In short: add up your total dynamic head — static lift, friction loss, and any extra discharge head — then multiply density, gravity, flow, and head to get hydraulic power. Divide by pump efficiency for brake horsepower, and round up to the nearest standard motor size. A typical residential booster needing 50 GPM against 66 ft of head at 65% efficiency needs about 1.28 brake horsepower, which calls for a 1.5 HP motor.
This tool is a planning aid for typical single-pump systems. For fire pumps, code-required sizing, or critical infrastructure, confirm with a licensed engineer.
How to size a water pump
Pump sizing comes down to two questions: how much water do you need to move (flow), and how much resistance does the pump have to push against (head)? Multiply those together with water's density and gravity to get the hydraulic power the water itself absorbs, then divide by the pump's efficiency to find the brake power the motor must actually deliver.
If you already know your exact flow and head and just want the power figure, the Pump Horsepower Calculator does that single step. This tool builds the head for you from its parts, using the same formula and the same result.
Worked example. Take a system needing 50 US GPM against 40 ft of static lift, 15 ft of pipe friction, and 5 ft of head at the discharge point, with a 10% safety margin and a typical 65% pump efficiency. TDH = (40 + 15 + 5) ft × 1.10 = 66 ft (20.12 m). Flow of 50 GPM is 0.003155 m³/s. Hydraulic power = 998.2 kg/m³ × 9.80665 m/s² × 0.003155 m³/s × 20.12 m ≈ 621 W, which is 0.83 water horsepower. Divide by 65% efficiency: 0.83 ÷ 0.65 ≈ 1.28 brake horsepower — rounded up to a 1.5 HP motor, the next standard size.
Why total dynamic head changes the answer
Static lift is simple geometry: the vertical distance water has to climb from its source to where it discharges. It doesn't depend on pipe size or flow rate at all.
Friction loss grows with pipe length, shrinks with pipe diameter, and grows roughly with the square of velocity — a long, narrow, or high-flow run costs far more head than a short, wide, low-flow one. If you haven't calculated this yet, the Total Dynamic Head Calculator walks through every component separately, including velocity head.
Additional head covers whatever pressure you still need once the water arrives — filling a pressure tank, reaching a sprinkler head, or clearing a backwash valve. It's easy to forget and easy to underestimate, which is why it gets its own field here.
Safety margin isn't physics — it's engineering judgment. Pipes roughen with age, minor losses (elbows, valves, meters) are rarely all itemized, and a 10–20% cushion keeps a pump from falling short a few years after installation.
Motor size vs. maximum flow at 100 ft of head
A quick reference for common motor sizes, assuming 100 ft of total dynamic head, 70% pump efficiency, fresh water at 20 °C, and no safety margin. Set the safety margin in the calculator to 0% to reproduce these figures exactly.
| Motor size | Max flow (US GPM) |
|---|---|
| 0.5 HP | 13.9 |
| 1 HP | 27.7 |
| 1.5 HP | 41.6 |
| 2 HP | 55.5 |
| 3 HP | 83.2 |
| 5 HP | 138.6 |
| 7.5 HP | 208.0 |
| 10 HP | 277.3 |
Worked examples
Submersible well pump
1 HP
10 GPM against 100 ft of well depth, 20 ft of friction, and 20 psi (46 ft) for the pressure tank, at 60% efficiency with a 10% margin, needs about 0.77 brake horsepower — a 1 HP motor.
Irrigation booster
0.5 HP
30 GPM with just 10 ft of lift and 25 ft of friction from a long lateral line, at 65% efficiency with a 10% margin, needs only about 0.45 brake horsepower — a 0.5 HP motor.
Sump-to-tank transfer
1.5 HP
100 GPM against 20 ft of lift and 10 ft of friction, at 70% efficiency with a 15% margin, needs about 1.24 brake horsepower — rounding up to a 1.5 HP motor.
What else affects the result
System curve vs. design point. This calculator gives the power needed at your design flow and head, but a real pump follows its own performance curve. The actual operating point is wherever that curve crosses your system's resistance curve — a pump specialist checks this against the manufacturer's curve, not just a single power number.
NPSH and cavitation. Sizing a motor correctly doesn't guarantee the pump won't cavitate on the suction side. If your pump draws from a well, sump, or low reservoir, check available suction head against the pump's requirement with the NPSH Calculator.
Motor service factor and duty cycle. Motors with a 1.15 service factor can handle brief overloads; pumps that cycle on and off frequently benefit from a slightly larger motor to avoid running near their limit continuously.
Fluid properties. Warmer water, seawater, or a process fluid with a different density and viscosity all shift the required power slightly — the water-type option above accounts for density directly.
Frequently asked questions
How do I size a water pump?
Add up the total dynamic head — static lift, friction loss and any extra discharge head — then decide the flow rate you need. Multiply density, gravity, flow and head to get hydraulic power, divide by pump efficiency to get brake horsepower, then round up to the nearest standard motor size.
What is Total Dynamic Head (TDH)?
TDH is the total resistance a pump must overcome: the vertical lift between source and discharge, the friction loss in pipe and fittings, and any extra pressure needed at the discharge point. It is usually expressed in feet or meters of head.
How much horsepower do I need for a pump?
It depends on flow, total dynamic head, and pump efficiency. As a rough guide, a pump moving 50 GPM against 66 feet of head at 65% efficiency needs about 1.28 brake horsepower, which rounds up to a 1.5 HP motor. Use the calculator above for your own numbers.
Should I add a safety margin when sizing a pump?
Yes. A 10 to 20 percent margin on total dynamic head is common practice, covering pipe roughening over time, minor losses that were not itemized, and small errors in the elevation or flow estimate.
Does water temperature affect pump sizing?
Only slightly. Water density changes a few percent between freezing and boiling, which shifts the required power by the same small amount. For most residential and irrigation systems this effect is minor compared with flow, head and efficiency.
What pump efficiency should I assume if I don't know the pump's rating?
Small centrifugal pumps typically run 55 to 70 percent efficient, submersible well pumps around 50 to 60 percent, and jet pumps closer to 40 to 50 percent. Check the manufacturer's pump curve for an exact figure whenever one is available.
Sources & references
- Hydraulic Institute. ANSI/HI 1.1-1.5 — Rotodynamic Pumps for Nomenclature, Definitions, and Applications.
- Hydraulic Institute. ANSI/HI 9.6.1 — Rotodynamic Pumps Guideline for Pump Piping.
- Kell, G. S. (1975). Density, thermal expansivity, and compressibility of liquid water from 0° to 150 °C. Journal of Chemical & Engineering Data, 20(1), 97–105.
- National Electrical Manufacturers Association (NEMA). MG 1 — Motors and Generators.
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)