Pump Power & NPSH Calculator
Power and energy, NPSH available, affinity laws and where the pump meets the system.
Try before you buy.
- Free preview: your inputs, the cavitation checks, the names of each table’s first rows and a watermarked chart, with the power, the NPSH and every figure hidden.
- Locked until you unlock it: download and copy.
- Unlock: Pro pass, ₹179 for 30 days or ₹79 for 1 week, a one-time payment that never renews. Unlock: Pro pass, $5.99 for 30 days or $2.99 for 1 week, a one-time payment that never renews.
Ways to unlock shows how to get the full result.
Printing this result is locked in the free preview.
Pump and system curves
How it was calculated
Locked in the free preview. Opens the ways to unlock this result.
Locked in the free preview. Batch runs unlock with a pass.
Locked in the free preview. Query results unlock with a pass.
Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the Pump Power & NPSH Calculator
Work out what a pump needs and what it does. Power: the hydraulic power ρgQH, the shaft power at the pump’s efficiency, the power drawn through the motor and any drive, a motor margin, the energy per year and its cost at your price per kWh (in your currency), and the specific speed — with the head typed in or worked out from suction and discharge gauge readings (ISO 9906 total head, with the velocity heads when you give the pipe bores).
NPSH available from the tank (open at your altitude, or closed at a pressure), the liquid’s vapour pressure at its temperature (from IAPWS-IF97 for water), the height of the liquid above or below the pump and the suction losses, checked against the pump’s NPSH required with the margin ratio or head you ask for, and the lowest liquid level that still meets it. Affinity laws for a new speed, a trimmed impeller or a scaled pump or fan (with a density change), and the operating point where a pump curve through three of its points crosses the system curve — also at a second speed, for a variable-speed drive.
Without a pass, the free preview shows your inputs, the cavitation checks, the names of each table’s first rows and a watermarked chart; the power, the NPSH, the other figures, the working, Copy result and the CSV come with a Pro pass.
How to use it
- Choose what to work out — power, NPSH available, the affinity laws or the pump and system curves — and SI or US customary units (every box with a unit has its own select; changing it converts what you typed).
- Pick the liquid: water at its temperature (density and vapour pressure from IAPWS-IF97) or your own density and vapour pressure.
- Enter the duty point and efficiencies; or the suction tank, the liquid level and the suction losses with the pump’s NPSH required; or the known point and the new speed or diameter; or three points of the pump curve and the system’s static and friction head.
- Read the answer, the checks and the table. Copy the result or download the CSV with a Pro pass, or after unlocking this result.
Examples
Water at 20 °C, pump 70 %, motor 90 %, 4,000 hours a year
Hydraulic 4.08 kW, shaft 5.83 kW, input 6.47 kW (63 % overall): 25,900 kWh a year — times your price per kWh
Check: ρgQH = 998.2 × 9.80665 × (50 ÷ 3600) × 30 = 4,079 W.
Sea level, water at 20 °C, surface 3 m below the pump, 0.5 m suction losses, NPSH required 4 m, margin ratio 1.3
NPSHa = 10.35 − 0.24 − 3 − 0.5 = 6.61 m: ratio 1.65, so it meets 1.3; the lift could grow to 4.41 m
As above, but the vapour pressure is 47.4 kPa
NPSHa = 2.16 m, below the 4 m required: the pump would cavitate. For the 1.3 margin the water surface must be at least 0.04 m above the pump.
