Net Positive Suction Head or NPSH for pumps can be defined as the difference between liquid pressure at pump suction and liquid vapor pressure, expressed in terms of height of liquid column. Suction head is the term used to describe liquid pressure at pump suction in terms of height of liquid column. When vapour pressure is also expressed in terms of equivalent height of liquid column, and subtracted from the suction head, the difference is npsh available at the pump suction.

where, h_{L} is the head loss between 0 and 1, p_{0} is the pressure at the water surface, p_{V} is the vapour pressure (saturation pressure) for the fluid at the temperature T1 at 1, Δz is the difference in height z1 − z0 (shown as H on the diagram) from the water surface to the location 1, and ρ is the fluid density, assumed constant, and g is gravitational acceleration.