Water pump HP calculator: horsepower from flow and head
This water pump HP calculator works out the horsepower and kilowatts a pump motor needs to lift water at a given flow rate. Enter the flow, the suction lift and delivery height to the tank, and the pipe length and diameter; the calculator adds the friction loss in the pipe, applies pump and motor efficiency and a safety margin, and suggests the next standard motor size.
It answers the usual question behind required water pump horsepower: “how many HP do I need to fill an overhead tank, run a borewell line or feed a garden?”. If you already know the total head from a data sheet or a pressure gauge, switch to I know the total head. Pair it with the pipe size calculator to pick the pipe, and the electricity bill calculator to cost running hours.
The maths runs in your browser. Results are engineering estimates; the pump maker's duty curve and a qualified installer decide the final selection.
How to use the Water Pump HP Calculator
- Enter the flow rate you need and pick its unit (L/min, L/s, m³/h, gpm or L/h).
- Enter the suction lift (height from the water surface up to the pump) and the delivery height (pump up to the top of the tank or outlet).
- Enter the total pipe length and inside diameter, choose the pipe material, and keep or change the 20% allowance for bends and valves.
- Check the pump efficiency, motor efficiency and safety margin; overwrite them with data-sheet values if you have them.
- Read the HP and kW, the next standard motor size and the step-by-step table, then copy the summary.
Water Pump HP Calculator features
Head built up for you
Static head, Hazen-Williams pipe friction, fittings and outlet pressure add up to total head.
Two entry modes
Heights and pipe, or a total head you already know.
Efficiency and margin
Hydraulic, shaft and electrical input power shown separately.
Standard motor size
Rounds up to common sizes from 0.25 to 200 HP.
Flow in any unit
L/min, L/s, m³/h, US gpm and L/h.
The pump horsepower formula
Hydraulic power is P = ρ × g × Q × H, with water density ρ = 1,000 kg/m³, g = 9.80665 m/s², flow Q in m³/s and head H in metres, giving watts. Divide by the pump efficiency for shaft power, and by the motor efficiency for electrical input. One mechanical horsepower is 745.7 W (often rounded to 746 W).
Worked example: 60 L/min is 0.001 m³/s. Lifting it 20 m needs 1,000 × 9.80665 × 0.001 × 20 = 196.1 W (0.263 HP). At 50% pump efficiency the shaft needs 392 W; adding a 20% margin gives 471 W or 0.63 HP, so a 0.75 HP motor is the next standard size. A shortcut some installers use is HP = L/min × head (m) ÷ (4,500 × efficiency), which gives about 0.53 HP of shaft power before the margin.
Pipe friction comes from the Hazen-Williams equation in SI form, h = 10.67 L Q1.852 ÷ (C1.852 d4.87), checked against the US form on the Engineering ToolBox Hazen-Williams page on 19 October 2026. Total head is the vertical lift plus this friction plus any pressure you need at the outlet.
Typical efficiencies and sizes
These ranges are common rules of thumb, not measured values for your pump, so every one is an input.
| Item | Typical range | Note |
|---|---|---|
| Small centrifugal pump | 30 to 60% | Lower at low flow or when far from best-efficiency point |
| Large centrifugal pump | 70 to 85% | Use the data sheet |
| Induction motor | 80 to 92% | Rises with size |
| Safety margin | 10 to 25% | For wear, voltage dips and future demand |
Limits and tips
- Suction lift is limited by atmospheric pressure: a surface pump cannot lift water more than about 7 to 8 m in practice, and long or narrow suction pipes reduce that. Deep wells need a submersible or jet pump.
- Friction rises steeply when the pipe is narrow. If the friction line is large, use a bigger pipe before buying a bigger motor.
- The result assumes clean water at normal temperature. Viscous, hot or dirty liquids need different calculations.
- Three-phase and single-phase motors differ in starting current and available sizes; for cable and supply sizing use the electrical load calculator, and to convert units use the power converter.
Water Pump HP Calculator FAQ
How do I calculate water pump HP?
Multiply flow in litres per second by head in metres by 9.81 to get watts of hydraulic power, divide by the pump efficiency, add a safety margin, then divide by 745.7 for HP. The calculator does these steps and shows each one.
What is the water pump HP calculation formula?
HP = (Q × H × 9.81) ÷ (745.7 × efficiency) with Q in L/s and H in metres, or P(kW) = 0.00981 × Q(L/s) × H(m) ÷ efficiency.
How many HP pump do I need for an overhead tank?
It depends on the flow and the total height. For 60 L/min lifting 20 m, about 0.6 HP including losses and margin, so a 0.75 HP pump. Taller buildings or long pipe runs need more.
What head should I enter?
Use total head: suction lift plus delivery height, plus friction in the pipes and fittings, plus any pressure you need at the outlet. One bar of pressure is about 10.2 m of head.
Should I include a safety margin?
Yes, typically 10 to 25%, so the motor is not running at its limit. Choose the next standard motor size above the result.
Is 1 HP equal to 746 W?
Mechanical horsepower is 745.7 W; 746 W is the usual rounding. Metric horsepower (PS) is 735.5 W, a little less.