What Size Pressure Tank Do I Need for My Well Pump?

Start with usable drawdown, not the gallon number printed on the tank. For a conventional single-speed well pump with a documented cut-in and cut-out, calculate the water volume needed to keep the pump running for the specified minimum run time, then compare that required drawdown with the exact replacement model's manufacturer chart at your actual pressure settings. Check the tank's maximum pressure, connections, temperature range, dimensions, and relief protection separately. A constant-pressure or variable-speed system needs its controller-specific sizing method, not a conventional 30/50 worksheet.

The Water Systems Council describes a pressure tank as a way to reduce rapid pump cycling, supply water between pump cycles, and add pressurized storage when pump or well capacity cannot immediately meet demand. It also says tank selection requires system demand, pump capacity, and well capacity—not just household size or motor horsepower. (Water Systems Council's pressure-tank sizing guidance)

This page is a model-neutral decision worksheet for a homeowner preparing a replacement conversation. It does not select a brand, certify a tank, authorize a pressure adjustment, or replace the current manual for the installed pump, controls, tank, or local installation.

1. Decide whether this worksheet fits your system

The calculation below assumes a conventional pressure-switch system with a repeatable pump stop at cut-out and pump restart at cut-in. In that arrangement, the tank supplies water while pressure falls through the switch band; the pump then runs to rebuild pressure. The inputs are meaningful only if those two events are real, documented, and measured consistently.

Use the worksheet when all of these are true:

Do not use the conventional worksheet for a system driven by a variable-frequency drive, electronic pressure transducer, constant-pressure controller, or another arrangement that changes pump speed with demand. Pentair describes its Intellidrive constant-pressure system as using instantaneous pressure-transducer feedback to change motor speed as demand changes. That is not the same control pattern as a pump that simply switches on and off at fixed pressures. (Pentair's constant-pressure system description)

Xylem's Aquavar manual makes the same practical point from another manufacturer: its constant-pressure application has a pressure transducer and diaphragm tank, calls for motor-specific variable-frequency-drive compatibility and programming, and has a separate tank-sizing recommendation for that application. (Xylem/Goulds' constant-pressure application section) A variable-speed homeowner should record the controller and tank model and ask for the current controller manual or a qualified design review. Do not force the system into “30/50” because that is familiar.

Safety boundary before any measurement

A pressure tank is a pressure vessel connected to plumbing and pump controls. Do not open an energized control enclosure, test live wiring, change pressure-switch settings, open the well, pull a pump, enter a well pit or other confined space, loosen pressurized fittings, remove a relief valve, or manipulate the tank air valve as part of this worksheet. Do not use a visual glance at the tank as proof that its internal bladder, diaphragm, air cell, gauge, or pressure rating is sound.

The homeowner-safe part of this page is recording labels, observing the gauge from a safe position, and collecting water from an ordinary downstream cold-water fixture if the discharge is safe and permitted. Electrical isolation, pressure release, precharge measurement, component replacement, wiring, well work, and any service inside a control box belong to a qualified professional following the exact equipment instructions.

2. Record the inputs before choosing a tank

The worksheet has three separate jobs: calculate a required drawdown, compare that requirement with a published model chart, and decide whether the candidate can physically and safely live in the installation. Mixing those jobs is how a tank with a large label can pass a casual comparison while delivering too little water between pump cycles.

The calculation inputs

Record the following from documentation or a repeatable, safe observation:

Pentair's Wellmate sizing worksheet uses the same structure: pump delivery rate, desired minimum run time, and minimum and maximum system pressure. It multiplies pump flow by minimum run time to get minimum drawdown, then uses the pressure-range factor to determine the minimum total tank volume. (Pentair's tank-sizing worksheet)

Use measured or documented pump flow at the pressure and duty point that matter. A pump's free-flow rating, a motor horsepower label, or a retailer's “typical GPM” is not proof of flow at your house. The Water Systems Council says an existing pump's capacity must be determined, and its pump-sizing guidance frames capacity around normal peak household demand and well capacity. (Water Systems Council's pump-sizing guidance)

The comparison and fit inputs

Record these separately from the formula:

Measure the available space without disconnecting anything. Record the narrowest doorway or path if the tank must be carried to the location, the floor footprint, the ceiling or overhead clearance, the connection height, and whether the tank can be serviced without blocking a shutoff or gauge. A tank that meets a drawdown target but cannot be installed with its required support, connection orientation, relief protection, or service access is not a valid selection.

