NPSH, NPSHA and NPSHR: What's the Difference?

Maintaining a sufficient NPSH margin is key to preventing pumping problems.

May 16, 2024

How to Choose

Article Updated on: November 10, 2026

When sizing a centrifugal pump for a dewatering project, guesswork won’t cut it. You can try to use whatever pump you have on hand, but if you get it wrong, the system won’t function effectively, and the pump could experience damaging cavitation, which could bring pumping operations to a halt.

To size a pump properly and reduce the risk of cavitation, it’s important to understand net positive suction head (NPSH) and how to ensure a sufficient margin between the net positive suction head available (NPSHA) and the net positive suction head required (NPSHR).

What is the difference between NPSHA and NPSHR?

NPSHA (net positive suction head available) is the pressure your system actually supplies to the pump, based on site conditions and setup. NPSHR (net positive suction head required) is the minimum pressure the pump manufacturer specifies to avoid damaging cavitation. The NPSHA must stay comfortably above the NPSHR; this gap is the NPSH margin.

What are NPSH, NPSHA AND NPSHR?

NPSH describes the pressure at the pump’s suction port, measured in feet or meters. It’s broken down into:

NPSHA: How much pressure is available. Technically, the stagnation pressure that is available over the liquid’s vapor pressure.
NPSHR: How much pressure is required. Technically, the minimum pressure at a specified rate of flow and speed for a specified pumped liquid that causes a 3% drop in the pumped head.

Factor	NPSH (General)	NPSHA (Available)	NPSHR (Required)
Definition	Pressure at the pump's suction port, measured in feet or meters	Pressure your system actually supplies at the suction port	Minimum pressure the pump needs to avoid a 3% head drop
Source	Umbrella term covering both NPSHA and NPSHR	Calculated based on site conditions and pump setup	Provided by the pump manufacturer
Who determines it	N/A	Best calculated by a pump expert or engineer	Set during pump design and testing
Can it change on site	Varies by component	Yes; It’s affected by atmospheric pressure, liquid temperature, piping and flow rate	No; It’s fixed by the manufacturer for a given pump and flow rate
Role in NPSH margin	Forms the basis of the margin calculation (NPSHA minus NPSHR)	The "supply" side of the margin	The "demand" side of the margin
Risk if mismanaged	Insufficient margin between NPSHA and NPSHR increases cavitation risk	Low NPSHA relative to NPSHR increases cavitation risk	High NPSHR relative to NPSHA increases cavitation risk

What is cavitation?

Cavitation is the rapid formation and implosion of vapor bubbles in the liquid. An early sign is vibration. If cavitation gets worse, the pump will start making loud noises as shockwaves from the implosions hit the impeller, pitting the metal. In addition to potentially damaging the pump, cavitation reduces the pump’s capacity.

A lower head drop, such as 0% or 1%, might eliminate cavitation entirely, but the Hydraulic Institute Standard guidelines use a 3% head drop value with the goal of preventing damaging cavitation as opposed to achieving zero cavitation.

How do you calculate the NPSH margin?

The difference between NPSHA and NPSHR (NPSHA minus NPSHR) is known as the NPSH margin. Maintaining a sufficient NPSH margin is key to avoiding damaging cavitation.

How much margin is enough? A general recommendation is at least 3.3 feet or 10% of NPSHA, whichever is greater. ANSI/HI 9.6.1-2024 Rotodynamic Pumps – Guideline for NPSH Margin provides target margins for different applications and pump designs.

The NPSHR is provided by the pump manufacturer, so there’s no calculation for the pump user to perform. Performing an NPSHA calculation, on the other hand, takes some effort and is best left to a pump expert or engineer. NPSH margins are typically tight, and a small miscalculation could result in real issues with the pump and pump setup.

To calculate the NPSHA, the formula is:

Site atmosphere pressure or absolute suction pressure + static head (this number is positive for flooded suction applications and negative for suction lift applications) – friction + velocity head at suction port – vapor pressure at the pumping temperature.

In some cases, acceleration head is another factor. An example is positive displacement pumping that involves sudden acceleration of liquid.

How does the NPSHA change over time?

Changes in the operating environment or the pumping setup can change the NPSHA, so regularly monitoring a pumping operation is critical. Adjustments to the pump system design may be needed if a pump performance issue arises.

Factors that can affect the NPSHA include:

Changes in atmospheric pressure due to a low-pressure or high-pressure weather system
A drop in the influent rate that causes an increase in static head
An increase in the flow rate, which can lower NPSHA and raise NPSHR
Changes in the temperature of the liquid, which alter the vapor pressure
A drop in the liquid level, which can increase the suction lift and decrease the NPSHA
A change in the liquid’s viscosity or specific gravity, which can alter friction losses
A change in the length or diameter of piping or the number of fittings, which can also alter friction losses

The value of expert guidance

Improperly calculating the NPSH margin and allowing the margin to drop too low are common, and potentially costly, mistakes. Working with a temporary pump vendor that has in-house Professional Engineers, such as United Rentals’ Fluid Solutions team, is the easiest way to avoid them from the start.

The vendor can also help you troubleshoot pumping problems. For example, if a system loses performance due to an insufficient NPSH margin, they might recommend:

Operating the pump at lower speed or operating two pumps instead of one
Using larger-diameter suction pipes or shorter pipes and hoses
Surcharging the liquid level in the suction source or benching down the pumps to reduce static head
Using a submersible pump or other intake pump if the suction lift is too high

When choosing a pump and designing a pumping system, attention to detail can mean the difference between a smooth dewatering operation and problems that reduce efficiency, damage equipment and cause unplanned downtime. The NPSH margin is one detail that’s critical to get right.

Frequently asked questions

Who calculates the NPSHA and who provides the NPSHR?

The pump manufacturer provides the NPSHR as a fixed specification. The NPSHA depends on site conditions and is best calculated by a pump expert or engineer, since small miscalculations can create real performance issues.

What is a safe NPSH margin?

A general recommendation for the NPSH margin is at least 3.3 feet or 10% of NPSHA, whichever is greater. ANSI/HI 9.6.1-2024 provides target margins specific to your pump design and application.

Can the NPSHA change during a dewatering project?

Yes, the NPSHA can change during a dewatering project. Atmospheric pressure, liquid temperature, flow rate, liquid level and piping configuration can all shift NPSHA, which is why ongoing monitoring matters on active jobsites.

What happens if the NPSHA falls below the NPSHR?

If the NPSHA falls below the NSPHR, the pump risks cavitation, which is the rapid formation and implosion of vapor bubbles that cause vibration, loud noise, impeller pitting and reduced pump capacity, potentially halting operations.