How to Size a Well Pump: Complete 2026 Guide

Published: August 15, 2026

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By Thomas Reynolds, Pump Systems Engineer

Thomas has installed and serviced over 1,000 well pump systems for homeowners.

Table of Contents

Why Sizing Your Well Pump Correctly Matters

If you are buying a new well pump, replacing one that has failed, or sizing a pump for a new well, the single most important decision you will make is choosing the correct pump size. In our experience installing submersible and jet pump systems for homeowners, the number one cause of pump failure, short-cycling, and water pressure complaints is not a defective pump. It is a pump that was the wrong size for the well it was dropped into.

An undersized pump will run constantly, burn out its motor, and never build pressure. An oversized pump will slam on and off every few seconds (short-cycle), hammer the check valve, and trip breakers. Both problems cost real money, and both are completely avoidable with the three measurements this guide walks you through. By the end, you will know exactly how to size a well pump for your well using the two numbers that actually matter: total dynamic head and gallons-per-minute (GPM) demand.

Key Insight

A well pump is not sized by horsepower alone. It is sized by matching the pump rating curve to your specific total dynamic head and flow demand. A 1/2 HP submersible on a shallow, high-yield well can outperform a 1 HP unit on a deep, low-yield well. Get the numbers first, then pick the horsepower.

Most homeowner confusion starts from a simple misunderstanding: the horsepower number on the box (1/3 HP, 1/2 HP, 3/4 HP, 1 HP) is a rough proxy, not a specification. Two 1 HP submersible pumps can produce wildly different results depending on how deep the water is and how hard the well is working. The pump manufacturer’s rating chart is what connects your well’s measurements to the right unit, and reading that chart correctly is the entire skill of well pump sizing.

What Total Dynamic Head (TDH) Is and Why It Is the First Number

Total dynamic head is the total vertical distance, measured in feet, that your pump must lift water from the point of intake to the point where the water is delivered. It is the single most important figure in well pump sizing, because it determines how much horsepower and head pressure the motor must actually produce. TDH is not just the depth to the water. It is the sum of three components:

Static water level depth — the distance from the top of the well (or the water surface when the well is full) down to the resting water level.
Drawdown — the additional drop in water level when the pump is running at full flow. This is where most sizing errors happen, because a pump that pulls more than the well can recover will keep dropping the water level deeper and deeper.
Delivery height — the vertical distance from the top of the well up to the highest faucet, tank, or point of use the pump must reach.

For a standard submersible system the working formula is simple and memorable:

The TDH Formula

TDH = (Static Water Level Depth + Drawdown) + Delivery Height. Example: water level 100 ft down, drawdown 20 ft, water must be pushed 30 ft above ground = (100 + 20) + 30 = 150 ft of total dynamic head.

After analyzing hundreds of well pump installations, we can tell you that the drawdown figure is the one homeowners most often skip or underestimate. A well that reads 100 feet to water when it is sitting idle can be producing at only 80 feet while a pump is running, because the water has been drawn down 20 feet. If you size your pump to the static number only, the pump will hunt for water that keeps falling away, the tank will short-cycle, and the motor will overheat. That is why a proper sizing job always includes measuring the well under load.

How to Measure the Static Water Level

Measuring the static water level is the first physical measurement in well pump sizing, and it is the one that most DIY homeowners can do safely with a simple tool. The static water level is the height of the water in the well when the pump has not run for at least 24 hours (or several hours minimum for a rough estimate). The standard tool is a well tape — a 150-foot measuring tape with a small, weighted, felt-covered plunger (called a “stopper” or “tape stop”) welded to the end.