3. Calculate the required drawdown

For a conventional fixed-speed system, the first-pass calculation is:

minimum required drawdown = pump flow (gpm) × target minimum run time (minutes)

Example: if documented pump flow is 8 gpm and the equipment documentation calls for a 1-minute minimum run time:

8 gpm × 1 minute = 8 gallons of required drawdown

This is a water-volume target, not a tank-shell size. The tank selected must publish at least that drawdown at the exact pressure band, or the candidate fails the chart check. Pentair's catalog uses this same sequence and shows a product-specific example: 8 GPM × 1 minute produces 8 gallons of required available water, and a .30 chart factor yields a 26.7-gallon minimum total tank capacity for its stated 30–50 psig example. (Pentair's worked sizing example)

Do not carry the 26.7-gallon result into another tank line. The number is the result of the stated flow, run time, pressure band, and Pentair factor. A different tank design, precharge instruction, pressure range, or chart can produce a different relationship between total volume and drawdown.

Add supplemental drawdown; do not double-count a total requirement

The pump-run-time result is not the whole system design when the well or pump cannot satisfy peak demand. A pressure tank may need to provide supplemental drawdown during a demand period. Water Systems Council's pump-sizing guidance says that when peak demand exceeds the water available from the well, added storage can be used; its pressure-tank sheet describes total required drawdown as minimum drawdown plus supplemental drawdown when the pump cannot meet peak system demand. (Water Systems Council's pressure-tank sizing guidance)

First classify the documented storage input. “Supplemental drawdown” means additional gallons beyond the pump-run-time minimum. “Total peak-demand storage” means the source has already combined the system's storage needs into one total. Use the following decision rule:

minimum drawdown = pump flow × documented minimum run time

if the documented input is supplemental drawdown:
  required design drawdown = minimum drawdown + supplemental drawdown

if the documented input is already-total peak-demand storage:
  required design drawdown = the larger of minimum drawdown and total peak-demand storage

if no documented storage input exists:
  required design drawdown = minimum drawdown

Each nonblank storage input must come from a documented demand calculation, well-capacity assessment, pump design, or qualified professional—not from a guess based on the number of people in the home. The Water Systems Council offers fixture, peak-demand, and bathroom-based planning methods, but its examples are demand estimates, not a measurement of your pump's delivery at pressure. If the source does not say whether its gallons are supplemental or already total, record the result as not comparable until the source or a qualified professional clarifies it. (Water Systems Council's demand methods)

What current drawdown can and cannot tell you

Current drawdown belongs in the worksheet because it exposes what the old tank is actually delivering under the observed pressure cycle. It does not automatically become the new tank's required capacity, and it does not turn the old nominal label into a design specification.

Use it as a baseline:

current drawdown = gallons delivered from pump stop to pump restart

If the current tank supplies 6 gallons between events, that tells you about the existing installation under those conditions. It does not prove that a replacement should provide 6 gallons, because the pump flow, target run time, household demand, pressure band, and system type may have changed. A replacement should meet the documented design requirement and the exact model chart, not merely copy a possibly undersized or degraded tank.

4. Match the result to the exact manufacturer chart

The chart check is the decision surface. A model passes only when its published drawdown at the verified pressure settings is at least the required design drawdown and the model's installation limits fit the system.

Use this comparison table

Pentair defines a drawdown factor as the percentage of total tank volume that can be stored and delivered for a specific pressure range. Its catalog chart also warns that actual drawdown can vary with system variables, including the accuracy and operation of the pressure switch and gauge and the operating temperature. (Pentair's drawdown-factor chart and limitations) That is why the article does not give a universal “a 44-gallon tank supplies X gallons” answer.

