HomeCalculatorsPhysicsFluid MechanicsNPSH Calculator
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 CVP VERIFIED · CVP protocol 1.0.0-proposed · Engineering assurance · Open-tank and flange NPSHa, optional margin vs NPSHr, + O3 mpmath tabulated NPSHa.
Input interpretation
Enter values to calculate.
Result
—
Verified scope
Open-tank and flange NPSHa, optional margin vs NPSHr, + O3 mpmath tabulated NPSHa.
Assurance
Engineering
Declared partition coverage
PASS · 5/5 declared partitions (open_tank, flange, margin, awkward, invalid-domain) · Matrix
Deferred
Not pump power, not orifice C_d, not Darcy f, and not Manning n.
Numerical scope
O2: NPSHa vs a separate-module identity (≤2 ULP). Not pump power, not orifice C_d, not Darcy f, and not Manning n. ≤2 ULP vs O3 applies only to the published tabulated NPSH vectors. Not pump power, not orifice C_d, not Darcy f, and not Manning n.
Known limitations
  • Core CVP does not include live graph, viewport, or pointer interaction.
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 · Source checked · v1.0.0 · CVP VERIFIED · CVP protocol 1.0.0-proposed · Engineering assurance · Open-tank and flange NPSHa, optional margin vs NPSHr, + O3 mpmath tabulated NPSHa.· View Manifest · CVP overview · Specification
Versions
Calculation 1.0.0 · CVP protocol 1.0.0-proposed · Evidence 2026-09-26.o2-o3
Verification revision
2026-09-26.o2-o3 · 1/1 property · digest 6800deb08ffc
Legacy regression
14/14 tests · Production surface contract 6/6
Trust layers
Verification VERIFIED · Production CURRENT · Overall CVP state: VERIFIED under CVP 1.0.0-proposed
Reference
O1 model · O3 expected_values · O3 numerical_behavior · O2 expected_values · O2 numerical_behavior
Interfaces
PASS · UI (SSR) / REST / MCP — ui-ssr is query-result HTML, not a live browser session. Error-path engine·REST·MCP 1/1 (status, code, calculation_version). SSR compared on URL-canonical requested calculations; empty query is idle (not an error) and JSON-typed object/array inputs are REST/MCP-only.
Supplemental domain review
Not performed
Named expert review
Not performed
CVP suite
4/4 golden · 7/7 CVP boundary · 9/9 invalid · 1/1 property · 1/1 metamorphic · 8/8 O3 · 2/2 cross-interface · 6/6 CVP contract · Manifest
Sources
Sources
Evidence
2 legacy golden · 7 legacy boundary · legacy regression suite · 4/4 oracle-backed golden · 9/9 invalid · Artifact integrity PASS
This calculator CURRENT · Public schema 1.0.0 matches · Semantic contract ✓ · Production attested · Public/cache ✓ · Origin ✓
Semantic contract
PASS
Full verification

Manifest identity, reference classes, interfaces, suite, and production records.

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 6/6 · 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: Alternate Depth Calculator, Bernoulli Equation Calculator, Broad-Crested Weir Calculator, Cavitation Number Calculator, Chezy Formula Calculator, Colebrook Friction Factor Calculator, Concentric Reducer Calculator, Critical Depth Calculator . Explore all Fluid Mechanics.

→ Alternate Depth Calculator Compute both rectangular depths at the same specific energy from E, Q, and width b. Runs locally. Not specific energy from y+v; not specific force M; not hydraulic jump y2; not critical depth yc as the product. › → Bernoulli Equation Calculator Incompressible Bernoulli along a streamline: solve P2, v2, or h2. Runs locally in your browser. › → Broad-Crested Weir Calculator Compute rectangular broad-crested weir discharge Q = C_d (2/3)^{3/2} b √g H^{3/2}. Default C_d=0.85. Runs locally. Not a thin-plate weir, not Manning, not Q=Av, and not an orifice. › → Cavitation Number Calculator Compute the cavitation number σ = (p − p_v) / (½ ρ v²). Runs locally. Not NPSH, not Thoma NPSH/H, not Weber, and not Euler Eu=Δp/(ρ v²). › → Chezy Formula Calculator Compute open-channel mean velocity V = C √(R S). Runs locally. Not Manning n, not Q=Av, not Hazen–Williams, and not Darcy. › → Colebrook Friction Factor Calculator Solve the Colebrook–White equation for Darcy friction factor f from Re and ε/D. Turbulent pipes Re≥4000. Runs locally. Not laminar 64/Re and not a Moody-chart lookup. › → Concentric Reducer Calculator Compute concentric-reducer loss h = K v₂² / (2g) with K = 0.5 (1 − A₂/A₁) referenced to the downstream speed. Runs locally. Not sudden expansion, not gradual contraction α, not vena-contracta C_c, not catalog K, and not Q=Av. › → Critical Depth Calculator Compute rectangular critical depth yc = (q²/g)^{1/3}. Runs locally. Not a Froude wrap, not specific energy, not a hydraulic jump, and not Q=Av. ›

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.