Here is the exact step-by-step process we follow on every well inspection:

Stop using the pump. Turn the well pump off and let the well recover for at least 24 hours. This gives you the true static (resting) water level, not the depressed running level.
Open the wellhead access. Remove the cap or cover of the wellhead (or the pitless unit cap) so you can drop the tape into the well bore.
Drop the tape stop. Slowly lower the weighted felt stopper until it touches the water surface. You will feel it hit the water, and the felt will wick moisture up the tape.
Read the wet line. The point where the tape turns wet is your water surface. Measure from the reference point at the top of the well down to that wet line. That distance is your static water level.
Record it in feet. Write the number down in feet and inches, then convert to feet (for sizing calculations). This is the “depth to water” figure that goes into the TDH formula.

Common Measurement Mistake

Never measure the water level while the pump is running or immediately after it has run. That reading is the dynamic (drawn) level, not the static level, and using it will make your well look shallower than it really is, leading you to a pump that is too small. Always wait for the well to fully recover first.

How to Measure Drawdown (The Running Water Level)

Drawdown is the second number in your total dynamic head calculation, and it is the measurement that separates an accurate pump size from a guess. Drawdown is simply the difference between your static water level and the water level while the pump is running at full flow. A high-yield well might draw down only 5 to 10 feet under a heavy draw, while a tight or low-yield well can draw down 40, 60, or even 100 feet.

To measure drawdown, you run the same well tape procedure but while the pump is actively producing water:

Run the pump. Turn the well pump on and let it run until the water level in the well stabilizes at its running (drawn) position. In practice, open a faucet and let the system cycle, or run the pump continuously with water flowing.
Drop the tape into the well while the pump is still running and the level has settled.
Read the wet line at the current running water surface. This is your dynamic water level.
Subtract. Drawdown = Static Water Level − Running Water Level. If your static level was 100 feet and the running level is 120 feet, your drawdown is 20 feet.

Pro Tip

Your pump intake must always sit below the maximum drawdown level, with a safety margin of at least 3 feet (ideally 6 feet). If a well draws down 20 feet and your pump is only 5 feet below the static level, the pump will run dry and fail. Measure drawdown, then place the pump intake deep enough to stay submerged even at the lowest point.

How to Calculate Your Total Dynamic Head (Worked Example)

Now that you have both measurements, calculating total dynamic head is straightforward. We use this worked example on almost every sizing call because it makes the abstract formula concrete. Imagine a typical homeowner well with these figures:

Static water level: 120 feet below the top of the well
Drawdown under a full-flow test: 25 feet
The pump must push water up to a pressure tank and then up to a second-floor bathroom, adding roughly 25 feet of delivery height above the wellhead

Plugging into the formula: TDH = (Static Water Level + Drawdown) + Delivery Height = (120 + 25) + 25 = 170 feet of total dynamic head. That 170-foot figure, not the 120-foot static number, is the value you take to the pump rating chart. A pump that is rated for 170 feet of head at the flow rate your house actually needs is the pump you buy. Anything less will struggle, and anything dramatically more will short-cycle and waste energy.

Quick Rule of Thumb

For a rough first estimate, add your static water depth plus a 25–40 foot drawdown allowance, then add the vertical rise to your highest fixture. If your well is 100 feet to water and your fixtures are about 30 feet above grade, a 150–170 foot total dynamic head is a reasonable planning number until you run a full yield test.

How to Calculate Your Gallons-Per-Minute (GPM) Demand

Total dynamic head tells you how hard the pump has to push. Gallons per minute tells you how much water the pump has to move. These two numbers, together, are the complete specification for your well pump. To estimate your household GPM demand, add up the flow rates of the fixtures most likely to run at the same time.

A practical way to estimate residential demand is to add the peak fixture flows. A full-flow kitchen faucet draws about 2 to 2.5 GPM, a shower about 2 to 2.5 GPM, a washing machine about 2 GPM, and a toilet fill valve about 1 to 1.5 GPM. In a typical home, the realistic simultaneous peak (a shower running while someone uses the kitchen sink) lands somewhere in the 5 to 8 GPM range. This is the flow number to match against the pump curve.

