Choose a Well Tank That Gives Your Pump Enough Runtime
Compare well tank drawdown at 30/50 and 40/60 PSI, calculate capacity from pump flow and runtime, and check model-specific precharge before buying.
Choose a bladder tank for a well by its published drawdown at your pressure settings, not the total gallons on its label. For a conventional on/off pump, required drawdown equals pump delivery in gallons per minute multiplied by required runtime in minutes. A pump delivering approximately 10 GPM with a one-minute runtime target needs approximately 10 gallons of drawdown—not merely a tank labeled “10 gallons.”
Enter pump delivery and required runtime, then select your pressure settings to compare three example tanks.
Compare Required Drawdown
Required drawdown
10 gallons
WX-250 is the smallest of these three tanks that meets this target at 40/60 PSI.
This checks drawdown only, not installation compatibility or controller requirements.
Defaults are the article’s example, not a pump recommendation. Use the pump’s operating flow and the runtime required by its instructions. Not for variable-speed sizing.
| Amtrol model | Tank volume | Drawdown 40/60 | Meets target |
|---|---|---|---|
| WX-202 | 20 gal | 5.4 gal | No |
| WX-203 | 32 gal | 8.6 gal | No |
| WX-250 | 44 gal | 11.8 gal | Yes |
Other Pressure Setting and Selection Limits
At 30/50 PSI, published drawdown is 6.2 gallons for WX-202, 9.9 gallons for WX-203 and 13.6 gallons for WX-250. At 40/60 PSI, it is 5.4, 8.6 and 11.8 gallons respectively.
These figures apply to the named models, not every tank of the same volume. Match precharge to the exact tank and controller instructions. If none meets the target, use a manufacturer’s sizing table to find sufficient drawdown; this tool does not extrapolate a tank size.
Sources: Pentair WellMate sizing guidance (flow × runtime); Amtrol August 2025 brochure (model drawdown).
The calculator compares conventional pressure-switch systems only. Confirm the runtime requirement in the pump and motor instructions. For a variable-speed well pump, identify the controller and follow its tank-sizing requirements instead.
Drawdown, Not Tank Volume, Spaces Out Pump Starts
A precharged well tank separates compressed air from water with a flexible bladder, air cell or diaphragm. As the pump fills the water side, it compresses the air. In a conventional pressure-switch system, pressure rises to cut-out and the pump stops. Opening a faucet lets the compressed air push water out until pressure falls to cut-in, when the pump restarts.
The water delivered between those settings is called drawdown. That usable volume—not total tank volume—is what spaces out pump starts. Amtrol’s installation manual explains this operating cycle.
The tank buffers demand; it does not increase the well’s sustainable yield or the pump’s delivery capability. A larger tank can reduce cycling, but it is not a cure for an inadequate water supply.
Size From Operating Flow and Required Runtime
Required drawdown in gallons equals pump delivery in gallons per minute multiplied by required runtime in minutes.
Pentair’s tank-sizing guidance uses this calculation. Use the pump’s delivery at the system’s operating head, established from its curve or an appropriate flow test—not an unrestricted-flow rating. Then select a tank with enough published drawdown at your actual cut-in/cut-out pressures.
A tank labeled “20 gallons” may fall well short of the approximately 10 gallons of drawdown needed in the example above:
| Example Tank | Total Volume | Drawdown, 30/50 PSI | Drawdown, 40/60 PSI |
|---|---|---|---|
| Amtrol WX-202 | 20 gal | 6.2 gal | 5.4 gal |
| Amtrol WX-203 | 32 gal | 9.9 gal | 8.6 gal |
| Amtrol WX-250 | 44 gal | 13.6 gal | 11.8 gal |
These are model-specific figures from Amtrol’s August 2025 brochure, not universal values for every tank of the same nominal size. For the approximately 10-gallon requirement, the WX-250 meets the target at either setting; the WX-203’s published drawdown falls below it at both.
The figures also show why increasing operating pressure can change the tank size needed. The tank’s precharge must match the manufacturer’s instructions for the chosen settings.
Do not treat one minute as a universal rule. That Amtrol brochure’s conventional sizing table uses one minute for pumps up to ¾ HP and two minutes for pumps over ¾ HP. Its variable-frequency-drive guidance instead gives priority to the drive manufacturer’s tank-size and precharge recommendations. The calculator’s one-minute default is an example, not a runtime recommendation for your pump.
Set Precharge by the Exact Tank Instructions
Precharge is the air pressure specified with the tank’s water side drained and unpressurized. A reading taken while water remains under system pressure does not establish the empty-tank precharge.
“Two PSI below cut-in” is not a safe universal specification. The two manufacturers give different instructions:
| Product | Published Precharge Guidance |
|---|---|
| Amtrol Well-X-Trol | 2 PSI below cut-in; for example, 38 PSI with a 40/60 switch setting |
| Pentair WellMate | 4 PSI below cut-in; check with pump power off and the tank drained |
Follow the instructions for the exact tank and controller rather than averaging these numbers or copying another installation. Use the Well-X-Trol manual linked above or Pentair’s WellMate FAQ, as applicable.
Do not simply add air to an old tank that has started short-cycling. Amtrol warns against repressurizing tanks with lost air, corrosion or diminished integrity and reserves precharge checks and adjustments for licensed professionals. Before tank work, pump power must be off and system water pressure released. These are explicit safety requirements in its manual.
Confirm Fittings, Installation Limits and Repairability
Products described casually as “bladder tanks” can use different flexible separators. That affects both the precharge specification and whether the separator can be replaced. Confirm that the proposed model is a potable-water well pressure tank intended to control pump cycling—not merely another vessel with a similar shape.
Before ordering, have the supplier or installer verify the tank’s published drawdown at your pressure settings. Also confirm port size, thread type and connection location: a replacement need not match the old tank’s fittings.
Check the approved orientation, dimensions, service access and support for the tank’s operating weight. The installation must also meet the model’s working-pressure rating and requirements for a relief valve, freeze protection and provision for leakage. Amtrol’s manual calls for installation close to the pressure switch to limit the effects of friction loss and elevation differences.
For repairability, ask whether the exact model has a replaceable separator and an available replacement kit. Pentair says its WellMate air cell can be replaced, but the tank must be removed and disassembled. That does not establish repairability for other brands or designs.
If rapid cycling prompted the replacement rather than a confirmed tank defect, start with water pressure tank troubleshooting. For a purchasing quote, record the existing tank model, observed cut-in/cut-out pressures, pump model and delivery rate, and controller type. Those details let the supplier check usable capacity and compatibility rather than simply quote another tank with the same nominal gallons.