Calculate the net positive suction head available (NPSHa) at a pump inlet and compare it against the pump's required NPSHr to check the margin against cavitation. Enter atmospheric and vapor pressure, fluid density, static suction head, and suction-line friction loss — all in SI units.
Net positive suction head (NPSH) is the single most important quantity for keeping a centrifugal pump out of cavitation. A pump can only move liquid that arrives at its impeller above the liquid's vapor pressure; if the available suction head falls below what the pump needs, the liquid flashes to vapor, the bubbles collapse violently on the impeller, and performance, efficiency, and the pump itself all degrade. This calculator computes the NPSH available from the suction-side energy balance and compares it against the manufacturer's required NPSH so you can verify a safe margin.
NPSHa is the total absolute head at the pump suction, above vapor pressure, expressed in metres of the pumped liquid:
NPSHa = (P_atm − P_vapor)·1000 / (ρ·g) + h_static − h_friction
The pressure term converts the gap between the surface pressure on the suction source and the liquid vapor pressure (both kPa absolute) into metres of head, with ρ in kg/m³ and g = 9.81 m/s². h_static is the elevation of the liquid surface relative to the pump centerline (positive for flooded suction, negative for a lift), and h_friction is the friction and fittings loss in the suction line. The result is the energy cushion the liquid carries above the point at which it would boil.
NPSH required (NPSHr) is a property of the pump, measured by the manufacturer and published on the pump curve as a function of flow rate. It is defined as the suction head at which the pump head has already dropped by 3% due to incipient cavitation. To run safely you need NPSHa > NPSHr with a margin.
Margin = NPSHa − NPSHr. A common rule of thumb is a margin of at least 0.5–1.0 m (or 1.1–1.3 × NPSHr for demanding services). This calculator flags margin ≤ 0 as a cavitation risk, 0 to 0.5 m as marginal, and above 0.5 m as an adequate margin — but always check the specific service and the pump vendor recommendation.
Vapor pressure rises steeply with temperature, and it sits directly in the NPSHa equation: hotter liquid has higher P_vapor, which shrinks the pressure head and reduces NPSHa. This is why pumping near-boiling water, light hydrocarbons, or liquids close to their bubble point is so cavitation-prone. Always evaluate P_vapor at the actual pumping temperature, not at ambient. For a boiler feed or a column-bottoms pump operating at saturation, the surface pressure and vapor pressure can nearly cancel, leaving only static head and friction to provide NPSHa — which is why such pumps are mounted well below the vessel.
When the liquid level is above the pump centerline (a flooded or static suction), h_static is positive and adds head — the preferred arrangement for difficult or hot services. When the pump must draw liquid up from below (a suction lift), h_static is negative and subtracts from NPSHa, making cavitation far more likely. To improve a marginal NPSHa you can lower the pump, raise the source level, cool the liquid, shorten or enlarge the suction line to cut friction, or select a pump with a lower NPSHr or an inducer.
NPSHa (available) is a property of your system — the suction-side energy balance of pressure, elevation, and friction at the pump inlet, above vapor pressure. NPSHr (required) is a property of the pump, measured by the manufacturer as the suction head at which the pump head has fallen 3% from cavitation. Safe operation requires NPSHa to exceed NPSHr with a margin.
Cavitation occurs when the local pressure at the impeller eye drops below the liquid vapor pressure, so the liquid flashes to vapor. The bubbles then collapse implosively in higher-pressure regions, eroding the impeller, causing noise and vibration, and cutting head and flow. It happens whenever NPSHa falls below NPSHr — from hot liquid, a suction lift, a clogged strainer, excessive friction, or running far out on the curve.
A common minimum is 0.5–1.0 m of margin (NPSHa − NPSHr), but critical, high-energy, or hydrocarbon services often use 1.1–1.3 × NPSHr or follow standards such as API 610. The Hydraulic Institute provides margin guidance by service type. Use a generous margin where vapor pressure or flow can vary, and always confirm with the pump vendor.
Because vapor pressure increases sharply with temperature. In the NPSHa equation the (P_atm − P_vapor) term shrinks as P_vapor rises, directly reducing the available head. Water at 100 °C has a vapor pressure near 101 kPa, so an open system gains almost no head from atmospheric pressure — leaving only static head and friction. Always evaluate vapor pressure at the actual pumping temperature.
Raise the source liquid level or lower the pump to add static head (or convert a lift to a flooded suction), cool the liquid to drop its vapor pressure, reduce suction-line friction by shortening, straightening, or enlarging the pipe and clearing strainers, or pressurize the suction vessel. On the pump side, choose a unit with a lower NPSHr, add an inducer, or run two pumps in parallel at lower individual flow.