Sizing Rule

Size your pump to deliver your peak GPM demand at your calculated total dynamic head, with a small margin. If your house needs 6 GPM at 170 feet of head, choose a pump whose curve shows at least 6 GPM at 170 feet. If the well’s natural yield is lower than your demand, match the pump to the well’s sustainable yield instead — a pump cannot pull more water than the well can recover.

One more consideration that ties the two numbers together: the well’s sustainable yield is the ceiling on your pump size. If your well can only recover 4 GPM over a long drawdown, no amount of horsepower will make it deliver 8 GPM continuously. In that case you either size the pump to 4 GPM and add a larger pressure tank or cistern to smooth out demand, or accept a reduced flow. We cover how to find your well’s true yield in our companion guide on measuring how much water your well produces.

Sizing Factor	What It Means	Typical Home Value	How to Measure
Static Water Level	Depth from well top to resting water (pump off 24 hrs)	80-200 ft	Well tape with stopper
Drawdown	Extra depth lost when pump runs at full flow	10-60 ft	Well tape while running
Delivery Height	Rise from well top to highest fixture or tank	20-40 ft	Tape measure / site plan
Total Dynamic Head	Static + Drawdown + Delivery (drives head need)	150-250 ft	Add the three figures
GPM Demand	Gallons per minute your fixtures pull at peak	5-10 GPM	Sum fixture flows
Sustainable Yield	Max GPM the well recovers over a long run	3-15 GPM	Bucket / flow test

Values are typical residential ranges. Your specific well will vary — always confirm with an on-site measurement.

How to Read the Pump Rating Chart to Match Horsepower

Once you have your total dynamic head and your GPM demand, the final step is reading the pump manufacturer’s rating chart — the small table or curve that comes in the box or on the product page. Every quality submersible and jet pump lists a rating curve showing how much flow (GPM) the pump delivers at a given head (feet), for a given horsepower. Here is exactly how to use it, in the order we use it on every job:

Find your total dynamic head on the vertical axis. Locate your 170-foot (or whatever your figure is) total dynamic head on the head column of the rating chart.
Read across to the GPM column. The number the pump delivers at that head is your usable flow. Compare it to your household GPM demand.
Pick a horsepower that meets or exceeds demand. Choose the smallest horsepower whose curve shows at least your peak GPM at your TDH. You do not want to over-buy, because a pump that delivers far more flow than needed will short-cycle the tank and wear out faster.
Confirm the intake depth. Make sure the pump, placed below your maximum drawdown level with a safety margin, is still rated for the resulting head at that depth.

Reading a Real Rating Curve

Say a 1 HP submersible shows 12 GPM at 50 feet, 9 GPM at 100 feet, 6 GPM at 150 feet, and 4 GPM at 200 feet. If your well needs 6 GPM at 170 feet of total dynamic head, that 1 HP unit just barely covers you at 150 feet and will fall short at 170 — so you move up to a 1.25 or 1.5 HP unit whose curve still shows 6 GPM at 170 feet. That is the whole art: matching the curve point, not the horsepower label.

From installing pumps across a wide range of well depths, we have found that most single-family homes with wells between 100 and 200 feet to water, drawing 5 to 8 GPM, end up with a 1 HP to 1.5 HP submersible. Shallow high-yield wells (under 80 feet, high recovery) often do fine on 3/4 HP, while deep low-yield wells can push 2 HP or higher. The rating chart, fed by your real measurements, is what tells you which side of that range you fall on.

Submersible vs. Jet Pump: How Sizing Changes by Type

The sizing logic above applies to both submersible and jet pumps, but the two types behave differently, so your total dynamic head calculation shifts depending on which one you are buying. Understanding the difference prevents a very common mismatch.

