The leak flow rate is determined by dividing the pre-measured bag volume by the time required to fill the bag, with the fill time converted from seconds to minutes. This gives the volumetric flow rate of the leak in standard cubic feet per minute (scfm), at approximately standard atmospheric conditions.

The formula assumes that the bag is held snugly over the leak and that all leaked air enters the bag (no bypass). Fill times under 5 seconds reduce accuracy because reaction-time measurement error becomes significant; fill times over 60 seconds are generally unnecessary since a smaller bag should be used for those cases.

\begin{equation}\label{eq:bag-method-flow-rate} Q_{leak} = \frac{V_{bag} \cdot 60}{t_{fill}} \end{equation}

Symbols

\(Q_{leak}\)	Compressed air leak flow rate \([\unit{ \scfm}]\)
\(V_{bag}\)	Pre-measured volume of the measurement bag \([\unit{ \cubicFoot}]\)
\(t_{fill}\)	Time for the bag to fill with leaked air \([\unit{ \second}]\)
\(60\)	Seconds per minute conversion \([\unit{ \second\per\minute}]\)