A deflationary gap (or a recessionary gap) occurs when an economy's actual output (Actual GDP) is below its potential output at full employment (Potential GDP). The gap is the measure of this shortfall. Therefore, the formula is the difference between the potential level and the actual level.
\[ \text{Deflationary Gap} = \text{Potential GDP} - \text{Actual GDP} \]
An inflationary gap, conversely, occurs when Actual GDP exceeds Potential GDP.