Pressure Tank Sizing Calculator: Find the Right Tank for Your Well Pump
A pressure tank that is too small makes your pump start and stop over and over, and that short cycling is one of the fastest ways to kill a well pump. A tank that is too big just costs more and takes up space.
This free pressure tank sizing calculator does the math for you. Enter your pump’s flow rate and pressure switch setting, and it tells you the minimum tank size, the right precharge and the standard tank to buy, in US or metric units.
Boreholeflow is reader supported. When you buy through links on this page, I may earn a commission at no extra cost to you. Read the full disclosure.
Assumes precharge 2 psi below cut-in and standard well tank sizes. Constant pressure (variable speed) systems need a much smaller tank; follow the maker’s guide.
How the Calculator Works
Pressure tank sizing comes down to one goal: the tank must hold enough water to let your pump run for a minimum time before it shuts off. Most pump makers want at least one minute of run time per cycle so the motor can cool and the starting surge does not wear it out.
The water your tank delivers between cut-out and cut-in is called drawdown. It is always much smaller than the tank’s total volume, because air takes up most of the space inside. The calculator follows three steps.
- Drawdown needed = pump flow rate × minimum run time. A 10 GPM pump running for 1 minute needs 10 gallons of drawdown.
- Drawdown factor comes from Boyle’s law and your switch setting. It is the share of the tank’s total volume you can actually use.
- Minimum tank volume = drawdown needed ÷ drawdown factor. The calculator then rounds up to the next standard tank size.
Drawdown Factor by Pressure Setting
Here are the factors the calculator uses, with precharge set 2 psi below cut-in. Notice how the usable share falls as the pressure goes up.
| Switch setting | Precharge | Drawdown factor | Usable water per 10 gal of tank |
|---|---|---|---|
| 20/40 psi | 18 psi | 0.34 | 3.4 gal |
| 30/50 psi | 28 psi | 0.30 | 3.0 gal |
| 40/60 psi | 38 psi | 0.26 | 2.6 gal |
| 50/70 psi | 48 psi | 0.23 | 2.3 gal |
Quick Sizing Chart
If you just want a fast answer, use this chart. It shows the minimum total tank volume for common pump sizes at a one minute run time.
| Pump flow | Minimum tank at 30/50 | Minimum tank at 40/60 | Standard size to buy (40/60) |
|---|---|---|---|
| 5 GPM (19 L/min) | 17 gal | 19 gal | 20 gal |
| 8 GPM (30 L/min) | 27 gal | 31 gal | 32 gal |
| 10 GPM (38 L/min) | 34 gal | 39 gal | 44 gal |
| 12 GPM (45 L/min) | 41 gal | 47 gal | 62 gal |
| 15 GPM (57 L/min) | 51 gal | 58 gal | 62 gal |
| 20 GPM (76 L/min) | 68 gal | 78 gal | 86 gal |
For pumps of 2 HP or more that need a two minute run, double the minimum volume. The calculator handles this when you change the run time setting.
A Worked Example
Say you have a 1 HP submersible pump delivering 12 GPM, with a 40/60 pressure switch. The pump needs 12 gallons of drawdown for a one minute run, and the drawdown factor at 40/60 is about 0.26.
Dividing 12 by 0.26 gives a minimum of about 47 gallons. The next standard size up is a 62 gallon tank, which delivers roughly 16 gallons per cycle and gives the pump a comfortable margin.
Common Sizing Mistakes
- Buying by total volume alone. A “20 gallon” tank delivers only about 5 to 7 gallons. Always size by drawdown.
- Raising the switch without upsizing. Moving from 30/50 to 40/60 cuts your drawdown by about 13 percent. See my guide to well pressure switch settings.
- Forgetting the precharge. A tank with the wrong air charge behaves like a smaller tank. Check it yearly with the system drained.
- Ignoring short cycling signs. If your pump already clicks on and off quickly, read my short cycling diagnosis guide before buying.
Recommended Pressure Tanks
Amtrol Well-X-Trol
The Well-X-Trol is the tank I find most often on well systems, and its diaphragm design and steel shell have a long track record. It comes in every standard size the calculator recommends, from 20 to 119 gallons.
For a full comparison of brands, read my best well pressure tanks guide and the head to head of Well-X-Trol vs Flexcon vs Water Worker. Prefer a printable version of this math? Get the free Pressure Tank Sizing Checklist.
Frequently Asked Questions
What size pressure tank do I need for a 10 GPM pump?
At 40/60 psi you need at least 39 gallons of total volume, so a 44 gallon tank is the right standard size. At 30/50 the minimum drops to about 34 gallons.
Is a bigger pressure tank better?
Bigger means longer pump runs and fewer starts, which helps pump life. Beyond the right size, the benefit is small compared with the extra cost and space.
What is pressure tank drawdown?
Drawdown is the amount of water the tank delivers between the pump shutting off and starting again. It is usually about a quarter to a third of the tank’s total volume.
How long should a well pump run each cycle?
Most makers recommend at least one minute, and larger pumps often need two. Shorter runs heat the motor and wear the switch.
Can I use two smaller tanks instead of one large one?
Yes. Tanks plumbed together on the same line add their drawdown, which is handy when space or doorways are tight.
