In short: a pump moving 100 GPM against 100 ft of head at 70% efficiency needs about 3.61 brake horsepower, which rounds up to a standard 5 hp motor. The exact figure scales directly with flow, head and the fluid's specific gravity, and inversely with pump efficiency — use the calculator above for your own duty point.
This is a planning aid for selecting a starting motor size. For a final purchase, confirm against the manufacturer's pump curve and consult a licensed engineer for critical or code-governed installations.
How to calculate pump horsepower
Pump horsepower sizing works in two stages. First, find the pure hydraulic energy the water needs — the water horsepower (WHP). Then divide by the pump's efficiency to find the larger figure the shaft actually has to deliver — the brake horsepower (BHP), which is what your motor is sized against.
BHP = WHP / η Q is flow in US gallons per minute, H is total dynamic head in feet, SG is the fluid's specific gravity (1.000 for fresh water), and η is pump efficiency as a decimal. The constant 3960 converts gallons, feet and minutes into horsepower.
This calculator converts whatever flow and head units you enter into GPM and feet, applies the fluid's specific gravity, and returns both figures — plus the standard NEMA motor size that comfortably covers your brake horsepower.
Worked example. A pump moves 100 GPM against 100 ft of total dynamic head, pumping fresh water (SG 1.000) at 70% efficiency. Water horsepower is (100 × 100 × 1.000) / 3960 = 2.53 hp. Brake horsepower is 2.53 / 0.70 = 3.61 hp. The nearest standard motor size at or above 3.61 hp is 5 hp. At a typical 90% motor efficiency, electrical input power is (3.61 × 0.7457) / 0.90 = 2.99 kW.
Why not just wire in the exact brake horsepower? Motors are only built in standard steps (1, 1.5, 2, 3, 5 hp and so on — see Pump Sizing Calculator for the full pump-and-motor picture). Sizing up to the next standard step keeps the motor away from its continuous rating limit and leaves headroom for minor curve or friction changes over time.
Why pump and motor efficiency change the answer
No pump converts 100% of shaft power into moving water — friction in the impeller, internal recirculation and mechanical losses all eat into it. That lost energy still has to come from somewhere, which is why brake horsepower is always higher than water horsepower. A pump running at 50% efficiency needs almost twice the shaft power of one running at 90% efficiency, for the identical flow and head. You can check your own pump's efficiency, or work backward from a known motor size, with the Pump Efficiency Calculator.
Motor efficiency is a second, separate loss stacked on top: it only affects how much electricity the motor draws from the wall to deliver that brake horsepower, and has no effect on the pump's own performance. That's why it's tucked into the calculator's advanced section rather than the main inputs.
Brake horsepower by flow and head
Required brake horsepower for fresh water at a typical 70% pump efficiency. Use this as a quick sanity check against the calculator above.
| Flow (GPM) | 50 ft head | 100 ft head | 150 ft head | 200 ft head |
|---|---|---|---|---|
| 10 | 0.18 | 0.36 | 0.54 | 0.72 |
| 25 | 0.45 | 0.90 | 1.35 | 1.80 |
| 50 | 0.90 | 1.80 | 2.71 | 3.61 |
| 100 | 1.80 | 3.61 | 5.41 | 7.22 |
| 200 | 3.61 | 7.22 | 10.8 | 14.4 |
| 500 | 9.02 | 18.0 | 27.1 | 36.1 |
Typical pump efficiencies by type
Starting points if you don't have a manufacturer's curve. Always confirm against the actual pump's published performance data.
| Pump type | Typical efficiency |
|---|---|
| Small submersible / open impeller | 40–55% |
| Standard end-suction centrifugal | 60–75% |
| High-efficiency end-suction | 75–85% |
| Multistage vertical turbine | 65–80% |
| Positive-displacement (rotary/piston) | 80–90% |
Worked examples
Residential well pump
1.46 BHP → 1.5 hp motor
A submersible well pump delivering 25 GPM against 150 ft of total dynamic head at 65% efficiency needs 1.46 brake horsepower — the next standard motor size is 1.5 hp, drawing about 1.28 kW at 85% motor efficiency.
Farm irrigation pump
10.1 BHP → 15 hp motor
An irrigation pump moving 500 GPM against 60 ft of head at 75% efficiency needs 10.1 brake horsepower. Because that falls between the 10 hp and 15 hp standard sizes, it rounds up to a 15 hp motor, drawing about 8.37 kW at 90% motor efficiency.
Seawater intake pump
9.14 BHP → 10 hp motor
An intake pump moving 200 GPM of seawater (SG 1.025) against 120 ft of head at 68% efficiency needs 9.14 brake horsepower — about 2.5% more than the same duty point in fresh water — rounding up to a 10 hp motor.
What else affects the horsepower you need
Getting total dynamic head right. Underestimating TDH is the most common sizing mistake — it hides in friction losses that grow with pipe length and flow velocity. Work through the components with the Total Dynamic Head Calculator before trusting a horsepower figure.
Net positive suction head (NPSH). Even a correctly sized pump can cavitate — and lose efficiency or fail early — if suction conditions aren't adequate. Check available NPSH with the NPSH Calculator alongside your horsepower sizing.
Variable-speed operation. Running a pump below full speed with a VFD cuts power roughly with the cube of speed, so a system that spends most of its time at partial flow can use a much smaller average horsepower than its peak rating suggests — see the Variable-Speed Pump Savings Calculator.
Motor service factor. Many motors carry a 1.15 service factor, meaning they can run briefly above nameplate horsepower without damage. That margin is a safety buffer, not license to undersize a motor for continuous duty.
Frequently asked questions
What is the difference between water horsepower and brake horsepower?
Water horsepower (WHP) is the pure hydraulic energy delivered to the fluid — flow times head. Brake horsepower (BHP) is the larger figure the pump shaft, and therefore the motor, must actually supply, because no pump is 100% efficient. BHP equals WHP divided by the pump's efficiency.
How do I find total dynamic head for this calculator?
Total dynamic head (TDH) is the sum of static lift, friction loss in the pipe, velocity head and any pressure head the pump must overcome. If you don't already have this figure, our Total Dynamic Head Calculator walks through each component.
What pump efficiency should I use if I don't know mine?
As a starting point, small submersible and open-impeller pumps typically run 40–55% efficient, standard end-suction centrifugal pumps 60–75%, and high-efficiency end-suction models 75–85%. Check the manufacturer's pump curve at your operating point for an exact figure.
Why is the recommended motor size bigger than the brake horsepower figure?
Motors are only manufactured in standard NEMA horsepower steps — 1, 1.5, 2, 3, 5 and so on — so the recommendation rounds your brake horsepower up to the next available size. That built-in margin also helps the motor avoid running at its absolute limit.
Does the fluid I'm pumping change the horsepower needed?
Yes. Horsepower scales directly with specific gravity, so denser fluids need proportionally more power to move. Seawater, at a specific gravity of about 1.025, needs roughly 2.5% more horsepower than fresh water at the same flow and head.
Sources & references
- Hydraulic Institute. ANSI/HI 1.3 Rotodynamic (Centrifugal) Pumps for Design and Application.
- National Electrical Manufacturers Association (NEMA). MG 1 Motors and Generators — standard horsepower ratings.
- International Bureau of Weights and Measures (BIPM). The International System of Units (SI).
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)