Water Well Pump Size Calculator

Motor screen {{ resultsReady ? formatHorsepower(computation.values.recommended_motor_hp) : '—' }} Well yield {{ resultsReady ? yieldStatusLabel : '—' }} Pipe velocity {{ resultsReady ? formatVelocity(computation.values.pipe_velocity_fps) : '—' }}

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Project preset:

Presets are editable screening examples, not well-test or pump-curve evidence.

System strategy:

For storage fill, use the sustainable fill rate here and size the pressure booster as a separate duty point.

Target pump flow:

Changing the unit preserves the same physical flow.

Sustainable well yield:

Enter 0 only when no defensible sustained-yield result is available.

Daily water use:

Use a representative household, livestock, or irrigation day.

Static water level:

Pumping lift also needs the drawdown measured or estimated below.

Pumping drawdown:

Static level plus drawdown produces the pumping water level used for lift head.

Pump setting depth:

The pump must remain below the expected pumping water level with manufacturer-required submergence.

Delivery pressure:

Use the required pressure under demand at the tank or service point.

Delivery elevation above wellhead:

Use 0 when the delivery point is approximately level with the wellhead.

Service pipe length after wellhead:

Pipe friction uses pump setting depth plus this service run.

Nominal pipe size:

Larger inside diameter reduces velocity and friction at the same flow.

Pipe condition:

Exact inside diameter, age, fittings, valves, and treatment equipment can change real loss.

Fitting and equipment allowance:

This visible allowance is added before the percentage reserve.

Head reserve:

Applied once to the head subtotal; keep it visible because it changes the pump duty point.

Wire-to-water efficiency:

The duty point is unchanged; only the horsepower screen uses this assumption.

Candidate capacity at this TDH:

The neutral 0 value disables the candidate fit check.

Planned atmospheric storage:

The neutral 0 value reports no storage buffer.

Pressure-tank screen:

Off keeps the documented one-minute, 40/60 PSI screening basis.

{{ custom_tank_settings ? 'Custom tank screen' : 'Standard tank screen' }}

Pressure switch:

Verify exact tank drawdown with manufacturer data.

Minimum pump run time:

Final cycling limits and tank selection follow pump, controller, and tank instructions.

Low-water protection:

Off records no protection claim; the ledger still flags when protection deserves review.

{{ low_water_protection ? 'Protection planned' : 'No protection claim' }}

Pump duty point

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Calculation method:

This screening model preserves full precision and rounds only for display.

TDH = (lift + pressure + elevation + pipe + fittings) × (1 + reserve)

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Review before model selection

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The chart renderer is unavailable. All head values remain available in the duty point and ledger.

Item	Value	Decision use	Copy
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A well system must satisfy demand without asking the aquifer to recover faster than it can. The pump, well, pipe, pressure tank, and any atmospheric storage therefore have to be considered together. Choosing a pump from horsepower or a zero-head flow rating alone misses the conditions under which it will actually operate.

The useful selection point is flow at total dynamic head (TDH). Flow is the amount of water required per minute. TDH is the lift and resistance the pump must overcome, expressed as feet of water head. A deeper pumping water level, higher delivery pressure, uphill service point, smaller or rougher pipe, more fittings, or longer pipe all increase head and reduce the flow available from a given pump curve.

Measurements that define a residential well pump duty point
Measurement	What it describes	Common error
Static water level	Depth to water before pumping	Using it as the operating lift after drawdown
Drawdown	Additional depth to water while the well is pumped	Guessing without a sustained test
Well yield	Sustained recovery or test rate	Confusing it with a short pressure-tank burst
Pump curve	Model-specific flow available at each head	Using the pump's headline GPM at the wrong head

Peak demand and sustainable yield can point to different designs. A direct-pressure system tries to serve fixtures at the selected duty flow. A low-yield well may instead fill atmospheric storage slowly at a rate the well can sustain, while a separate booster serves short peaks. Merely installing a larger submersible pump can increase drawdown, uncover the intake, and shorten equipment life.

Pressure-tank volume is not the same as usable drawdown. Only part of a captive-air tank's labeled volume is released between cut-out and cut-in pressure, and the available fraction changes with pressure settings and precharge. Variable-speed systems follow manufacturer-specific tank rules rather than the fixed-speed screening approach used here.

This is preliminary sizing, not an installation specification. Final selection requires the well test, the manufacturer's complete pump curve and efficiency range, correct motor and cable data, local plumbing and electrical requirements, sanitary protection, and review by a qualified well or pump professional.

How to Use This Tool:

Define the required flow and measured water levels before comparing any pump model.

Choose a Project preset for an editable example, then select Direct pressure or Storage fill as the system strategy.
Enter Target pump flow, Sustainable well yield, and representative Daily water use. Use 0 for yield only when no defensible sustained test exists.
Enter Static water level, Pumping drawdown, and Pump setting depth. The pump setting must be below the computed pumping water level.
Set the pressure required under demand and the delivery elevation relative to the wellhead.
Open Advanced to describe service length, pipe size and condition, fitting allowance, head reserve, efficiency, storage, pressure-switch settings, run time, and low-water protection.
If a manufacturer curve is available, enter its candidate capacity at the calculated TDH. Do not enter a catalog maximum measured at a different head.
Review the duty point, well-yield status, pipe velocity, submergence, tank screening, and candidate-curve result together before selecting equipment.

