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Well Pump Sizer

Select the correct residential well pump by mathematically resolving fixture Peak Demand for target GPM flow and graphing well lift physics to map Total Dynamic Head (TDH).

System Demands

Total Water Fixtures

count

Sum of all sinks, showers, toilets, dishwashers, washing machines, and outdoor spigots.

Drawdown Water Level Depth

feet below surface

The maximum depth the water reaches when the pump is running continuously.

Pressure Tank Cut-In Rating

Pump Output Specification

Required Flow Rate

12.0GPM

1 GPM assigned per installed plumbing fixture

Total Dynamic Head (TDH)

273FT Head

TDH Calculation Anatomy

Drawdown Vertical Lift

150.0'

Tank PSI Head Equivalent 50 PSI x 2.31

+ 115.5'

Pipe Friction Loss 5% drop pipe

+ 7.5'

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What is Aquifer Hydraulics: Engineering Pump Capacity?

You cannot guess a well pump size, nor can you simply buy a larger pump 'just to be safe'. Incorrectly sizing a submersible well pump will quickly destroy the equipment and the well. Choosing a pump that operates at too high of a GPM for your aquifer will suck the well dry and ruin the impellers. Conversely, installing an oversized pump for a small home will cause violent 'short cycling' (turning on and off every 15 seconds), which instantly strips the motor bearings and melts the starter control box. You must mathematically match a specific pump curve (GPM vs TDH) exactly to the physical load of the house and the physical depth of the aquifer.

Mathematical Foundation

Total Dynamic Head (TDH)

TDH = D_{drawdown} + P_{head} + F_{friction}

TDH

= The ultimate metric for pump selection. The total mechanical resistance the pump must physically overcome, measured in equivalent feet of vertical lift.

D_{drawdown}

= Drawdown Depth: The lowest vertical distance the aquifer water level falls to while the pump is actively running (NOT the static resting water level).

P_{head}

= Pressure Head: The pneumatic resistance of the home's pressure tank, converted to vertical feet (Tank Cut-In PSI × 2.31 ft).

F_{friction}

= Pipe Friction: The hydraulic resistance of water dragging against the inside of the casing drop pipe (typically estimated at 5% of the drawdown depth).

Laws & Principles

The Rule of 2.31 (PSI to Lift Conversion): It takes exactly 1 PSI of mechanical pressure to push a column of water 2.31 feet straight up into the air. Therefore, if your home's basement pressure switch is set to turn on at 50 PSI, the pump is NOT just fighting gravity up the 150-foot well casing—it must generate enough force to push the water an extra 'invisible' 115 vertical feet against the trapped air bladder in the tank.
The 1-GPM Fixture Rule: A standard engineering estimation for peak residential water demand specifies roughly 1 Gallon Per Minute (GPM) of pump capacity for every water fixture in the home. A typical 3 bed / 2 bath home with a kitchen sink, dishwasher, washing machine, and two outdoor hose bibbs possesses around 10 to 12 fixtures, requiring a ~10 or 12 GPM pump design.
Static vs Drawdown Isolation: Never calculate a pump based on where the water sits when the well is turned off (Static Water Level). When the pump activates, it rapidly vacuums water out of the 6-inch casing faster than the surrounding rock aquifer can weep water back in. The water level plunges drastically. You must calculate lift based purely on the 'Drawdown Water Level'—the deepest point the water stabilizes at during a heavy 30-minute stress test.
Mapping the Pump Curve: A pump manufacturer sticker maps operational limits based entirely on TDH (Total Dynamic Head), not straight depth. A 10 GPM pump rated for '50ft of head' will produce exactly 0 GPM if dropped into a well that possesses 200ft of active TDH. It will spin endlessly but lack the force to push the water past the pitless adapter.

Step-by-Step Example Walkthrough

" A technician is sizing a new 230V submersible pump for an active farmstead. The house and barn combine for 18 total fixtures. The driller's log notes the static water level is at a shallow 50 feet, but under heavy stress, the drawdown level plunges to 150 feet. The plumber is installing a high-pressure 50/70 PSI tank switch in the basement. "

1. Calculate Flow Demand: 18 fixtures × 1.0 = 18 required peak GPM.
2. Isolate Gravity Drawdown Lift: The pump must lift water from the 150-foot drawdown mark (Ignore the 50-foot static mark).
3. Convert PSI to Head Pressure: The switch kicks on at 50 PSI. 50 PSI × 2.31 feet = 115.5 feet of invisible Head Resistance.
4. Calculate Frictional Drag: 150 ft of plastic drop pipe × 0.05 (5%) = 7.5 feet of friction loss.
5. Summate Total Dynamic Head (TDH): 150 (gravity) + 115.5 (tank) + 7.5 (pipe) = 273 Total Dynamic Head.

Final Result: The technician requires an 18 GPM pump. They cannot just buy any 18 GPM pump. They must reference a manufacturer's Performance Curve Chart and select a specific motor (likely a 1.5 HP or 2.0 HP model) that is physically tuned to deliver 18 GPM while actively fighting against 273 feet of TDH resistance. If they choose a 1/2 HP pump, it will never reach the 50 PSI cut-off and will run until the motor burns out.

Quick Answer: How do you size a well pump?

