System guide
Well Pressure Tanks: How They Work, Sizing & Common Problems
The pressure tank is the part that decides how often your pump starts. It's also the part most likely to fail first — and the easiest to check.
On this page
A well pressure tank holds water against a cushion of compressed air. When you open a faucet, the air pushes water out so the pump doesn't have to start right away; when pressure falls to the switch's cut-in setting, the pump refills the tank to cut-out. Only a fraction of the tank's volume is usable water — roughly 30% at 30/50 psi and 26% at 40/60 with a correct precharge.[1] Size the tank so the pump runs at least a minute per start, set the air precharge 2 psi below cut-in, and replace the tank if water appears at its air valve.
How a pressure tank works
Modern well tanks are pre-pressurized: a rubber diaphragm or bladder separates a sealed air charge from the water. Amtrol's manual describes the cycle in two steps — the pump fills the tank and compresses the air until the pressure switch turns the pump off; as you use water, pressure drops until the switch starts the pump again. The water delivered between those two points is the drawdown.[2]
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.
Three numbers control how that cycle behaves:
The three settings that matter
- Cut-in
- Pressure at which the switch starts the pump (e.g., 30 psi).
- Cut-out
- Pressure at which it stops the pump (e.g., 50 psi).
- Precharge
- Air pressure in the empty tank. Amtrol and Pentair specify 2 psi below cut-in.[2][3] Precharge explained.
Types of well tanks
| Type | How air is kept | What to know |
|---|---|---|
| Diaphragm tank | A flexible diaphragm fixed inside the shell keeps a sealed air charge above the water | A widely used residential design (Amtrol's Well-X-Trol line is a diaphragm tank); the air charge is set through a tire-style valve. |
| Bladder tank | Water is held inside a bag; air surrounds it | Same principle; some larger models allow bladder replacement. |
| Air-over-water (conventional) tank | No separator — air sits directly on the water and slowly dissolves into it[4] | Needs a way to replenish air, and waterlogs over time. The Driller's example: an 85-gallon tank of this kind delivers about 8.7 gallons at 30/50, versus about 25 gallons for a precharged tank of the same size.[1] |
How much water a tank really delivers
A tank's gallon rating is its total volume, not the water you get between pump starts. The table below is calculated with the Boyle's-law method published in The Driller[1], assuming the precharge is 2 psi below cut-in and a sea-level atmosphere. Manufacturer tables can differ slightly, and tanks lose drawdown when the precharge is low.
| Total volume | 20/40 psi (18 psi precharge) | 30/50 psi (28 psi) | 40/60 psi (38 psi) |
|---|---|---|---|
| 20 gal | 6.9 | 5.9 | 5.2 |
| 30 gal | 10.3 | 8.9 | 7.7 |
| 40 gal | 13.8 | 11.8 | 10.3 |
| 50 gal | 17.2 | 14.8 | 12.9 |
| 60 gal | 20.7 | 17.7 | 15.5 |
| 80 gal | 27.6 | 23.6 | 20.6 |
| 100 gal | 34.5 | 29.5 | 25.8 |
| 120 gal | 41.3 | 35.4 | 31.0 |
Notice the trade-off: a higher pressure setting gives better pressure at the faucet but less water per cycle from the same tank. That's one reason a switch upgrade can make a pump start more often. See 30/50 vs. 40/60.
Sizing in one paragraph
The tank should give the pump enough stored water to run a minimum time per start. Water Well Journal's method: multiply the pump's flow in gallons per minute by the minimum run time — 1 minute for pumps under 2 HP, 2 minutes at 2 HP and above — then divide by the drawdown factor for your pressure setting.[5] A 10 GPM pump at 30/50 needs about 10 gallons of drawdown, which works out to a tank of roughly 34 gallons total volume. Full method and examples: how to size a well pressure tank, or use the tank size calculator.
Common problems
- TroubleshootingHow to tell if a pressure tank is badWaterlogged, losing air or corroded — five checks and when to replace.
- How-toPressure tank prechargeWhat PSI the tank should be, how to check it, and when not to add air.
- How-toWhat size pressure tank do I need?Size by pump GPM, run time and switch setting — with worked examples.
- TroubleshootingWell pump short cyclingPump turns on and off every few seconds — tank, check valve, leak or switch.
Where the tank sits and what's on the tank tee
Most tanks sit in a basement, utility room or well house, next to the pressure switch. Amtrol recommends installing the tank as close as possible to the pressure switch, and if the tank is placed at the end of a long pipe run, relocating the switch with it or adjusting the setting for the pressure drop.[2] A typical tank tee holds:
- the pressure switch and its ¼-inch sensing nipple,
- a pressure gauge,
- a relief valve — Amtrol requires one, set to open at excessive pressure (125 psig or more for its residential tanks), to protect the tank if the switch fails to stop the pump,[2]
- a drain valve used for flushing and for checking the precharge.
A typical submersible-pump system. Jet-pump systems put the pump above ground instead (see the pump-type comparison).
- Pump and motor — sits below the water level and pushes water up.
- Check valve — keeps water from draining back down the well when the pump stops. Often built into the pump; more may be installed higher up.
- Pitless adapter — where the line leaves the casing, below the frost line.
- Pressure tank — an air cushion stores water under pressure so the pump doesn't start for every glass of water.
- Pressure switch and gauge — the switch turns the pump on at the cut-in pressure and off at the cut-out pressure.
- House supply — filters, softeners and treatment equipment usually sit on this line.
- Electrical supply — a dedicated breaker feeds the pressure switch, which switches power down the well. Three-wire pumps also have a control box on this circuit.
Pressure tank safety
Burst hazard
Amtrol's installation manual warns that a tank that is corroded, damaged or has lost integrity can burst if its precharge is adjusted or it is re-pressurized, and that a loss of air or a shorter pump cycle than when it was new can be a sign of internal corrosion.[2] Before any work on the tank, turn off power to the pump and release the water pressure. Never cap or plug a relief valve.
Lifespan and replacement cost
Tank makers don't publish a guaranteed service life, and it depends on water chemistry and how hard the tank cycles. HomeGuide, which compiles installed-cost data, reports that tanks last 10 to 15 years on average and cost $800 to $1,500 to replace, installed:[4]
| Tank size | Tank only | Installed |
|---|---|---|
| 20–29 gal | $200–$500 | $450–$1,000 |
| 30–39 gal | $300–$850 | $550–$1,350 |
| 40–59 gal | $350–$1,100 | $600–$1,600 |
| 60–79 gal | $500–$1,300 | $750–$1,800 |
| 80–99 gal | $600–$1,800 | $850–$2,300 |
| 100–119 gal | $850–$2,000 | $1,100–$2,500 |
Sources
- The Driller. How to Calculate and Control Pressure Tank Drawdown — Bob Pelikan (Feb. 1, 2006). Accessed Sept. 28, 2026.
- 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.
- HomeGuide. How Much Does It Cost to Replace a Well Pressure Tank? (article dated July 25, 2025). Accessed Sept. 28, 2026.
- Water Well Journal. Tank Selection by the Numbers — David L. Kill, PE (April 19, 2022). Accessed Sept. 28, 2026.
We cite manufacturer documentation and public agencies first. Read how we choose and check sources.