Tool
Pressure Tank Precharge Calculator
Enter your switch setting (or constant-pressure setpoint) and, if you have it, your tire-gauge reading. The calculator shows the target and what your reading means.
Pressure tank precharge calculator
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.
The rules this calculator uses
| System | Rule | Source |
|---|---|---|
| Pressure switch (on/off) | Cut-in − 2 psi, tank empty | Amtrol Well-X-Trol manual;[1] Pentair switch instructions[2] |
| Franklin SubDrive / MonoDrive | About 70% of setpoint (table: 50 psi → 35 psi) | Franklin Electric AIM manual[3] |
| Grundfos SQE with CU 301 | 70% of the pressure setting | Grundfos CU 301 manual[4] |
For a reading you enter, the calculator calls anything within 2 psi of the target "in range." That tolerance is our editorial choice, based on typical tire-gauge precision — not a manufacturer specification.
Other well calculators: see all tools.
How to take the reading
- Turn off the pump's breaker.
- Open a faucet near the tank until water stops and the gauge reads zero — the Water Systems Council describes the same steps.[5]
- Read the tank's air valve with a tire gauge.
Don't pump up a damaged tank
Amtrol warns that adjusting the precharge of a tank that is corroded, damaged or has lost integrity can make it burst.[1] If the reading is low on an older tank, see how to tell if a pressure tank is bad first.
This tool is for educational guidance and does not replace an onsite diagnosis.
Sources
- Amtrol. Well-X-Trol Residential Pre-Pressurized Well Tanks — Installation & Operation Instructions (WX-101 through WX-350). Accessed Sept. 28, 2026.
- Pentair. Adjusting Well Pump Pressure Switches (instruction sheet U80-129). Accessed Sept. 28, 2026.
- Franklin Electric. Submersible Motors: Application, Installation & Maintenance (AIM) Manual, 60 Hz. Accessed Sept. 28, 2026.
- Grundfos. CU 301 Installer's Manual (SQE constant-pressure control). Accessed Sept. 28, 2026.
- Water Systems Council (wellcare). Pressure Tank Pre-charge — information sheet. Accessed Sept. 28, 2026.
We cite manufacturer documentation and public agencies first. Read how we choose and check sources.