SEC. 01 / Free tool · Pressure tank sizing
What size pressure tankdo you need?
Size it from your pump, not your house. Put in the flow rate and your pressure switch setting, and this works out the drawdown you need and the tank that delivers it.
Free· no signup·Math checked against Amtrol tables
On the pump nameplate or your original well record. Most homes are 5 to 15 GPM.
Printed inside the pressure switch cover. 40/60 is the most common.
Drawdown you need
10.0 gal minimum
20.0 gal for a 2-minute run (longer pump life)
Total tank volume
37.4 gal minimum
74.7 gal to hit the 2-minute target
Off-the-shelf, that is a 44-gallon tank to clear the minimum, or a 86-gallon for the 2-minute run most drillers size for. Check that model's published drawdown chart before you buy, because the number on the label is a model name and not always the measured volume.
At a 40/60 switch, only 27%of a tank's volume comes out as water - the rest stays compressed air. Set the air pre-charge to 38 PSI (2 below cut-in) with the pump off and the tank drained. Manufacturer drawdown charts are figured with the pre-charge at cut-in, so expect roughly 4% less water in practice.
SEC. 02 / Drawdown
The number on the label is not what you get.
A bladder tank holds water and compressed air together, and the air stays put. The water that comes out between pump starts is the drawdown, and it is a fraction of the number painted on the shell.
How big a fraction depends entirely on your pressure switch. Raise your cut-out pressure and your effective tank gets smaller, because the air cushion has to be squeezed harder to push the same water out.
This catches people every day. A 44-gallon tank sounds like it holds 44 gallons. It gives you about 12.
| Switch (PSI) | Usable fraction | From a 44 gal tank |
|---|---|---|
| 20/40 | 37% | 16.1 gal |
| 30/50 | 31% | 13.6 gal |
| 40/60 | 27% | 11.8 gal |
| 50/70 | 24% | 10.4 gal |
Figured on a correctly pre-charged tank with a healthy bladder. Manufacturer charts are calculated with the pre-charge at cut-in, so a tank charged the recommended 2 PSI lower gives up roughly 4% less than these figures.
SEC. 03 / Short cycling
An undersized tank kills the pump, not the tank.
A submersible motor pulls several times its running current the instant it starts, and the only thing carrying that heat away is water moving past the motor. It needs to run long enough to cool itself down. Franklin Electric puts a hard floor on it in their submersible motor installation manual: run the motor at least one minute every time it starts.
They also cap how often it can start. A single-phase motor between 1 and 5.5 HP is rated for 100 starts in 24 hours. Smaller motors up to 0.75 HP get 300. A tank that is too small burns through that allowance before dinner.
Same pump, same bathtub, two tanks
20 gallon tank
8 starts
5.4 gallons per cycle. The pump runs 32 seconds at a time, which is under Franklin's one-minute floor on every single start.
86 gallon tank
2 starts
23 gallons per cycle, and the motor runs well past two minutes. Same pump, same water, a quarter of the wear.
Filling a 40-gallon tub with a 10 GPM pump on a 40/60 switch.
The pump does not fail the day you install the wrong tank. It fails years early, and by then nobody connects the two. Replacing a pressure tank costs a few hundred dollars. Pulling and replacing a submersible pump costs thousands.
SEC. 04 / The math
Two steps, both worth checking.
First, how much water you need between starts. That is your pump's flow rate times the minimum run time, which is one minute for a pump under 2 HP and two minutes at 2 HP and above.
required drawdown (gal) = pump GPM x run time (min)
Second, how much tank it takes to hold that. Only part of a tank comes out as water, and the fraction falls out of Boyle's law once you know the switch setting.
usable fraction = (cut-out - cut-in) / (cut-out + 14.7)
tank volume (gal) = required drawdown / usable fraction
Worked through for a 10 GPM pump on a 40/60 switch: 10 gallons of drawdown, divided by 0.27, is 37 gallons of tank. The next size you can buy is a 44.
Where these numbers come from
The sizing procedure and the 2 HP run-time breakpoint are Water Well Journal's, from “Tank Selection by the Numbers.” The one-minute floor and the starts-per-day limits are Franklin Electric's, from their submersible motor installation manual.
We check the drawdown formula against Amtrol's published Well-X-Trol tables. It reproduces them: Amtrol prints 6.2 gallons for a 20-gallon tank at 30/50 and 5.4 at 40/60, and the formula gives 6.18 and 5.35.
SEC. 05 / Field notes
Tank sizing questions.
Start with your pump's flow rate, not the size of your house. The tank has to hold enough water that the pump runs at least a full minute every time it starts. A 10 GPM pump therefore needs 10 gallons of drawdown, and because a tank only gives up about a quarter to a third of its volume as water, that works out to roughly a 44-gallon tank on a 40/60 switch. Enter your own numbers in the calculator above for a figure that matches your setup.
A bladder tank is part water and part compressed air, and the air never leaves. The water you actually get between pump starts is the drawdown, and at a 40/60 switch it is about 27% of the tank's total volume. A tank labeled 44 gallons delivers somewhere near 12 gallons at the tap. That is normal and it is how the tank is designed to work, but it catches people out when they buy by the number on the label.
About 37 gallons of total volume at a 40/60 switch to clear the one-minute minimum, so you buy the next size up, which is a 44-gallon tank. If you want the two-minute run that most drillers prefer, you need roughly 75 gallons and would buy an 86. Both figures come out of the calculator above, and both assume a pump under 2 HP.
A submersible motor draws several times its running current at startup, and the only thing carrying that heat away is water moving past the motor. Franklin Electric's submersible motor manual tells installers to run motors at least one minute per start for exactly that reason, and caps a 1 to 5.5 HP single-phase motor at 100 starts per 24 hours. An undersized tank blows through that budget in an afternoon. The pump does not fail that day; it fails years early.
No. Your pressure switch sets your pressure, and a larger tank does not change it. What a larger tank buys you is more water between pump starts, which means fewer starts, a cooler motor, and longer life for the switch and the pump. If you want more pressure, that is a switch adjustment or a constant-pressure system.
Two PSI below your cut-in pressure, checked with the power off and the tank drained of water. A 40/60 switch wants 38 PSI. Check it once a year with a tire gauge, because a tank that has lost its charge waterlogs and starts short cycling even though it is the right size on paper.
Some people do, and the plumbing is not exotic. The parts that go wrong are the ones that are easy to underestimate: cutting into a live pressurized line, getting the pre-charge right on the new tank, and re-wiring the switch without creating a hazard. Most states also want well work done by a licensed contractor. If you would rather not find out the hard way, we list licensed well contractors by area.
Need someone to put it in?
Swapping a tank means cutting into a pressurized line and setting the pre-charge correctly on the new one. Find licensed well contractors near you.
Want the background rather than the number? Read the well pressure tank guide for tank types, lifespan and replacement cost. If you already have a tank and want to know what it holds, the well water pressure guide runs the calculation the other way round. Do not know your pump's flow rate? It is often on the original well record.
SEC. 06 / For drilling contractors
Sizing tanks all day? Put it on the quote.
DrillerDB runs quoting, scheduling, well logs, state reports and invoicing for drilling companies across the country, off the same well data behind this tool.
Disclaimer: These figures are standard industry sizing math and are not a substitute for a professional evaluation of your system. Check the drawdown chart for the specific tank model you buy, and consult a licensed well contractor before replacing equipment.