Method: For an open or vented water source: NPSHA = atmospheric pressure head + suction static head − suction friction loss − vapor pressure head. Suction static head is positive when the liquid surface is above the pump suction datum and negative when the pump must lift water from below. Atmospheric pressure is estimated from site altitude using the standard-atmosphere relationship. Water vapor pressure is calculated from water temperature. Vapor pressure rises rapidly as water temperature increases, reducing NPSH available. If pump NPSHR is entered: NPSH margin = NPSHA − NPSHR. A positive mathematical margin alone does not establish an adequate pump design. The appropriate NPSH margin depends on pump type, operating point, service conditions and manufacturer or applicable Hydraulic Institute guidance. This calculator is intended for an open or vented water supply. Closed or pressurized vessels require the actual absolute pressure above the liquid and should be analysed separately.