100 m³/h at 50 m and 20 kW at 1,450 rpm, slowed to 1,160 rpm
80 m³/h at 32 m and 10.2 kW: flow × 0.8, head × 0.64, power × 0.512
Pump: 40 m shut-off, 36 m at 36 m³/h, 25 m at 72 m³/h. System: 15 m static and 8 m friction at 54 m³/h. Pump 70 %
Runs at 66.5 m³/h and 27.1 m (7.01 kW). At 80 % speed: 43.1 m³/h at 20.1 m (3.37 kW) — the static head makes the flow fall faster than the speed
Common uses
- Sizing a motor and estimating the running cost of a pump
- Checking whether a pump will cavitate on a suction lift or with hot water
- Working out the duty after slowing a pump with a drive, or trimming its impeller
- Finding the operating point of a pump in a system, and at reduced speed
- Turning a pump test’s gauge readings into total head and efficiency
- Pumps and hydraulic machines coursework, with each step of the working
Formulas
- Hydraulic power: P_h = ρ g Q H; shaft power P = P_h ÷ η_pump; input power P ÷ (η_motor × η_drive)
- Total head from gauges (ISO 9906): H = (p_d − p_s) ÷ ρg + (z_d − z_s) + (v_d² − v_s²) ÷ 2g
- NPSH available: NPSHa = (p_surface − p_vapour) ÷ ρg + z − h_f, with absolute pressures and z negative for a suction lift
- Standard atmosphere: p = 101,325 × (1 − 2.25577 × 10⁻⁵ h)^5.25588 Pa at an altitude h in metres
- Affinity laws (same shape): Q ∝ N D³, H ∝ N² D², P ∝ ρ N³ D⁵; for a trimmed impeller in the same casing, approximately Q ∝ D, H ∝ D², P ∝ D³
- Specific speed: N√Q ÷ H^(3/4) (rpm, m³/s, m; about 51.6 times larger in rpm, US gpm and feet)
- Operating point: the pump curve H = a + bQ + cQ² through your three points meets the system curve H = H_static + kQ²; at a speed ratio r the pump curve becomes a r² + b r Q + c Q²
NPSH required and the margin
Pump makers publish the NPSH required as NPSH3: the suction head at which the pump has already lost 3 % of its head to cavitation (the ISO 9906 and Hydraulic Institute test). At that point the impeller is cavitating, so the NPSH available must stay above it with a margin. ANSI/HI 9.6.1 recommends margin ratios by application and pump; take the ratio or the head margin from your pump maker or your specification and enter it here. Hot liquids need special care: water’s vapour pressure is 2.3 kPa at 20 °C but 47 kPa at 80 °C and 101 kPa at 100 °C.
Sources
- ISO 9906 Rotodynamic pumps — Hydraulic performance acceptance tests (total head, NPSH3, power and efficiency)
- ANSI/HI 9.6.1 Rotodynamic Pumps Guideline for NPSH Margin
- IAPWS-IF97, the industrial formulation for the properties of water and steam
- ISO 2533 standard atmosphere (pressure with altitude)
Limitations
- Efficiencies are yours to enter: a pump’s efficiency changes along its curve and with wear, and a motor’s with its load. Read them from the makers’ curves at the duty point.
- The operating point fits a smooth curve through three points; check it against the maker’s curve, especially beyond your last point.
- The affinity laws assume geometric and dynamic similarity; large trims, very low speeds and viscous liquids depart from them. The NPSH required does not scale simply with speed: ask the pump maker.
- NPSH available needs the suction losses at the actual flow; underestimate them and the margin is smaller than shown. Results are estimates, not a professional design — have a qualified engineer check anything that matters.
Privacy
On this device: nothing is sent
Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.
Frequently asked questions
What do I get without a pass?
Without a pass, Pump Power & NPSH Calculator shows your inputs, the cavitation checks, the names of each table’s first rows and a watermarked chart, with the power, the NPSH and every figure hidden. Until you unlock it, the result can’t be downloaded or copied. A Pro, Premium or Ultimate pass, a one-time payment that never renews, unlocks the full result. The pricing page lists the passes and their prices.
How do I work out the power a pump needs?
Multiply the density, g, the flow and the head: ρgQH is the power the liquid receives (4.08 kW for 50 m³/h of water at 30 m). Divide by the pump’s efficiency for the shaft power, and by the motor’s (and any drive’s) for the power from the supply. Choose a motor for the largest shaft power along the curve you may run at, plus your margin.
What is the difference between NPSH available and NPSH required?
NPSH available is what your suction system gives the pump: the absolute pressure on the liquid minus its vapour pressure, as head, plus the height of the liquid above the pump and minus the suction losses. NPSH required is what the pump needs, from its maker’s curve (NPSH3). Available must be greater than required, with a margin, or the pump cavitates.
How much does slowing a pump save?
In a system that is all friction, the affinity laws hold along the system curve: 80 % speed gives 80 % of the flow at about 51 % of the power. With static head the saving at the same speed is smaller and the flow falls further — in the example, 80 % speed drops the flow from 66.5 to 43.1 m³/h. Use the pump and system curves mode to see your own system.
Do the affinity laws work for fans?
Yes: flow follows the speed, pressure the square of the speed and power its cube, and pressure and power also follow the density, so a fan moving hot or high-altitude air needs less power. Enter both densities to include that.
Why does the calculator ask for the head from gauges?
On site you usually read a suction and a discharge pressure rather than the head. ISO 9906 defines the pump’s total head from them: their difference as head, plus the height between the gauges, plus the change of velocity head when the suction and discharge pipes differ in size.