_0 (2006). (4) Wikibooks, Human Physiology, The respiratory system, https://en.wikibooks.org/wiki/Human_Physiology/The_respiratory_system : tidal, inspiratory reserve, expiratory reserve and residual volume and vital capacity in plain words. The chart is our own. Licence: own work
The drawing: Drawn for this site on 2026-10-03 from plain geometry. Own original chart; nothing traced or copied. The heights are schematic. Licence: own work
How it works
The trace shows quiet breaths, one deepest breath in and one deepest breath out. Dashed lines mark the levels it reaches. Each block between two lines is a volume; each tall dashed bar is a capacity.
Read the full guide
The volumes
A volume lies between two neighboring levels. The block beside the quiet breaths is the tidal volume. Above it is the inspiratory reserve volume and below it the expiratory reserve volume. Under the deepest breath out is the residual volume, the air that never leaves.
The capacities
A capacity is two or more volumes added together. Read each dashed bar against the column of blocks beside it: the bar is as tall as the volumes it is the sum of. The heights are schematic, not measured amounts.
Volumes never overlap. Capacities do: vital capacity contains inspiratory capacity.
A simple spirometer cannot measure residual volume, so it cannot give functional residual capacity or total lung capacity either.
Questions and answers
What are the four lung volumes?
Tidal volume, inspiratory reserve volume, expiratory reserve volume and residual volume. Tidal volume is the air moved in one quiet breath. The reserve volumes are the extra air a deepest breath in or out can move. Residual volume stays in the lungs.
What are the four lung capacities?
Inspiratory capacity is tidal volume plus inspiratory reserve volume. Functional residual capacity is expiratory reserve volume plus residual volume. Vital capacity is tidal volume plus both reserve volumes. Total lung capacity is all four volumes together.
What is the difference between a lung volume and a lung capacity?
A volume is one amount of air that does not overlap with any other volume. A capacity is the sum of two or more volumes.
Why can't a spirometer measure residual volume?
A spirometer measures air that moves in and out of the lungs, and residual volume is the air that cannot be breathed out. It is found by indirect methods such as helium dilution, nitrogen washout or body plethysmography.
How do I label the respiratory capacities on the diagram using the terms provided?
Find the levels of the trace first. Inspiratory capacity runs from the bottom of a quiet breath up to the deepest breath in, and functional residual capacity from that same bottom down to no air. The chart here is the site's own drawing.