Q {\displaystyle Q} is the volumetric flow rate (e.g. CFM, GPM or L/s), D {\displaystyle D} is the impeller diameter (e.g. in or mm), N {\displaystyle N} is the shaft rotational speed (e.g. rpm), H {\displaystyle H} is the pressure or head developed by the fan/pump (e.g. psi or Pascal), and P {\displaystyle P} is the shaft power (e.g. W).[2]