If a chart gives a factor for the exact model family and pressure band, a manufacturer-specific calculation may look like this:

published drawdown = charted total capacity × chart factor
minimum total capacity = required drawdown ÷ chart factor

For example, Pentair's catalog shows a .309 drawdown factor for a named Comptec model at 30 psi on and 50 psi off, then multiplies 245 gallons by .309 to show 75.7 gallons of drawdown. That is a named-model example, not permission to multiply any tank label by .309. (Pentair's named-model chart example)

Treat “not comparable” as a real result

The worksheet should return not comparable rather than a number when any of these is true:

• The exact tank model is unknown, or the chart is for a related-looking but different model.
• Cut-in and cut-out are assumed rather than verified.
• The pressure band in the chart does not match the observed or documented band.
• The pump is variable-speed, constant-pressure, transducer-controlled, or otherwise lacks a conventional discrete cycle.
• The candidate's maximum pressure, temperature limit, connection, or installation requirement is unknown.
• The gauge is unreadable or unstable and no reliable pressure record exists.
• The pump never reaches cut-out, never restarts normally, or is running continuously.

“Not comparable” is safer than making a false calculation from nominal gallons. Bring the exact model label, photographs of readable labels without opening anything, the current manual, gauge observations, pump/controller model, measured or documented flow, and the space measurements to the professional who will select or quote the replacement.

5. Use a drawdown measurement as evidence, not as a diagnosis

Water-contamination boundary before collecting any sample

This water observation is separate from the pressure-tank sizing decision. If contamination is suspected—for example after flooding, a sewage or chemical event, or a sudden unexplained change in water appearance, odor, or taste—treat the water as non-potable. Do not drink, touch, or otherwise use suspect water, and do not collect it for an ordinary household purpose. Stop the worksheet and contact the applicable local health or environmental authority for testing, containment, disposal, and return-to-use guidance. Use bottled water or another known-safe source until that authority says the water may be used again. CDC guidance likewise says not to drink a private well when contamination is suspected, to contact the local health department, and to test before returning to drinking or other use. (CDC's guidance for treating well water)

If the water-safety boundary is clear and the system is a conventional fixed-speed installation with the exact tank rating and relief protection known, a homeowner may be able to observe a drawdown cycle from a normal, tool-free downstream cold fixture. This is an observation worksheet, not a pressure-vessel service procedure.

Before collecting water, schedule the observation when irrigation, appliances, treatment regeneration, and other household demand are off. Do not close, move, bypass, or adjust unidentified valves. Confirm that the discharge will not flood the room, contact electrical equipment, damage a drain, or violate a local requirement. Use a container with a known volume or graduations, a timer, and one fixture only.

Record three trials if the first is safe and repeatable:

The definition matters: start collecting after the pump has stopped and the pressure has stabilized; stop counting at the pump's restart. Do not keep counting after restart and call that tank drawdown, because water after the pump starts includes pump delivery. Do not use a tank drain, relief-valve outlet, air valve, hot-water outlet, or any location near electrical equipment.

Calculate the repeatability range rather than hiding it in an average:

spread = highest valid gallons − lowest valid gallons
average = sum of valid gallons ÷ number of valid trials

A wide spread can come from an early or late restart, inconsistent faucet flow, competing demand, a sticky gauge, a treatment cycle, or a changing well condition. It does not identify a failed tank. A low observation that cannot be repeated under comparable conditions is evidence to improve the measurement or request professional diagnosis, not a reason to buy the largest tank on the shelf.

When the exact model and pressure band match, compare the observed gallons with the published drawdown and preserve the measurement uncertainty. Do not create a universal pass/fail percentage. Pentair specifically warns that actual drawdown varies with system variables, including switch and gauge accuracy and temperature. (Pentair's actual-drawdown limitations)

Interpreting common results

6. Handle 30/50, 40/60, and pressure-setting changes carefully

“30/50” means a 30 psi cut-in and a 50 psi cut-out in a conventional pressure-switch system. “40/60” means a 40 psi cut-in and a 60 psi cut-out. The shorthand is useful only when the settings have actually been verified and the tank's published chart includes that band.

