In short: at sea level with 60°F water, a 10 ft suction lift and 2 ft of friction loss, available NPSH is about 21.3 ft. Against a pump needing 10 ft (NPSHr), that's an 11.3 ft margin — a healthy buffer against cavitation. Hot water and high altitude both shrink that margin fast.
This is a planning aid using standard atmosphere and vapor pressure models. For a critical or high-value installation, verify NPSHr against your pump manufacturer's actual published curve.
How to calculate available NPSH
NPSHa measures how much pressure cushion exists above the point where water would start turning to vapor inside the pump. It's built from two pressures and two head terms:
Worked example. Take the default case: sea level (Patm = 14.696 psi), 60°F water (Pvapor = 0.255 psi, ρ = 999.0 kg/m³), a 10 ft suction lift, and 2 ft of friction loss. The pressure term (Patm − Pvapor)/(ρg) works out to 33.34 ft. Add the −10 ft lift and subtract the 2 ft friction loss: 33.34 − 10 − 2 = 21.34 ft of NPSHa — matching the calculator's default result.
NPSHa is a property of your installation, not your pump. The pump only contributes NPSHr — the minimum NPSHa it needs at a given flow rate, from its manufacturer's curve. Sizing a pump correctly means checking that your site's NPSHa clears that pump's NPSHr with margin to spare, at the flow rate you actually intend to run.
NPSHa by altitude
Available NPSH at 60°F, 10 ft suction lift, 2 ft friction loss — atmospheric pressure is the only thing changing.
| Altitude | Atmospheric pressure | NPSHa | Margin |
|---|---|---|---|
| Sea level (default) | 14.696 psi | 21.34 ft | 11.34 ft |
| 1,000 ft | 14.173 psi | 20.13 ft | 10.13 ft |
| 3,000 ft | 13.171 psi | 17.82 ft | 7.82 ft |
| 5,280 ft · Denver | 12.100 psi | 15.35 ft | 5.35 ft |
| 8,000 ft | 10.916 psi | 12.62 ft | 2.62 ft |
NPSHa by water temperature
Available NPSH at sea level, 10 ft suction lift, 2 ft friction loss — vapor pressure is the only thing changing.
| Temperature | Vapor pressure | NPSHa | Margin |
|---|---|---|---|
| 40°F | 0.121 psi | 21.62 ft | 11.62 ft |
| 60°F (default) | 0.255 psi | 21.34 ft | 11.34 ft |
| 80°F | 0.506 psi | 20.84 ft | 10.84 ft |
| 100°F | 0.947 psi | 19.94 ft | 9.94 ft |
| 140°F | 2.882 psi | 15.72 ft | 5.72 ft |
| 180°F · boiler feed | 7.497 psi | 5.11 ft | −4.89 ft |
Worked examples
Shallow well pump
24.0 ft
Sea level, cold 50°F water, an 8 ft suction lift and 1.5 ft friction loss give 24.0 ft NPSHa. Against a small pump's 8 ft NPSHr, that's a comfortable 16.0 ft margin.
Boiler feed pump
8.1 ft
Near-boiling 200°F feedwater, even with a 2 ft flooded suction and low 1.5 ft friction loss, gives just 8.1 ft NPSHa. Against a 6 ft NPSHr pump, the margin is only 2.1 ft — marginal, and typical of why hot-water pumps need extra care.
Denver well pump
10.3 ft
At 5,280 ft elevation with a deep 15 ft suction lift, NPSHa drops to 10.3 ft. Against a pump needing 10 ft, the margin is razor-thin at 0.3 ft — this installation needs either less lift, less friction, or a lower-NPSHr pump.
What else affects the result
Suction pipe diameter. A narrower suction pipe raises both friction loss and velocity, both of which eat into NPSHa. Suction lines are usually sized generously — often one size larger than the pump's discharge — for exactly this reason.
Entrance and fitting losses. Foot valves, strainers, elbows and reducers on the suction side all add to friction loss beyond straight-pipe losses. Include their equivalent length when estimating the friction figure above.
Vapor pressure margin at other liquids. This calculator assumes fresh water. Other liquids — glycol mixtures, brine, or water with dissolved gases — have different vapor pressure curves and may need a wider safety margin.
Frequently asked questions
What is NPSH and why does it matter?
NPSH (net positive suction head) measures how much pressure margin exists above water's vapor pressure at a pump's suction inlet. If the pressure there drops to the vapor pressure, the water flashes into vapor bubbles that collapse violently downstream — cavitation — which damages the impeller and can stop the pump from working entirely.
What is the difference between NPSHa and NPSHr?
NPSHa (available) is what your specific installation provides — it depends on atmospheric pressure, water temperature, suction lift and pipe friction. NPSHr (required) is what a particular pump needs at a given flow rate to avoid cavitation, published on the manufacturer's pump curve. A safe installation needs NPSHa to exceed NPSHr with a margin.
How much NPSH margin is enough?
A commonly used rule of thumb is a margin of at least 3 ft (about 1 m), or roughly 20 to 35% above NPSHr, whichever is larger — though the Hydraulic Institute's ANSI/HI 9.6.1 standard gives more precise guidance by pump type and application. Always check your specific pump manufacturer's recommendation for critical applications.
Why does altitude reduce available NPSH?
Atmospheric pressure drops as altitude increases, and that atmospheric pressure is one of the two pressures driving water into the pump's suction. At 5,280 ft (Denver's elevation), atmospheric pressure is about 12.1 psi versus 14.7 psi at sea level — roughly 6 fewer feet of available NPSH before any other factor is considered.
Why does hot water make cavitation more likely?
Water's vapor pressure rises steeply with temperature — it's essentially zero when cold and equals full atmospheric pressure at the boiling point. Since NPSHa is driven by the gap between atmospheric pressure and vapor pressure, that gap shrinks fast as water heats up, which is why boiler feed pumps and hot water circulators are far more prone to cavitation than cold-water pumps.
How do I fix a pump with insufficient NPSH?
Raise the water source relative to the pump (or lower the pump) to add flooded suction head, use a larger-diameter suction pipe to cut friction loss, shorten the suction line and reduce fittings, lower the water temperature if possible, or select a pump with a lower NPSHr at your required flow rate.
Sources & references
- Hydraulic Institute. ANSI/HI 9.6.1 — Rotodynamic Pumps for Pump Piping.
- Stull, D.R. (1947). Vapor Pressure of Pure Substances. Industrial & Engineering Chemistry — source of the Antoine-equation vapor pressure model used here.
- ICAO Standard Atmosphere — barometric pressure vs. altitude model.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)