The usable capacity is the difference between the air stored at the charging (inlet) pressure and the air remaining at the cut-out (outlet) pressure, expressed in standard cubic feet. The tank volume in gallons is first converted to cubic feet, then scaled by the pressure differential relative to atmospheric pressure.

\begin{equation}\label{eq:receiver-tank-usable-capacity} V_{usable} = \frac{V_{tank}}{k_{gal}} \cdot \frac{P_{in} - P_{out}}{P_{atm}} \end{equation}

Symbols

\(V_{usable}\)	Usable air storage capacity \([\unit{ \scf}]\)
\(V_{tank}\)	Tank volume \([\unit{ \gallon}]\)
\(k_{gal}\)	Gallons per cubic foot (7.48) \([\unit{ \gallon\per\cubicFoot}]\)
\(P_{in}\)	Charging (inlet) pressure \([\unit{ \psi}]\)
\(P_{out}\)	Cut-out (outlet) pressure \([\unit{ \psi}]\)
\(P_{atm}\)	Atmospheric pressure (14.7 psia at sea level) \([\unit{ \psi}]\)