NPSH calculator
Water at 200000 Pa absolute suction pressure, with a vapor pressure of 2000 Pa and a velocity head of 0.5 m, has just over twenty metres of NPSH available. Enter the five values and compare the result with the pump's NPSH required. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Suction pressure
- 200,000
- Vapor pressure
- 2,000
- Density
- 1,000
- Gravity
- 9.81
- Velocity head
- 0.5
- Pressure head
- 20.1834862385 m
- Npsh
- 20.6834862385
Net positive suction head (NPSH) is the extra pressure head, above the liquid vapor pressure, that remains at a pump suction to keep the fluid from flashing into vapor. This NPSH calculator converts absolute suction pressure, liquid vapor pressure, fluid density, gravitational acceleration, and a velocity head into NPSH in metres using the SI identity NPSH equals (P_s minus P_v) divided by (ρ times g), plus the velocity head.
What net positive suction head means and the SI identity
Net positive suction head is the margin of pressure head at a pump inlet above the vapor pressure of the liquid being pumped. When that margin collapses, local pressure at the impeller eye drops to P_v, vapor cavities form, and the machine cavitates: noise, vibration, eroded vanes, and a steep drop in developed head.
Search intent clusters around NPSH calculator, net positive suction head, NPSHa, pump cavitation head, and suction pressure head: people who already have station pressures and a density and want a numeric NPSH in metres they can compare with a manufacturer's NPSH required, not a unit converter and not a full system-curve solver. This landing evaluates that identity only. It does not look up vapor-pressure tables, estimate friction from a pipe schedule, or decide whether NPSH available exceeds NPSH required.
Keep units consistent: pascals with kilograms per cubic metre and metres per second squared yield metres.
Where an NPSH calculator is used in design, labs, and software
Process and mechanical engineers use available NPSH to decide whether a centrifugal pump will run without cavitation on a given suction vessel, flooded pit, or lift from a sump. A first-pass estimate from station absolute pressure, fluid vapor pressure at the pumped temperature, density, local g, and a measured or estimated velocity head is enough to compare with the pump curve NPSH required before a full pipe-loss model is built.
Software tests for pump-selection modules need a frozen expected NPSH for water at atmospheric suction and for a vacuum suction so that a refactor of a hydraulics library cannot silently change the hydrostatic conversion. Field technicians checking a boiler-feed or cooling-water pump can turn a suction-pressure transmitter and a density into metres of NPSH when the velocity head is known or conservatively set to zero.
Keep requests small, pin the rounded npsh in tests, and you get reproducible net positive suction head for homework, LMS plugins, and edge workers without standing up a pump-system solver of your own.
What you can do with it
Centrifugal pump suction checks
Convert a station absolute suction pressure, liquid vapor pressure, density, and velocity head into available NPSH in metres before comparing it with the pump's NPSH required.
FAQ
How do I calculate net positive suction head (NPSH)?
In SI, available NPSH in metres is (P_s − P_v) / (ρ g) plus the suction velocity head. Send absolute suction pressure and vapor pressure in pascals, density in kg/m³, gravity in m/s², and velocity head in metres.
Why must density and gravity be positive?
Both sit in the denominator of the pressure-head term.
Which units should I use for NPSH?
Pressures in pascals, density in kilograms per cubic metre, gravity in metres per second squared, and velocity head in metres. The returned npsh is in metres (unit m). The calculator does not convert bar, psi, or feet of head.
Does this include pipe friction or NPSH required?
No. It evaluates available NPSH from the five SI inputs only. Friction losses, static elevation z, and comparison with manufacturer NPSHr belong in a full pump-system calculation.
Should I enter absolute or gauge suction pressure?
