What the tank is for
People assume a pressure tank is water storage. It is not, mostly. Its job is to give the pump somewhere to push water into so that small draws — a toilet refilling, someone rinsing a cup — are served from the tank rather than by starting a motor. Every start is wear, and the number of starts per day is what determines whether a submersible pump lasts fifteen years or three.
Inside a modern tank is a rubber bladder or diaphragm with water on one side and compressed air on the other. The air is what does the pushing. When the pump fills the tank, it squeezes the air; when a tap opens, the air expands and forces water out until the pressure falls to cut-in, at which point the switch starts the pump again.
Why drawdown is a fraction, not the tank size
Only the water that leaves between cut-out and cut-in is usable. Above cut-out the pump has stopped; below cut-in it has started again. So the drawdown is set by how much the air volume changes across that pressure band, which is a gas law question rather than a plumbing one.
Drawdown ≈ tank volume × (1 − (cut-in + 14.7) ÷ (cut-out + 14.7))
The 14.7 converts gauge pressure to absolute, which is what the gas law needs. On a 30/50 switch that gives 1 − (44.7 ÷ 64.7), or about 30.9 percent. An 86 gallon tank therefore holds back about 60 gallons and gives up about 26.6, which is close to what manufacturers publish for tanks of that size on that switch.
| Switch setting | Drawdown as a fraction | From a 20 gal tank | From an 86 gal tank |
|---|---|---|---|
| 20 / 40 | 36.5% | 7.3 gal | 31.4 gal |
| 30 / 50 | 30.9% | 6.2 gal | 26.6 gal |
| 40 / 60 | 26.8% | 5.4 gal | 23.0 gal |
| 30 / 60 | 40.2% | 8.0 gal | 34.6 gal |
Two things fall out of that table. Higher pressure settings give less drawdown from the same tank, because the same 20 psi band is a smaller proportional change at the top of the scale. And widening the band raises drawdown substantially — going from 30/50 to 30/60 adds thirty percent more usable water without buying anything. Whether your system tolerates the higher cut-out is a separate question involving the pump, the fittings and everything downstream, and it is one to put to whoever services the well.
Sizing from run time
The usual rule of thumb runs the other way: decide how long the pump should run per start, multiply by its delivery rate, and that is the drawdown you need. Divide by the fraction and you have the tank.
A 10 GPM pump wanting a one minute run needs 10 gallons of drawdown. On a 30/50 switch that is 10 ÷ 0.309, or about 32 gallons of tank. Tanks come in fixed sizes so you round up, and rounding up is always the right direction — a larger tank costs a little more once and buys longer runs forever.
The run time to aim for is not a universal figure. Small submersibles are commonly given a minute; larger motors, and three-phase equipment, are often given considerably more, and the manufacturer of the pump is the authority on its own motor. If you do not know, the direction is clear enough: longer is easier on the equipment.
Precharge is the maintenance item nobody does
The air charge in the tank is set with the system depressurised and the tank drained, measured at the Schrader valve on top with an ordinary tire gauge, and set conventionally a couple of psi below the cut-in pressure. If the precharge drifts — and it does, slowly, through the valve and through the bladder — the drawdown falls and the pump starts cycling more often long before anything obviously breaks.
A tank that gives almost no drawdown at all, so that the pump starts and stops within a few seconds, has usually either lost its charge entirely or ruptured its bladder. The quick test is to press the Schrader valve on a drained tank: air means the bladder is probably intact, water means it is not. Rapid cycling is the symptom that should send you to check this before you buy anything.
Pressure and flow at the fixtures are downstream questions. If the pressure at the tank is fine and at the far end of the house it is not, the loss is in the pipe run and the pressure drop calculator will show you where it goes. If you are working out what a run holds for flushing or disinfection after a well repair, the pipe volume calculator covers that. Plumbing work is regulated, and the requirements are not the same from one town to the next. There are several model codes, jurisdictions adopt different ones and then amend them, and the local authority having jurisdiction decides what applies to your building. Most permanent work needs a permit and an inspection. Anything that touches the water service, the gas line, or a backflow-prevention point is licensed work in essentially every jurisdiction. What is on this page is common working practice and arithmetic, not a requirement you can hold up to an inspector.
Questions people ask
What size pressure tank do I need for a 10 GPM well pump?
It depends on the run time you want and the pressure switch setting, not on the pump alone. Ten gallons per minute with a one minute target run needs 10 gallons of drawdown, which on a 30/50 switch means roughly a 32 gallon tank, on 20/40 roughly a 27 gallon tank, and on 40/60 roughly a 37 gallon tank. The same pump, the same target, three different tanks. Pick the switch setting first, then size the tank, and round up to the next size the tank is made in.
Why does my well pump start every few seconds?
Almost always because the tank has lost its air charge or the bladder has failed, so there is nothing storing energy between cut-in and cut-out. Check the precharge with the system off and the tank drained: it should read a couple of psi below cut-in on a tire gauge. If pressing the valve gives water rather than air, the bladder is ruptured and the tank is finished. Less commonly the pressure switch has drifted so the band is very narrow, or a leak downstream is bleeding pressure continuously. Rapid cycling destroys pump motors quickly, so it is worth chasing straight away rather than living with it.
Should I set the precharge to the cut-in pressure exactly?
The usual practice is a couple of psi below cut-in, and there is a reason for the gap. If the precharge equals or exceeds cut-in, the bladder can bottom out against the tank as the last of the water leaves, which is hard on it and can produce a brief surge of air at a fixture. A few psi of margin keeps a little water in the tank at the switch point. Set it with the tank drained and the pump off, because a pressurised tank reads the water pressure rather than the air charge and you will get a meaningless number.
Is a bigger tank always better?
Bigger is easier on the pump, and that is the main thing tanks are for, so the answer is usually yes up to the point where cost and floor space stop you. There are two counterweights. Water sits in a large tank longer between uses, which matters more on a system with treatment or in a seasonal property, and the physical size of an 80 gallon tank is a genuine constraint in a small utility room. Two smaller tanks plumbed in parallel add their drawdown together and are a normal way to get more storage where one large one will not fit.
Does the calculator work for a constant pressure system?
Not really. A variable speed constant pressure pump throttles its output to match demand and holds a set pressure rather than swinging between cut-in and cut-out, so the drawdown arithmetic that governs a conventional system does not describe what it does. Those systems use a small tank, sized by the manufacturer, and the sizing logic belongs to the controller rather than to a pressure band. Use this page for a conventional pressure switch system.