Pump Horsepower Calculator
Flow and head to water, brake HP, and motor frame.
What it does
Pump motors fail two ways: too small and they burn out, too big and money burns instead. Water horsepower from flow times head sets the floor; efficiency and standard frames set the purchase. Three numbers end the guessing.
The tool above divides flow-head product by 3960 for water HP, adds 65 % pump efficiency for brake HP, and rounds up to standard motor frames. Metric mode mirrors it in liters, meters, and kilowatts.
DOE pumping guidance and Hydraulic Institute standards teach this same chain from hydraulic power through efficiency to motor selection. Head means everything resisting flow: lift plus friction plus pressure, all in consistent units.
Size for the worst credible duty point, not the average afternoon. Dirty filters, low tanks, and long pipe runs all raise head after install day, so confirm running amps and keep margin on the frame.
How to use
- Pick imperial (gpm, ft, HP) or metric (L/min, m, kW).
- Enter required flow.
- Enter total head in feet or meters.
- Read water, brake, and motor sizes.
- Buy the motor step at or above brake.
- Confirm amps, wire, and tank after install.
Formula
- flow
- Flow in gpm (L/min metric).
- head
- Total head in ft (m metric).
- 3960
- Divisor converting gpm-ft to HP.
Metric water kW equals 9.81 x L/min x head_m / 60000. Motor steps follow standard frames in each system. Efficiency and head estimates dominate accuracy.
Worked example
Example: 20 gpm at 100 ft
20 gpm against 100 ft of total head.
- Well pump needs 20 gpm at 100 ft of head.
- Water HP: 20 x 100 / 3960 = 0.51 HP.
- Brake at 65 % pump efficiency: 0.51 / 0.65 = 0.78 HP.
- Motor: first standard at or above 0.78 is 1 HP.
- Buy 1 HP or larger with matching voltage.
Reference table
| Imperial HP | Metric kW | Use |
|---|---|---|
| 0.5 | 0.37 | Small jet and sump |
| 0.75 | 0.55 | Large sump, small well |
| 1 | 0.75 | Common residential well |
| 1.5 | 1.1 | Deep well, pool |
| 2 and up | 1.5 and up | Irrigation and commercial |
Limits
- Planning math at fixed 65 % efficiency.
- Head estimates dominate; friction tables needed.
- No NPSH, water hammer, or controls logic.
- Confirm amps and wiring after install.
Common mistakes
- Sizing to water HP without efficiency.
- Forgetting friction in total head.
- Rounding the motor down to save money.
- Ignoring worst-case tank levels.
- Skipping amp checks after install.
Try it yourself
Quick checks. Answers sit tucked under each question.
Motor for 10 gpm at 50 ft?
Water is 10 x 50 / 3960 = 0.13 HP. Brake is 0.13 / 0.65 = 0.19 HP. Motor pick is 0.5 HP.
Motor for 37.9 L/min at 15.2 m?
Water is 9.81 x 37.9 x 15.2 / 60 / 1000 = 0.09 kW. Brake is 0.14 kW. Motor pick is 0.37 kW.
Questions
Water vs brake vs motor HP?
Water HP moves the water; brake HP adds pump losses at 65 % here; motor rating covers the brake with standard-size steps. Wiring and breakers follow the motor, not the water.
Where does 3960 come from?
3960 converts gpm-feet to horsepower (weight of water over time). Metric uses 9.81 times flow times head for kilowatts. Same physics, different units.
What counts as total head?
Vertical lift plus pipe friction plus pressure head, all in feet of water. Measure lift, estimate friction from tables, and convert psi at 2.31 ft each.
Why assume 65 % efficiency?
65 % wire-to-shaft planning figure for small end-suction and jet pumps. High-efficiency or larger units beat it; worn ones miss it. Makers publish curves for exact picks.
Is one duty point enough?
No. Flow and head swing with tank levels, filter dirt, and valve positions. Size for the worst credible case, then confirm amps after install.
Do metric motors match?
Yes for the math: 0.746 kW per HP with motor frames in each system. A 1 HP pick mirrors a 0.75 kW pick, which the metric example shows.