How-to

Well Pressure Tank Precharge: What PSI Should Your Tank Be?

The air charge in an empty tank decides how much water you get between pump starts. It takes a tire gauge and ten minutes to check — and one warning to respect.

By Well Pump Handbook EditorialPublished How we research

On this page
  1. Precharge calculator
  2. What precharge is, and what "psi" on the gauge means
  3. Why the precharge matters
  4. How to check the precharge
  5. Adjusting it: who, when and how
  6. Constant-pressure systems use a different rule
  7. Common precharge questions
  8. Sources
Short answer

For a standard pressure-switch system, set the tank's air precharge 2 psi below the pump's cut-in pressure, measured with the tank empty: 18 psi for a 20/40 switch, 28 psi for 30/50 and 38 psi for 40/60. Amtrol, Pentair and the Water Systems Council all describe it this way.[1][2][3] Constant-pressure (variable-speed) systems are different: Franklin Electric and Grundfos specify about 70% of the pressure setpoint for their controllers.[4][5]

Precharge by pressure switch setting (tank empty)
Switch setting (cut-in/cut-out)Precharge
20/40 psi18 psi
30/50 psi28 psi
40/60 psi38 psi
Other settingscut-in − 2 psi

Pressure tank precharge calculator

What controls your pump?

This tool is for educational guidance and does not replace an onsite diagnosis. Rules used: cut-in − 2 psi (Amtrol, Pentair); 70% of setpoint (Franklin Electric SubDrive/MonoDrive table, Grundfos CU 301). Always follow your own tank's and controller's manuals.

What precharge is, and what "psi" on the gauge means

Precharge is the air pressure inside the tank when it holds no water. Your system pressure gauge shows water pressure; the tank's air pressure only matches it when the tank has water in it. That's why precharge is always measured with the pump off and the tank drained — a reading on a full tank just tells you the system pressure.

What happens inside a bladder pressure tank during one pump cycleThree tanks. Empty, the air side is at the precharge pressure of 28 psi. At the 30 psi cut-in only a little water remains. At the 50 psi cut-out, water has compressed the air and fills about a third of the tank. The water released between cut-out and cut-in is the drawdown. air Empty air at 28 psi air water Pump starts cut-in 30 psi air water Pump stops cut-out 50 psi draw-down

Illustrative levels for a 30/50 switch with a 28 psi precharge. The usable water between cut-out and cut-in is the drawdown; everything else in the tank is air or water that never leaves.

For the bigger picture — tank types, sizes and failure signs — see the well pressure tank guide.

Why the precharge matters

The Water Systems Council lists the reasons to get it right: a precharge that is too high can interrupt water flow, and one that is too low causes excessive cycling that can wear out the pressure switch, pump or tank early.[3] The Driller's worked examples put numbers on the "too low" case for an 85-gallon tank on a 30/50 switch:[6]

Effect of precharge on usable water — 85-gallon tank, 30/50 switch
PrechargeDrawdown per cycleLabel
28 psi (correct)25.1 galPublished example
20 psi≈20.4 galOur calculation, same formula
10 psi≈14.5 galOur calculation, same formula
No precharge8.7 galPublished example

Less drawdown means more pump starts for the same water use. A precharge set above cut-in has the opposite problem: the tank runs empty before pressure reaches cut-in, so there's a short stretch where pressure falls with no stored water behind it — the interruption the Water Systems Council warns about.

How to check the precharge

Safe to check

  1. Turn the pump's breaker off.
  2. Open a faucet near the tank and let it run until water stops and the system gauge reads zero.[3]
  3. Remove the cap from the tank's air valve.
  4. Read the pressure with a tire gauge. Note it next to your switch setting.
  5. Replace the cap, close the faucet, turn the breaker back on.

A digital or dial tire gauge that reads in 1 psi steps is accurate enough. If the reading is within a psi or two of your target, leave it alone.

Adjusting it: who, when and how

Read this before adding air

Amtrol's manual says not to adjust the precharge or re-pressurize a tank except for the adjustments required at installation — especially if the tank is corroded, damaged or has diminished integrity — because it can burst. It also warns that losing air, or a pump cycle that has become shorter, can signal internal corrosion. Precharge adjustments must be made at ambient temperature.[1]

In practice:

  • New tank or new switch setting: the installer sets the precharge during installation. Amtrol's procedure is to drain the tank, check the precharge, then release or add air to 2 psi below the switch cut-in.[1] The Water Systems Council also tells installers to check precharge before installing, because tanks aren't all shipped with the right charge for your system.[3]
  • Older tank that has lost air: treat it as a warning sign rather than a maintenance item. Have it evaluated; replacement is often the right call.
  • Changing from 30/50 to 40/60: the precharge has to change too (28 → 38 psi). Amtrol's manual includes a specific danger warning about raising the precharge on a tank already in service.[1] That's a job for the person changing the switch.

Constant-pressure systems use a different rule

A variable-speed controller holds one target pressure instead of cycling between cut-in and cut-out, so the "cut-in minus 2" rule doesn't apply.

Precharge for constant-pressure controllers (from manufacturer manuals)
ControllerManufacturer guidanceExample
Franklin Electric SubDrive / MonoDrivePrecharge table equal to about 70% of the pressure setting[4]50 psi setting (factory) → 35 psi
Grundfos SQE with CU 301Precharge = 70% of the pressure setting; if the precharge is higher than the setting, the system can't control pressure[5]50 psi setting → 35 psi
Other drivesFollow the drive's manual—

More on these systems: constant-pressure well systems.

Common precharge questions

Can I check the precharge without draining the tank?

No. With water in the tank, the air-valve reading equals the water pressure at that moment. It tells you nothing about the empty-tank charge.

How often should I check it?

Franklin Electric tells owners of its constant-pressure controllers to check the tank precharge regularly.[4] For any system, once a year is a sensible routine — the CDC recommends checking your well every spring for mechanical problems, so do it then.[7]

Does altitude change the target?

No. Tire gauges and system gauges both read gauge pressure, so "2 psi below cut-in" works at any elevation. Altitude slightly changes the tank's drawdown (The Driller's Lake Tahoe example gains about a gallon on an 85-gallon tank), not the setting.[6]

Sources

  1. Amtrol. Well-X-Trol Residential Pre-Pressurized Well Tanks — Installation & Operation Instructions (WX-101 through WX-350). Accessed Sept. 28, 2026.
  2. Pentair. Adjusting Well Pump Pressure Switches (instruction sheet U80-129). Accessed Sept. 28, 2026.
  3. Water Systems Council (wellcare). Pressure Tank Pre-charge — information sheet. Accessed Sept. 28, 2026.
  4. Franklin Electric. Submersible Motors: Application, Installation & Maintenance (AIM) Manual, 60 Hz. Accessed Sept. 28, 2026.
  5. Grundfos. CU 301 Installer's Manual (SQE constant-pressure control). Accessed Sept. 28, 2026.
  6. The Driller. How to Calculate and Control Pressure Tank Drawdown — Bob Pelikan (Feb. 1, 2006). Accessed Sept. 28, 2026.
  7. Centers for Disease Control and Prevention. Guidelines for Testing Well Water. Accessed Sept. 28, 2026.

We cite manufacturer documentation and public agencies first. Read how we choose and check sources.

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