Pump Head Calculator
Static lift plus losses, giving total head and shaft power.
Inputs
Result
Formula
| Velocity | Q ÷ A with Q in m³/s |
| Reynolds number | Re = vd ÷ ν, ν = 1.006×10⁻⁶ m²/s at 20°C |
| Friction factor | λ from Swamee-Jain, ε = 0.15 mm for steel |
| Friction loss | λ × (L ÷ d) × (v² ÷ 2g) |
| Minor loss | friction × share |
| Total head | static + friction + minor + outlet pressure |
| Shaft power | ρgQH ÷ η |
| Pressure | 1 MPa ≈ 102 m of water |
20 m static, 10 m³/h through 50 m of 50 mm bore with 10 m outlet pressure at 65% efficiency: 1.41 m/s, Re about 70,300, λ = 0.0283, 2.88 m friction, 0.86 m minor, 33.75 m total head and 1.41 kW shaft power — a 1.63 kW motor.
FAQ
Why does pump head always come up short?
Three reasons: sizing on storey height without pipe loss, where 50 m at 1.4 m/s costs 2.5 m and fittings push it to 3.5; ignoring residual pressure, since a shower needs 0.1-0.15 MPa or 10-15 m; and reading the shut-off head off the nameplate, which at the actual duty point is 20-30% lower.
Variable speed or fixed speed?
Variable speed where demand swings. It matches speed to load and power falls with the cube of speed, saving 30-50%, while a fixed-speed unit can only start and stop, wearing the motor and swinging the pressure. It costs 50-100% more up front and brings harmonics. Domestic water and chilled water favour variable speed; fire pumps must be fixed — codes do not allow it.
What causes cavitation?
When pressure at the suction falls below the vapour pressure of water, bubbles form and collapse in the high-pressure zone, sending shock waves at thousands of atmospheres into the impeller. It shows as noise, vibration, falling flow and a pitted, honeycombed impeller. Prevent it with a lower suction lift, a larger suction pipe, fewer inlet bends and adequate NPSH margin.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.