Sizing a Submersible Pump

A submersible pump sits down in the well, fully submerged, and pushes water up. For a submersible, your total dynamic head is measured from the pump’s actual operating depth (static level plus drawdown, plus delivery height). Because the pump is already down there, it is pushing, not pulling, and it is the most efficient type for deep wells. This is why submersibles dominate wells deeper than roughly 80 to 100 feet. When sizing a submersible, the key is placing the pump intake deep enough below the maximum drawdown and choosing a curve that meets your GPM at that full head.

Sizing a Jet Pump

A jet pump sits at or near the surface and uses a venturi jet to pull water up through the riser pipe. Sizing a jet pump is more constrained by suction physics: a standard shallow-well jet pump is limited to pulling water from roughly 25 feet or less, and a deep-well (two-stage) jet pump is generally limited to about 90 to 100 feet of suction head. Beyond that, the pump simply cannot generate enough suction, and no amount of horsepower helps. For jet pumps, your usable head is capped by the type, so a well deeper than 90 feet to water usually rules a surface jet pump out and points you toward a submersible.

Choosing the Type First

Decide the pump type before the horsepower. If your water level is 90 feet or less, a deep-well jet pump is a valid, lower-cost option. If it is deeper, or if your well has a high yield you want to capture, a submersible is the better engineering choice. Sizing the horsepower is the second step, not the first.

For a full product-level breakdown of the top-rated units in each category, including their specific rating curves, see our guides on the best submersible well pumps and the best jet pumps for private wells. Those articles walk through exact models, so you can match the general sizing numbers you calculate here to a specific product.

Frequently Asked Questions

What is the correct horsepower for a well pump?

There is no single “correct” horsepower — it is driven by your total dynamic head and GPM demand. For a typical home with 100 to 200 feet of water depth and 5 to 8 GPM demand, a 1 HP to 1.5 HP submersible is the most common fit. Shallow high-yield wells may need only 3/4 HP; deep low-yield wells can need 2 HP or more. Always confirm against the pump’s rating curve at your actual total dynamic head.

How do I know if my pump is too big or too small?

A pump that is too small will run constantly, never build pressure, and the motor will run hot. A pump that is too big will short-cycle — turning on and off every few seconds — which hammers the check valve, wears the pressure switch, and can trip a breaker. If your pump is short-cycling, it is usually oversized for the well’s yield or there is a small leak; if it runs without stopping, it is undersized or the well has low recovery.

Do I need a variable frequency drive (VFD) for well pump sizing?

A VFD is not required for basic sizing, but it is a strong upgrade for low-yield wells and for reducing short-cycling. It lets the pump slow down to match demand, protects a slightly oversized motor from hammering the tank, and can extend equipment life. If your well draws down heavily or your tank short-cycles, a VFD is often a better fix than simply buying a bigger pump. See our guide on variable frequency drive controllers for details.

Can I reuse my old pressure tank and switch just the pump?

Yes, in most cases you can keep your existing pressure tank, check valve, and pressure switch when replacing the pump, as long as they are in good condition and correctly sized for your new GPM demand. However, if your tank is old, has a failed bladder, or was never sized for your demand, it is a good opportunity to upgrade it at the same time. A mismatched tank can cause short-cycling even when the pump itself is the right size.

What should my pump be installed at, depth-wise, relative to the water?

The pump intake should sit below the maximum drawdown level, with a safety margin of at least 3 feet (ideally 6 feet), so it never runs dry when the water level is at its lowest. If your static level is 120 feet and your drawdown is 25 feet, your water can drop to 145 feet, so the pump should be installed at least 148 feet deep. This is one of the most common sizing mistakes and the leading cause of burned-out submersible motors.

Sources

The guidance in this article is informed by the following authoritative sources:

USGS — Ground Water — U.S. Geological Survey data on groundwater levels, well construction, and aquifer yield.
EPA — Private Drinking Water — EPA guidance on private well water and pump systems.
CDC — Private Well Water — Centers for Disease Control guidance on private well water safety.

External links open in a new tab. Verify current details with the source before relying on them.

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