Interpreting Results:

Operating TDH and Target flow form the duty point to locate on a manufacturer's pump curve. Recommended motor HP rounds the estimated input power up to a standard motor class; it does not identify a pump model or prove that the impeller operates efficiently at that point.

Yield margin means target flow is no more than 85% of known well yield. Above 85% through 100% is Tight yield; above yield calls for a lower storage-fill rate or a different direct-pressure design.
Pipe velocity is clear below or at 8 ft/s, cautioned above 8 ft/s, and high above 10 ft/s.
Modeled submergence below 20 ft is a caution. The actual minimum comes from the pump manufacturer and the expected low pumping level.
A candidate curve value below 95% of target is short. Values above 180% of target or more than 110% of known yield are also cautioned.

Technical Details:

The calculation normalizes flows to gallons per minute, lengths to feet, pressure to PSI, and volume to gallons. Pumping water level is static level plus drawdown. Pipe length includes the pump setting depth and the service run, while fittings are represented as an additional head allowance.

Formula Core:

Total dynamic head adds pumping lift, pressure head, delivery elevation, pipe friction, and fittings, then applies the selected reserve.

\begin{matrix} H_{pump} & = & H_{static} + H_{drawdown} \\ H_{pressure} & = & 2.31 \times P \\ H_{base} & = & H_{pump} + H_{pressure} + H_{elevation} + H_{friction} + H_{fittings} \\ H_{operating} & = & H_{base} \times (1 + \frac{r}{100}) \end{matrix}

Pressure conversion uses 2.31 ft of water head per PSI. A negative delivery elevation reduces head, but the combined base TDH must remain positive.

Pipe friction uses the Hazen–Williams form with flow in GPM, total pipe length in feet, inside diameter in inches, and a material coefficient of 150 for smooth PVC, 140 for poly or mixed plastic, or 120 for older or rough steel.

H_{friction} = \frac{4.52 L Q^{1.85}}{C^{1.85} d^{4.87}}

Pipe constants used by the friction model
Input	Choice	Model value
Nominal pipe size	1 in	1.049 in inside diameter
Nominal pipe size	1-1/4 in	1.380 in inside diameter
Nominal pipe size	1-1/2 in	1.610 in inside diameter
Nominal pipe size	2 in	2.067 in inside diameter
Nominal pipe size	2-1/2 in	2.469 in inside diameter
Pipe condition	Smooth PVC	C = 150
Pipe condition	Poly or mixed plastic	C = 140
Pipe condition	Older or rough steel	C = 120

Hydraulic power follows flow times head. Estimated input power divides that water horsepower by the entered wire-to-water efficiency, then rounds upward through the screened standard classes 0.5, 0.75, 1, 1.5, 2, 3, 5, 7.5, 10, 15, and 20 HP.

\begin{matrix} {HP}_{water} & = & \frac{Q \times H_{operating}}{3960} \\ {HP}_{input} & = & \frac{{HP}_{water}}{η / 100} \end{matrix}

Tank and Yield Rules:

Required pressure-tank drawdown equals pump flow multiplied by the desired run time. The labeled tank-volume screen divides this drawdown by an ideal captive-air fraction calculated from cut-in, cut-out, and a precharge set 2 PSI below cut-in, all converted to absolute pressure. The default screen uses 40/60 PSI and 60 seconds.

\begin{matrix} V_{draw} & = & Q \times \frac{t}{60} \\ F & = & P_{precharge,abs} \times (\frac{1}{P_{in,abs}} - \frac{1}{P_{out,abs}}) \\ V_{tank} & = & \frac{V_{draw}}{F} \end{matrix}

Daily recovery is known yield multiplied by 1,440 minutes. Recovery use above 65% is flagged as tight. For direct pressure, any target-flow excess over yield is the peak deficit. When atmospheric storage is entered, storage buffer hours equal stored gallons divided by that deficit in gallons per hour.

All calculations keep full precision; displayed values are rounded. Hazen–Williams friction, assumed efficiency, idealized tank drawdown, and fixed caution thresholds are screening approximations. Actual pump curves, pipe schedules, fittings, water temperature, worn pipe, voltage, motor service factor, variable-speed control, and local code can change the final design.

Safety and Accuracy Notes:

A pump that can meet the arithmetic duty point may still be unsuitable. Verify continuous submergence, motor cooling flow, minimum and maximum operating range, check-valve placement, wire size, overcurrent protection, pressure ratings, dry-run protection, sanitation, and local permits.

Use a measured pumping level and sustained yield from a representative test whenever available.
Confirm the exact duty point on the manufacturer's curve rather than relying on the screened horsepower class.
Have electrical and well work reviewed and performed by appropriately qualified people.

Worked Examples:

Deep well with a yield conflict

A 10 GPM target, 8 GPM well yield, 220 ft static level, 55 ft drawdown, 340 ft pump setting, 50 PSI delivery pressure, 15 ft elevation, and the default pipe assumptions produce about 469.0 ft of operating TDH. At 55% efficiency, the screened motor class is 3 HP and modeled pipe velocity is about 2.15 ft/s. The duty flow still exceeds known yield, so a 3 HP label alone does not resolve the design; the result calls for a lower sustained fill rate, storage strategy, or better field evidence.

References:

State of Michigan Water Well Manual, Michigan Department of Environment, Great Lakes, and Energy, 2023.
Group B Water System Design Guidelines, Washington State Department of Health, September 2018.
Irrigation Water Pumps, North Dakota State University Extension.