= Total lung capacity
{title2=TLC}
\b[Total lung capacity], TLC, is the volume of gas in the <lungs> after maximal inspiration. It combines <residual lung volume> with <vital capacity>, or equivalently <functional residual capacity> with <inspiratory capacity>:
$$
\mathrm{TLC}=\mathrm{RV}+\mathrm{VC}=\mathrm{FRC}+\mathrm{IC}.
$$
The equivalent subdivision is $\mathrm{TLC}=\mathrm{RV}+\mathrm{ERV}+V_T+\mathrm{IRV}$, using <expiratory reserve volume>, <tidal volume> and <inspiratory reserve volume>.<Standardisation of the measurement of lung volumes, 2023 update by Nirav R. Bhakta and others>[Definitions and subdivisions of lung volume; Figure 1]{ref}
A constructed example assigns 1.2 L to <residual lung volume>, 1.0 L to <expiratory reserve volume>, 0.5 L to <tidal volume> and 3.3 L to <inspiratory reserve volume>. Their sum gives TLC 6.0 L. The <vital capacity> is 4.8 L, <functional residual capacity> is 2.2 L, and <inspiratory capacity> is 3.8 L. Each identity gives the same total because the compartments share consistent endpoints.
A <spirometry> trace alone measures changes at the mouth; it does not establish the volume remaining after complete expiration. Adding an arbitrary constant to the entire gas-volume trace leaves every observed inhaled and exhaled increment unchanged. An independent estimate of <residual lung volume> or <functional residual capacity> is therefore needed to fix that offset. The measurement standard obtains capacities by combining an FRC measurement with linked volume maneuvers.<Standardisation of the measurement of lung volumes, 2023 update by Nirav R. Bhakta and others>[Background and purpose; Measurement of FRC]{ref}
This distinction also explains why <vital capacity> and <total lung capacity> are not interchangeable descriptors. Two hypothetical systems can both have a 4 L <vital capacity>, while having <residual lung volume> 1 and 2 L respectively. Their total capacities are then 5 and 6 L. The arithmetic separates the amount that can be exchanged during the maneuver from the amount present at full inspiration.
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