Use the Well Pump Sizing Calculator to pinpoint your exact required model. Enter the Total Plumbing Fixtures in your home (to establish your Gallons Per Minute demand), the Drawdown Water Level of your well (the lowest depth the water hits during pumping), and select your desired Pressure Tank PSI. The calculator will instantly resolve your Total Dynamic Head (TDH), combining gravity lift and pneumatic tank resistance so you can select the correct horsepower matching your GPM on the manufacturer's curve.

Well Pump Destructions

The 'Bigger is Better' Short Cycle

A homeowner with a small 2-bathroom cabin (requiring only 7 GPM) decides to buy the biggest pump at the hardware store: a massive 25 GPM, 2.0 HP model, hoping for "better water pressure." Because the pump outputs water vastly faster than the home's small pressure tank can absorb it, the tank slams up to 60 PSI and shuts the pump off in under 10 seconds. When a faucet is opened, the pressure drops instantly, firing the pump back on 10 seconds later. This violent 'short-cycling' rips the starting capacitor apart and melts the motor windings within two months. A smaller 7 GPM, 1/2 HP pump was required for smooth, continuous run cycles.

The Static vs. Drawdown Trap

A DIYer drops a tape measure down their well and hits water at 40 feet (the Static Level). They incorrectly size their Total Dynamic Head (TDH) and horsepower based on lifting water only 40 feet. Once the pump turns on, it rapidly drains the casing until the aquifer stabilizes at 120 feet deep (the Drawdown Level). Suddenly, the small pump doesn't have the torque to lift water 120 feet vertically while pushing against a 40 PSI pressure tank. The water abruptly stops flowing upstairs, and the pump spins endlessly at the bottom of the well until the impeller stack melts from friction.

PSI to Vertical Head Conversion

Mapping The Hidden Resistance

Tank Head Resistance = Switch PSI Cut-In × 2.31 ft

Gravity isn't the only force your pump fights. Pushing water into a sealed steel pressure tank requires massive force against an air bladder. Upgrading your pressure switch from 40 PSI to 50 PSI forces the pump to work significantly harder—mathematically equivalent to forcing the pump to push water up an extra 23-foot vertical cliff.

Pro Tips for Pump Selection

Do This

✓Verify the aquifer yield first. It does not matter if your massive new home calculator requires a 25 GPM pump. If your driller's log states the rock aquifer only recharges at 5 GPM, placing a 25 GPM pump in the well will suck the shaft completely dry in minutes, burning up the motor. For low-yield wells, you MUST use a small pump filling a massive atmospheric holding tank.
✓Use a torque arrestor. Submersible pumps spin at 3450 RPM. When a 1-HP motor kicks on, the massive instant rotational torque causes the entire pump to violently twist and slam against the inside of the steel casing. Installing cheap rubber torque arrestors stops the pump from slowly chafing a hole through its own electrical wiring.

Avoid This

✗Don't mix 2-wire and 3-wire equipment. A 2-wire well pump has the starting capacitor built directly into the motor under the water. A 3-wire pump requires a separate starter control box mounted on the basement wall. You cannot splice a 3-wire system to a 2-wire pump—the voltages are incompatible, and it will instantly fry the equipment.
✗Don't set the pump at the absolute bottom. Never rest your pump on the floor of the well. The lowest 15 feet of a well is typically a 'sump' filled with mud, heavy sediment, and drilling tailings. Dropping a pump into this zone will cause it to inhale rocks and instantly destroy the plastic impellers.

Pipeline Lift & Total Dynamic Head (TDH) Matrix

Static Depth to Water	Drawdown Tolerance Limit	Estimated Target Horsepower
< 50 Feet	High (Stable Yield)	1/2 HP (10 GPM Max)
50 - 150 Feet	Moderate	3/4 HP to 1.0 HP
150 - 300 Feet	Vulnerable to Over-Drafting	1.5 HP to 2.0 HP Heavy Duty
300+ Feet	Extreme (Deep Aquifer Only)	3.0 HP+ Commercial Grade

Frequently Asked Questions

What does Total Dynamic Head (TDH) mean?

TDH is the true measurement of resistance your pump must overcome. You cannot simply size a pump by the depth of the well. TDH calculates the physical vertical gravity lift of the water, plus the resistance of the house pressure tank (acting as a massive pneumatic wall), plus the friction of water dragging against the inside of the drop pipe.

Why can't I just buy the highest GPM pump available?

An oversized pump will ruin your system through 'short-cycling'. If a massive 30 GPM pump feeds a standard residential pressure tank, it fills the tank so fast that it reaches cutoff pressure and turns off in 10 seconds. Motors get extremely hot when turning on. Continually turning on and off multiple times a minute will burn out the starting capacitor and melt the motor windings prematurely.

What is the difference between Static Level and Drawdown Level?

The Static Level is where the water rests silently when the well hasn't been used all night. The Drawdown Level is where the water level plummets to when the pump has been running hard for an hour. Because the pump is evacuating water faster than the rocks can refill the hole, the pump must lift water from a significantly deeper point. You must always size according to the deeper Drawdown depth.

Do I have to use a 40/60 PSI switch?

No. Standard switches are set to 40 PSI (turn pump on) and 60 PSI (turn pump off). You can upgrade to a high-pressure 50/70 switch to achieve better shower pressure. However, you must recalculate your Total Dynamic Head if doing this, as forcing the pump to push against 50 PSI instead of 40 PSI requires significantly more horsepower to overcome the pneumatic resistance.