Absolute. The formula takes the absolute pressure at the suction flange or impeller eye and subtracts the liquid's vapor pressure, both in pascals. The same rule applies to water at atmospheric suction or under vacuum.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/phys/npsh-net-positive-suction-head \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"suction_pressure":200000,"vapor_pressure":2000,"density":1000,"gravity":9.81,"velocity_head":0.5}'const res = await fetch("https://api.kit.forhosting.com/phys/npsh-net-positive-suction-head", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"suction_pressure": 200000,
"vapor_pressure": 2000,
"density": 1000,
"gravity": 9.81,
"velocity_head": 0.5
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/phys/npsh-net-positive-suction-head",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"suction_pressure": 200000,
"vapor_pressure": 2000,
"density": 1000,
"gravity": 9.81,
"velocity_head": 0.5
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/phys/npsh-net-positive-suction-head", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"suction_pressure":200000,"vapor_pressure":2000,"density":1000,"gravity":9.81,"velocity_head":0.5}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"suction_pressure":200000,"vapor_pressure":2000,"density":1000,"gravity":9.81,"velocity_head":0.5}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/phys/npsh-net-positive-suction-head", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"suction_pressure": 200000,
"vapor_pressure": 2000,
"density": 1000,
"gravity": 9.81,
"velocity_head": 0.5
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "phys.npsh_net_positive_suction_head",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
max_abs_suction_pressure | 1000000000000 |
max_abs_vapor_pressure | 1000000000000 |
max_abs_density | 1000000000000 |
max_abs_gravity | 1000000000000 |
max_abs_velocity_head | 1000000000000 |
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Density and gravity must be strictly positive: a non-positive density or g would divide by zero or invert the hydrostatic conversion, so those requests fail with invalid_input and are not billed. Successful API calls cost $0.002.
What net positive suction head means and the SI identity
Hydraulic textbooks write the available NPSH in metres as the pressure head corresponding to the difference between absolute suction pressure and vapor pressure, plus the kinetic contribution already expressed as a velocity head: NPSH equals (P_s minus P_v) divided by the product of density and gravity, plus v_head. P_s is the absolute pressure at the suction flange or impeller eye in pascals, P_v is the saturation pressure of the liquid at the actual temperature in the same unit, ρ is mass density in kilograms per cubic metre, g is gravitational acceleration in metres per second squared, and v_head is v squared over two g in metres.
Inputs, units, response fields, and invalid values
Preferred names are those canonical fields. Vapor pressure and velocity head must be non-negative: a negative saturation pressure is not physical, and velocity head is v squared over two g. Magnitudes above one trillion are rejected as outside the supported range. Failed validation is not billed. POST a JSON body with five required finite numbers: suction_pressure, the absolute suction pressure P_s in pascals; vapor_pressure, the liquid vapor pressure P_v in pascals; density, the fluid mass density ρ in kilograms per cubic metre; gravity, gravitational acceleration g in metres per second squared; and velocity_head, the suction velocity head in metres. Aliases ps, p_s, p_suction, suction, and pressure_suction are accepted for suction pressure; aliases pv, p_v, p_vapor, vapour_pressure, and p_vapour for vapor pressure; aliases rho, fluid_density, rho_kg_m3, and mass_density for density; aliases g, g_accel, and gravitational_acceleration for gravity; and aliases v_head, hv, kinetic_head, and velocity_head_m for velocity head, so spreadsheets and typed forms can reuse the same contract. Each value may arrive as a JSON number or as a decimal numeric string, including scientific notation; booleans, objects, arrays, empty strings, NaN, and infinities are rejected. Density and gravity must be strictly positive because they sit in the denominator of the pressure-head term; zero and negative values fail with invalid_input naming the field. Suction pressure may sit below vapor pressure; then pressure_head is negative and NPSH may be negative, which is a valid warning that the suction will cavitate rather than an input error. On success the body echoes the five inputs rounded to twelve decimal places, plus pressure_head in metres, npsh in metres, unit m, a formula string, and method npsh_available_si.
Where an NPSH calculator is used in design, labs, and software
Teaching labs ask students to convert a suction-gauge reading to absolute pressure, look up P_v for water at the lab temperature, and recover NPSH in metres; a deterministic oracle lets instructors generate keys for arbitrary P_s, P_v, ρ, g, and v_head without a commercial hydraulics suite.
Integration use cases
Hydraulics homework and lab keys
Generate deterministic NPSH answers for water and other liquids so students can check (P_s − P_v)/(ρ g) + v_head against manometer and vapor-pressure data.
Pump-selection software fixtures
Pin expected available NPSH in unit tests for atmospheric and vacuum suction without embedding a full pipe-loss or cavitation library.
Technical questions
What does the NPSH calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.