This calculator determines the absolute pressure at the pump impeller. NPSHA must exceed the NPSHR (net positive suction head requirement; specified by the pump manufacturer) or caviation and/or loss of prime will occur.

The site elevation is used to calculate the atmospheric pressure available. The friction pressure losses come from the suction (intake) line, and include the entrance, screen and other losses in bends. The vertical height is measured by the vertical distance between the free water surface and the impeller eye.