Standard volumetric flow (scfm) is defined at atmospheric pressure. The standard density is computed at the same temperature as the supply density but at atmospheric pressure only. Dividing mass flow (lbm/min) by standard density (lbm/ft³) yields the equivalent volumetric flow at standard conditions (scfm).

\begin{equation}\label{eq:orifice-method-standard-density} \rho_{std} = \frac{P_{atm} \cdot 144}{R_{air} \cdot T_R} \end{equation}

Symbols

\(\rho_{std}\)	Air density at standard atmospheric conditions \([\unit{ \pound\per\cubicFoot}]\)
\(P_{atm}\)	Local atmospheric pressure \([\unit{ \psia}]\)
\(144\)	Unit conversion: square inches per square foot \([\unit{ \squareInch\per\squareFoot}]\)
\(R_{air}\)	Specific gas constant for dry air (53.34 ft·lbf per lbm·°R) \([\unit{ \foot\lbf\per\lbm\per\degreeRankine}]\)
\(T_R\)	Air temperature in degrees Rankine \([\unit{ \degreeRankine}]\)