Free Calculator · Published October 2026
Pressure Tank Sizing Calculator for Well Pumps
Enter the pump’s gallons per minute and the pressure switch cut-in and cut-out. The calculator returns the drawdown you need, the air precharge, and a common nominal tank size so the pump runs at least a minute before it stops.
Built by SubcontractorHub — the software plumbing contractors use to quote, finance, and run every job.
Book a DemoWhat Size Pressure Tank Does This Pump Need?
The pump’s flow sets the drawdown. The pressure switch sets how much of the tank you can actually use.
The pump’s delivery at the pressure switch, not the household’s fixture count.
Cut-in is when the pump starts. Cut-out is when it stops.
Longer run time means a larger tank and fewer motor starts.
📋 Important: All calculator results are ballpark estimates
The figures shown are approximate estimates from Boyle’s law with precharge 2 psi below cut-in. They are not a substitute for the manufacturer’s drawdown chart or a licensed well contractor. Actual acceptance volume varies by tank design. Always confirm precharge, switch setting and tank size on site.
How Pressure Tank Sizing Works
Drawdown is the number that matters
A pressure tank is a small reservoir between the well pump and the house. When a faucet opens, the tank gives up water and pressure falls. When pressure hits cut-in, the pump starts. When it hits cut-out, the pump stops. The gallons delivered in that window are the drawdown.
The gallon figure on the tank is total volume, air plus water. At a typical 30/50 switch only about 30 percent of that volume is usable water. Buying a “20 gallon” tank because the house “needs 20 gallons” undersizes the system by a factor of three, and the pump short-cycles. The well pump size calculator is the companion check for flow and total dynamic head.
Drawdown factors by switch setting
These factors assume a precharge 2 psi under cut-in and atmospheric pressure of 14.7 psi. They are planning figures from Boyle’s law, not a single brand’s catalog.
| Switch | Precharge | Drawdown factor |
|---|---|---|
| 20/40 | 18 psi | 34% of total volume |
| 30/50 | 28 psi | 30% of total volume |
| 40/60 | 38 psi | 26% of total volume |
| 50/70 | 48 psi | 23% of total volume |
A higher switch raises household pressure and shrinks drawdown. Moving from 30/50 to 40/60 on the same tank cuts usable water, so the pump starts more often unless the tank grows with the switch.
Worked examples
| Case | Drawdown needed | Nominal tank |
|---|---|---|
| 7 gpm pump, 30/50, 1 minute | 7 gal | 32 gal |
| 10 gpm pump, 30/50, 1 minute | 10 gal | 44 gal |
| 10 gpm pump, 40/60, 1 minute | 10 gal | 44 gal |
| 12 gpm pump, 30/50, 2 minutes | 24 gal | 86 gal |
Where the tank sits in a treatment train
The pressure tank is part of the well, not part of the softener. Sediment filters, iron filters and softeners go downstream of it. A backwashing filter also needs enough flow from the pump to lift the bed, which is a separate check from tank size. The well water treatment system selector puts those stages in order once the tank and pump can support them.
Constant-pressure or variable-speed packages use a much smaller tank because the drive, not a large drawdown, limits cycling. This calculator is for a conventional pressure switch and a bladder or diaphragm tank. If the job is a variable-speed drive, follow that manufacturer’s tank spec instead of these drawdown factors.
What a dealer should record on the job
Write down pump gpm, cut-in, cut-out, measured precharge, and the tank’s nominal size on the customer record. The next tech who hears a pump clicking every ten seconds can compare those numbers before replacing a motor that was only short-cycling. That is the same record a water treatment dealer uses when the iron filter downstream cannot backwash because the pump was never sized for it.
Frequently Asked Questions
How do you size a well pressure tank?
Size the tank on drawdown, not on the gallons printed on the shell. Drawdown is the usable water between the pressure switch cut-in and cut-out. Multiply the pump's flow in gallons per minute by the minimum run time you want, usually at least one minute, and that product is the drawdown you need. Divide by the drawdown factor for the switch setting. At a 30/50 switch with the precharge 2 psi below cut-in, the factor is about 30 percent, so a pump that moves 10 gpm needs about 10 gallons of drawdown and a tank of roughly 34 gallons total, which lands on a common 44 gallon nominal tank.
What is drawdown on a pressure tank?
Drawdown is the gallons of water the tank can give up between the moment the pump shuts off and the moment it starts again. It is always smaller than the tank's total volume, because a precharged air cushion takes up the rest. Raising the pressure switch, from 30/50 to 40/60 for example, shrinks drawdown on the same tank, so the pump cycles more often unless you move up a tank size.
What pressure should a well pressure tank be set to?
Set the air precharge 2 psi below the pressure switch cut-in, with the tank empty of water. On a 30/50 switch that is 28 psi. On a 40/60 switch it is 38 psi. Check it with a tire gauge on the air valve at the top of the tank after the pump has been off and a faucet has drained the pressure. A precharge that is too low makes the tank waterlogged and the pump short-cycles. A precharge above cut-in means the pump starts before the tank has given up its water.
What size pressure tank do I need for a house?
It depends on the pump's gallons per minute and the pressure switch, not on the number of bathrooms by itself. A common residential submersible around 7 to 12 gpm on a 30/50 switch, sized for a one-minute minimum run, usually lands on a 32, 44 or 62 gallon nominal tank. A 40/60 switch needs a larger tank for the same pump because the drawdown factor is smaller. Use the calculator with the pump's actual flow rather than a bathroom guess.
Why does my well pump short cycle?
Short cycling means the pump starts and stops every few seconds. The usual causes are a waterlogged tank (precharge lost, or a failed bladder), a tank that is too small for the pump's flow, or a pressure switch set so high that drawdown has collapsed. Each start is the hardest moment for the motor, so a pump that clicks on and off all day fails early. Confirm precharge first, then compare tank drawdown with the pump's gpm.
Does a bigger pressure tank increase water pressure?
No. The pressure switch sets the pressure. A larger tank only stores more water between cycles, so the pump runs longer and rests longer. If the showers are weak, look at the switch setting, the pump, a clogged filter, or a pressure regulator, not at tank volume alone. Moving from 30/50 to 40/60 raises pressure and at the same time reduces drawdown, so the tank often has to grow when the switch is raised.
Keep the Pump, the Tank and the Treatment on One Job
Quote the well equipment and the treatment train together, and keep switch settings, precharge and filter service on the customer record instead of a notebook in the truck.
Get a Free DemoAll calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements. Drawdown factors assume a bladder or diaphragm tank with precharge 2 psi below cut-in. Confirm the manufacturer chart and measure precharge with a gauge before you leave the job.