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Physics calculator

NPSH Calculator

Compute NPSH available from an open tank or a suction flange, and compare with manufacturer NPSHr. Runs locally. Not pump power, not an orifice, not a weir, not Q=Av, and not Darcy.

Instant result
Result
—

Enter values to calculate.

Inputs—
Mode—
Formula—
Trust summary Engine tested · Source checked · 14/14 tests · Production surface contract 4/4 · v1.0.0
Input interpretation
Enter values to calculate.
Result
—
Model
Net positive suction head available (open tank or suction flange) and optional NPSHr margin.
Scope
NPSHa = (p_s − p_v)/(ρ g) + v_s²/(2g) + z_s; pressures absolute; SI
Verification
Engine tested · 14/14 tests · Production surface contract 4/4 · Source checked · v1.0.0
Named expert review
Optional · Not performed
Evidence
2 golden · 7 boundary · 5 property · Production surface contract 4/4 · Artifact integrity PASS
Production
Embedded snapshot: STALE · Last attested schema matched 1.0.0 snapshot / local build · Semantic contract ✓ · Last attestation PASS · current evidence changed · re-attestation required · Public/cache ✓ · Origin ✓ · Live production status PASS (0 stale; 163 CURRENT) @ 2026-09-17T06:27:56.932Z
Semantic contract
PASS

Formulas

Core equations used by this calculator.

FlangeNPSHa = (p_s − p_v)/(ρ g) + v_s²/(2g) + z_s
Open tankNPSHa = (p_atm − p_v)/(ρ g) + z − hf
Marginmargin = NPSHa − NPSHr
iSI. Pressures are absolute. Default ρ = 1000 kg/m³. Default g = 9.80665 m/s². Default p_atm = 101325 Pa. You supply p_v; this page is not a steam table. hf is an input, not a Darcy solve. This page is not pump power, not an orifice, not a weir, not Q=Av, and not Darcy–Weisbach.

How to use

1

Choose open tank or flange

Open tank uses surface elevation z and suction-line hf. Flange uses absolute p_s at the suction tap. This is not pump power.

2

Enter vapor pressure

p_v is absolute, in Pa, at the pumping temperature. Optional NPSHr from the manufacturer returns margin.

3

Read NPSHa

NPSHa is in m. Cavitation is flagged when NPSHa < NPSHr.

Example calculations

Common configurations with formula and result.

ϟ

Flooded water tank

p_v=2339 Pa, z=2 m, hf=0.5 m, default ρ and p_atm

NPSHa = (p_atm − p_v)/(ρ g) + z − hf
NPSHa≈11.594 m
ϟ

Suction flange

p_s=90 kPa abs, p_v=2339 Pa, v_s=2 m/s, z_s=0

NPSHa = (p_s − p_v)/(ρ g) + v_s²/(2g)
NPSHa≈9.143 m

NPSH calculator specification

Version 1.0.0 · Engine tested

Calculation status

Review policy · Evidence

Definition
Flange: NPSHa = (p_s − p_v)/(ρ g) + v_s²/(2g) + z_s. Open tank: NPSHa = (p_atm − p_v)/(ρ g) + z − hf. Margin = NPSHa − NPSHr.
What it calculates
Net positive suction head available (open tank or suction flange) and optional NPSHr margin.
Inputs
  • mode?
  • p_v
  • z|p_s
  • hf?
  • p_atm?
  • v_s?
  • z_s?
  • NPSHr?
  • rho?
Outputs
  • NPSHa
  • kind
  • NPSHr?
  • margin?
  • cavitation?
Formula
NPSHa = (p_s − p_v)/(ρ g) + v_s²/(2g) + z_s
Assumptions
  • NPSHa = (p_s − p_v)/(ρ g) + v_s²/(2g) + z_s; pressures absolute; SI
  • Open tank: p_s=p_atm (default 101325 Pa), z_s=z−hf, v_s=0; hf is an input
  • Not pump power, not orifice, not weir, not Q=Av, not Darcy, not a catalog NPSHr curve, not steam p_v(T)
Units
  • SI; pressures in Pa absolute; NPSH in m; ρ in kg/m³
Boundary conditions
  • missing p_v, or missing both p_s and z → MISSING_REQUIRED_INPUT
  • margin mode without NPSHr → MISSING_REQUIRED_INPUT
  • hf, v_s, p_v, or NPSHr < 0 → VALUE_OUT_OF_RANGE
  • rho, g, or p_atm ≤ 0 → VALUE_MUST_BE_POSITIVE
  • p_s with z/hf/p_atm, Q/H/P/η, C_d, A, L, f, or gauge pressure → MIXED_INPUT_ENCODING
  • pump, orifice, weir, Manning, Darcy, or Q=Av as mode → INVALID_MODE
Example
p_v=2339 z=2 hf=0.5 → NPSHa=(101325−2339)/(1000·9.80665)+2−0.5
Validation cases

3 published on this page · 14/14 tests · Production surface contract 4/4 · View evidence

  • p_v=2339 z=2 hf=0.5 → NPSHa≈11.594 m
  • p_s=90000 p_v=2339 v_s=2 → NPSHa≈9.143 m
  • p_v=2339 z=2 Q=0.05 → MIXED_INPUT_ENCODING
Sources
Calculation version
1.0.0

Background

Interpretation and common distinctions.

Compute net positive suction head available, with an optional manufacturer NPSHr margin.

Supported and not supported

Supported — open-tank NPSHa · suction-flange NPSHa · optional NPSHr margin / cavitation flag · API physics.fluid.npsh

Not supported — pump power or affinity, orifice discharge, weir overflow, Q=Av identities, Darcy–Weisbach (hf is an input), steam p_v(T), manufacturer catalog curves

Agent / API notes

Capability id: physics.fluid.npsh · tool id: npsh · pin 1.0.0.

{ "p_v": 2339, "z": 2, "hf": 0.5 }

Flange: { "p_s": 90000, "p_v": 2339, "v_s": 2 }. Margin: add "mode": "margin" and "NPSHr". Optional "rho" (default 1000) and "p_atm" (default 101325). Errors: MISSING_REQUIRED_INPUT, INVALID_NUMBER, VALUE_MUST_BE_POSITIVE, VALUE_OUT_OF_RANGE, MIXED_INPUT_ENCODING, INVALID_MODE.

Calculator URL stays /calc/physics/npsh. There is no /calc/fluid. NPSH is an industry acronym; the title expands it.

Other calculators in this family: Pump power, Darcy–Weisbach, Flow rate .

P Pump power › hf Darcy–Weisbach › Q Flow rate ›
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Frequently asked questions

Key distinctions behind the calculation.

Is this pump power?

No. Shaft and hydraulic power P=ρgQH/η is /calc/physics/pump-power. Sending Q, H, P, or η is MIXED_INPUT_ENCODING.

Does this compute Darcy hf?

No. Open-tank hf is an input. Pipe friction from a given f is /calc/physics/darcy-weisbach. Sending f or pipe L is MIXED_INPUT_ENCODING.

Do you look up p_v(T) or NPSHr from a catalog?

No. You supply vapor pressure and, if you want a margin, manufacturer NPSHr. Steam tables and pump curves are out of scope.

Where does this run?

Locally in the browser by default. REST and MCP call the same fluid-engine npshAvailable wrapper.