NPSHA is net positive suction head available which is defined as; Absolute Pressure head + Suction static head - Vapour pressure - Frictional losses in the piping, valves & fittings whereas NPSHR is net positive suction head required which is defined as; the NPSH at which the pump's total head (first stage head in multi stage pumps) has decreased by 3% due to resultant cavitation & low suction head within the pump