Inputs
Results
Enter values and click Compute to see results.
Example scenarios
| Scenario | Units | Alt | Hs | L | D | Q | ε | Ksum | NPSHr | Temp | Method |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1: Water, short pipe, flooded | SI | 0 m | +2 m | 5 m | 0.05 m | 5 m³/h | 0.000045 m | 2.0 | 3.0 m | 20 °C | Darcy |
| 2: Water, long suction, minor losses | SI | 500 m | -1 m | 25 m | 0.08 m | 12 m³/h | 0.00015 m | 8.0 | 4.0 m | 25 °C | Darcy |
| 3: Water, direct headloss | US | 0 ft | +4 ft | — | — | 10 gpm | — | — | 12 ft | 68 °F | Direct |
| 4: Water, Hazen–Williams | SI | 200 m | +1 m | 12 m | 0.06 m | 10 m³/h | — | — | 3.5 m | 30 °C | Hazen |
Formulas used
NPSH available (referenced to pump suction, head units):
NPSHa = (Patm − Pv)/(ρ g) + Hs − hf [+ v²/(2 g)]
Patm: absolute atmospheric pressure at site.Pv: fluid vapor pressure at temperature.Hs: static suction head (positive flooded, negative lift).hf: suction line head losses (pipe + fittings).v: velocity at suction nozzle if velocity head is included.
Darcy–Weisbach losses:
hf = ( f·L/D + Ksum ) · v²/(2 g), with
v = 4Q/(π D²) and Swamee–Jain: f = 0.25 / [log10( ε/(3.7D) + 5.74/Re0.9 )]², Re = ρ v D / μ.
Hazen–Williams (SI) for water:
hf = 10.67 · L · Q1.852 / ( C1.852 · D4.871 ) (Q in m³/s, D in m, L in m).
Atmospheric pressure by altitude (troposphere approximation):
P = 101.325 · (1 − 2.25577×10⁻⁵ · h)5.25588 [kPa] with altitude h in meters.
Water properties by temperature (empirical fits): density and dynamic viscosity; vapor pressure by Antoine equation (1–100 °C).
How to use this calculator
- Select unit system and fluid preset. For water, temperature determines properties automatically.
- Choose atmospheric input by altitude or enter absolute pressure.
- Enter static suction head and pick the friction method. Provide pipe data as needed.
- Set NPSH required and preferred safety margin (percent or absolute).
- Click Compute; review NPSH available, margin to required, and PASS/FAIL.
- Export your summary using CSV or PDF for documentation and review.
FAQs
Net Positive Suction Head is the absolute suction head above vapor pressure. Adequate NPSH prevents cavitation, protecting performance, efficiency, and pump reliability.
Some definitions include velocity head at the suction nozzle. Many manufacturers define NPSH at the pump suction, so including v²/2g is reasonable when comparing with curve values.
Darcy–Weisbach is general and accurate. Hazen–Williams is convenient for water. If detailed data is unavailable, enter a conservative direct head loss from design guidelines.
Typical margins are 0.5–1.0 m or 10–20% depending on service, variability, and uncertainty. Follow project standards and manufacturer recommendations for critical applications.
Yes. Choose Custom to input density, viscosity, and vapor pressure. Ensure units and properties reflect your fluid and temperature to avoid significant errors.
Quick reference
- 1 psi = 6.894757 kPa
- 1 ft = 0.3048 m
- 1 gpm = 0.0000630902 m³/s
- γ = ρ g, g ≈ 9.80665 m/s²
Assumes single suction line. Check unusual geometries and transients separately.