# Critical pressure ratio The critical pressure ratio is defined as: $\frac{P_{c}}{P_u}=\left (\frac{2}{k+1} \right )^{\left ( \frac{k}{k-1} \right )}$ Where: $P_c$ is the critical pressure (bara) $P_u$ is the upstream pressure (bara) $k$ is the ratio of specific heats for an ideal gas [[Choked flow|Choking]] will occur when downstream pressure $\leqslant P_c$.