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NPSH Available Calculator

Estimate source-boundary net positive suction head available from absolute liquid-surface pressure, vapor pressure, static suction head, and suction-line losses.

Why it matters: Adequate NPSH margin is necessary to reduce cavitation risk, but the manufacturer NPSH-required curve and project margin remain controlling.

User inputs

Enter field measurements

8 inputs
Select I-P or SI units. Numeric entries are converted when the selection changes.
psia or kPa absAbsolute gas pressure acting on the suction liquid surface. This calculator uses a liquid-surface source boundary, not a pump-flange pressure measurement.
psia or kPa absAbsolute vapor pressure at the actual liquid temperature and concentration.
ft or mPositive when the liquid surface is above the pump datum; negative for suction lift.
ft or mFriction and fitting head loss from the liquid surface to the pump suction datum. Do not add a separate suction velocity-head term to this source-boundary form.
ratioSpecific gravity at pumping temperature.
ft or mNPSH required from the current manufacturer curve at the evaluated flow.
ft or mProject- or manufacturer-specific margin above NPSH required.

Calculated result

Engineering evaluation

Entered NPSH margin met
Net positive suction head available (NPSHa)39.8 ft
Absolute-pressure head above vapor pressure (Ha−Hvp)
32.8 ft
Margin above entered NPSH required (M)
27.8 ft
Entered required margin (Mreq)
3 ft

Engineering interpretation

Using the liquid-surface source-boundary method, estimated NPSH available is 39.8 ft, providing 27.8 ft above the entered manufacturer NPSH required. The entered project margin is 3 ft.

Important limitations

  • This is a liquid-surface source-boundary calculation. Do not enter pump-flange pressure or add a separate suction velocity-head term. Confirm the manufacturer NPSH-required definition, pump datum, operating flow, fluid temperature, dissolved gases, and project margin policy.

Motor-driven equipment

Estimate electrical demand and energy use

Use nameplate information and an operating schedule to screen motor input power, current, annual energy, and operating cost. Results update automatically.

Optional companion calculation
Choose the motor nameplate output-power convention.
Rated shaft output, not electrical input.
Used to estimate current; it does not change the hp-based energy result.
Use efficiency at the evaluated load point when available.
Use measured or defensible operating load rather than motor nameplate size alone.
Used only for the estimated current calculation.
Assumes all selected motors operate concurrently at the entered load.
Replace the profile with the actual occupied, process, or control schedule.
$/kWh
Use the applicable marginal utility energy charge; demand charges are not included.

Estimated result

Total motor input power (Pin,total)6.21 kW
Current per motor (I)
9.18 A
Total coincident current (Itotal)
9.18 A
Daily energy (Ed)
50 kWh/day
Annual operating hours (Hy)
2,080 h/year
Annual energy (Ey)
12,925 kWh/year
Annual energy cost (Cy)
$1,551/year

Equations and assumptions

Nameplate-based screening method

Pin=PratedLηmI=1000Pin3VPFEy=NPinhddyCy=Eyce

Horsepower is converted using 1 hp = 0.7457 kW. This estimate assumes steady average load and the entered operating-point efficiency. Voltage and power factor affect estimated current, but hp, load, efficiency, quantity, and schedule establish the energy estimate. VFD speed profiles, cycling, standby power, demand charges, and process variation require separate evaluation.

Technical basis: U.S. Department of Energy — Determining Electric Motor Load and Efficiency

Engineering knowledge center

Understand the calculation—not just the answer

2026-08-07 · Free Library
Governing relationships
Ha−Hvp = 2.31(Pa−Pv)/SG
NPSHa = (Ha−Hvp)+Hs−Hf
M = NPSHa−NPSHr
criterion M≥Mreq
Current substitution

NPSHa = 32.8+(10)−3 = 39.8 ft; M = 39.8−12 = 27.8 ft.

Calculation sequence

Step-by-step method

  1. Use absolute pressure at the suction liquid surface and convert the amount above vapor pressure to fluid head.
  2. Add static suction head, using a negative value for lift.
  3. Subtract source-to-pump suction losses and compare NPSH available with entered NPSH required and margin.

Equation legend

Inputs and calculated quantities

Pa
Absolute pressure at liquid surface — psia or kPa abs
Pv
Liquid vapor pressure — psia or kPa abs
Hs
Static suction head — ft or m
Hf
Suction piping losses — ft or m
SG
Fluid specific gravity — ratio
NPSHr
Manufacturer NPSH required — ft or m
Mreq
Required NPSH margin — ft or m
NPSHa
Net positive suction head available — ft
Ha−Hvp
Absolute-pressure head above vapor pressure — ft
M
Margin above entered NPSH required — ft
Mreq
Entered required margin — ft

Continue the evaluation

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Engineering calculation disclaimer

These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.

Read the full use and limitation statement