Irrigation Pump Selection Calculator
Calculate required pump horsepower from total dynamic head and flow rate using Hazen-Williams friction loss.
About this calculator
Sizing an irrigation pump comes down to answering one question: how much energy does the pump need to add to the water to get it from the source to the sprinkler head at the right pressure? This calculator adds up three components of head — static lift (the vertical distance from the water source to the highest point in the system), operating pressure converted to feet of head (multiplying PSI by 2.31), and friction loss in the mainline pipe — into a single Total Dynamic Head figure. Friction loss isn't a flat guess; it's computed with the Hazen-Williams equation, the standard empirical friction-loss formula used throughout water-system design, which scales with flow rate raised to the 1.852 power and divides by pipe diameter raised to the 4.87 power, so a pipe that's slightly undersized or a run longer than expected can spike losses dramatically rather than linearly. From TDH, water horsepower (GPM × TDH ÷ 3960) gives the theoretical work being done on the water, and dividing by pump efficiency yields the real brake horsepower the motor shaft must deliver.
From there, the calculator matches brake horsepower against a table of standard motor frame sizes (0.5 HP up to 300 HP) and returns the next size up — never spec the exact brake HP figure, since pumps rarely run at their best-efficiency point continuously. It also reports pipe velocity, which should land between 3 and 7 feet per second; faster flows waste energy on friction and increase the risk of water hammer, while slower flows suggest an oversized pipe. The annual energy cost estimate multiplies brake horsepower by a fixed 0.746 kW/HP conversion, so it assumes the pump runs near this single operating point for the full input hours — real-world costs will vary with starts, stops, and seasonal demand changes.
Inputs
Results
Total Dynamic Head (ft)
112.4
Recommended motor (HP)
3
Figures current as of 2026. Source: The Engineering ToolBox, "Hazen-Williams Water Flow Formula"
How to Use This Calculator
- Enter required flow rate in GPM and static lift in feet from source to highest point.
- Set operating pressure in PSI at the sprinkler or emitter and mainline pipe length in feet.
- Enter pipe diameter in inches and Hazen-Williams C factor for your pipe material.
- Set pump efficiency (%) and annual run hours for energy cost estimation.
- Review Total Dynamic Head (ft), Recommended motor (HP), and Annual energy cost ($) to select the right pump.
How the result changes with Operating pressure (PSI)
| Operating pressure (PSI) | Total Dynamic Head (ft) | Recommended motor (HP) |
|---|---|---|
| 20 | 66.2 | 1.5 |
| 30 | 89.3 | 2 |
| 60 | 158.6 | 3 |
| 100 | 251 | 5 |
What each input means
- Required flow (GPM)
- Total system flow rate needed in gallons per minute.
- Static lift (ft)
- Vertical distance water must be lifted from source to highest point.
- Operating pressure (PSI)
- Required pressure at the sprinkler head or emitter.
- Pipe length (ft)
- Total length of mainline pipe.
- Pipe diameter (in)
- Inside diameter of mainline pipe in inches.
- Hazen-Williams C
- Pipe roughness coefficient. PVC=150, new steel=140, old steel=100.
- Pump efficiency (%)
- Pump efficiency at operating point. Typical: 60-80%.
- Energy cost ($/kWh)
- Electricity cost per kilowatt-hour.
- Annual run hours
- Expected pump operating hours per year.
What each result means
- Total Dynamic Head (ft)
- Sum of static lift, operating head, and friction loss.
- Friction loss (ft)
- Pipe friction loss via Hazen-Williams equation.
- Water horsepower
- Theoretical power needed to move water (WHP = GPM x TDH / 3960).
- Brake horsepower
- Actual shaft power required accounting for pump efficiency.
- Recommended motor (HP)
- Next standard motor size above brake HP.
- Pipe velocity (ft/s)
- Water velocity in mainline. Ideal range: 3-7 ft/s.
- Annual energy cost ($)
- Estimated yearly electricity cost for pump operation.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersRequired flow (GPM) = 50, Static lift (ft) = 20, Operating pressure (PSI) = 40, Pipe length (ft) = 500 = 9 input(s) provided
- Calculate Total Dynamic HeadTotal Dynamic Head = staticLiftFt + operatingHeadFt + totalFrictionLoss112.4 = 112.4
- Calculate Recommended motorRecommended motor3 = 3
- Calculate Friction lossFriction loss = frictionPer100 * (pipeLength / 100)0 = 0
- Calculate Water horsepowerWater horsepower = (flowGPM * tdh) / 39601.42 = 1.42
Figures and sources
- Hazen-Williams friction-loss equation and pipe roughness (C) coefficients (2026) — The Engineering ToolBox, "Hazen-Williams Water Flow Formula"
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does pipe diameter matter so much more than pipe length for friction loss?
The Hazen-Williams formula this calculator uses raises pipe diameter to the 4.87 power in the denominator, while pipe length only scales the result linearly (per 100 ft). That means cutting pipe diameter in half increases friction loss roughly 29-fold, while doubling the pipe length only doubles it. A slightly undersized mainline is a far bigger energy penalty than a slightly longer run.
What is the difference between water horsepower and brake horsepower?
Water horsepower (WHP = GPM × TDH ÷ 3960) is the theoretical power delivered to the water itself, with no losses. Brake horsepower divides WHP by pump efficiency, so it's the real mechanical power the motor shaft has to supply to overcome the pump's internal losses. Brake HP is always higher than water HP for any efficiency below 100%, and it's the figure this calculator uses to pick a motor.
Why does the calculator recommend a specific motor size instead of just the brake horsepower number?
Motors are only manufactured in standard frame sizes — this calculator checks brake HP against a list running 0.5 HP up to 300 HP and returns the next size at or above that figure. Sizing to the exact brake HP would leave no margin, and motors aren't built in arbitrary fractional sizes anyway, so the recommended motor is always a real, purchasable size.
What pipe velocity range should I be targeting, and why does the calculator flag it?
The calculator reports velocity in feet per second from your flow rate and pipe diameter, with 3-7 ft/s considered the practical target range. Velocities above that waste energy on friction and raise the risk of water hammer when valves close quickly, while velocities well below it usually mean the pipe is larger (and more expensive) than the flow actually requires.
How accurate is the annual energy cost estimate?
It multiplies brake horsepower by the fixed 0.746 kW/HP conversion, your entered electricity rate, and annual run hours, which assumes the pump operates continuously at this single calculated operating point for the full duration. Real systems cycle on and off, face seasonal demand changes, and see efficiency drift over time, so treat this figure as a baseline estimate rather than a precise utility bill forecast.
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