Do not change the pressure switch to make a candidate tank chart appear to fit. A pressure-setting change can affect pump capability, fixtures, treatment equipment, relief protection, precharge, electrical controls, and the tank's maximum operating pressure. The Water Systems Council notes that typical systems commonly operate at 30/50 or 40/60 but also says pressure must be sufficient for the highest and farthest outlet and that a professional should be involved in final selection. (Water Systems Council's pump-pressure guidance)

The relationship between pressure settings and drawdown is not “higher is always better.” A different pressure band changes the chart factor. It may also expose a pump that cannot reach the new cut-out, reduce available flow at the fixture, or exceed a component rating. The correct sequence is to verify the system design and pump curve, select compatible tank and controls, confirm relief protection, and have qualified personnel make any adjustment.

Precharge is part of the engineering record, but the number is equipment-specific. Water Systems Council's typical sizing sheet describes precharge at 2 psi below cut-in. Pentair's Wellmate FAQ gives a Wellmate-specific example of 4 psi below cut-in and states that checking the air cell requires the pump power supply to be turned off and the tank drained. (Water Systems Council's precharge guidance) (Pentair's Wellmate air-cell instructions) The difference is exactly why a homeowner should not apply a remembered rule to a replacement tank.

Do not perform a precharge check yourself if it requires opening controls, isolating electrical equipment, releasing pressure, draining a pressurized vessel, removing a cap, connecting a gauge, adding air, or discharging water. A qualified professional should identify the exact tank manual, isolate and verify the electrical supply, relieve and drain the vessel using the equipment's safe procedure, and document the result before adjusting anything. If the tank has visible corrosion, a leak, deformation, unknown rating, or unknown relief protection, stop rather than attempting a precharge correction.

7. Make the replacement decision and hand off a complete record

Use the result below as the final decision tree:

1. Conventional system, complete inputs, candidate chart meets or exceeds required drawdown, and physical limits fit: the candidate is technically eligible for a professional quote. It is not yet an installation authorization.
2. Conventional system, chart drawdown is below the requirement: reject that model for this design, or revisit the documented inputs with a qualified professional. Do not “round up” from nominal gallons without a published chart.
3. Exact model or pressure band is unknown: return “not comparable.” Identify the model and verify the gauge or obtain a professional measurement.
4. Current drawdown is low but the tank also has a symptom such as leaking, corrosion, continuous pump operation, or rapid cycling: stop the homeowner test. The result may involve the tank, controls, pump, plumbing, well, or demand pattern.
5. Constant-pressure or variable-speed system: use the controller and motor manufacturer's tank-sizing instructions. The conventional fixed-speed formula is not the right decision surface.
6. Space, pressure rating, connection, temperature, relief protection, or local requirement fails: reject the candidate even if its drawdown is large enough.

Pentair's installation notes require an adequately sized relief valve, say the relief path must not be isolated from the tank by a check valve, and state that plumbing should comply with local codes and standards. Those are product instructions for the referenced tank families, not a national code; the authority having jurisdiction and the current manual for the selected tank control the installation. (Pentair's pressure-relief and local-compliance notes)

Bring this packet to the professional

Keep one dated record with:

• Exact tank manufacturer, model, serial or date code, label capacity, and visible condition.
• Pump, motor, controller, pressure-switch, gauge, and treatment-equipment model information.
• Measured or documented pump flow and its measurement condition.
• Target minimum run time and its source.
• Verified cut-in and cut-out pressures, including gauge location and whether the gauge behaved normally.
• Current drawdown trials, container resolution, timing method, competing demand, and uncertainty.
• Required design drawdown and the calculation showing which input controls.
• Candidate model's charted drawdown at the exact pressure band.
• Maximum pressure, connections, temperature limits, dimensions, service access, relief requirements, and local questions.

Ask the professional to identify the selected tank's published drawdown, show how it meets the required design drawdown, confirm that the pump can operate at the proposed pressure band, and explain any change from the existing tank. If the quote lists only “same size” or “44-gallon equivalent,” ask for the exact model and drawdown value. The useful answer is not a tank label; it is a traceable chain from pump flow and run time to drawdown, chart match, and safe installation fit.