Well Pressure Tank Size Calculator
This free well pressure tank calculator runs the math most sizing advice skips: Boyle's Law. A "36-gallon" pressure tank does not hold 36 usable gallons — it holds an air cushion and a bladder of water, and only the volume the air can push out between your pump's cut-in and cut-out pressures is usable. On a common 40/60 switch that is only about a quarter of the tank. Undersize the tank and the pump cycles every few seconds of draw — short-cycling — which is the leading killer of well pumps.
Enter your pump's output in GPM (from the label, or a bucket test) and your pressure-switch setting, and the calculator computes the required drawdown — one gallon per GPM, guaranteeing the one-minute minimum run that protects the motor — then divides by the acceptance factor with all pressures converted to absolute, the detail that separates the real formula from the rules of thumb. The result is the minimum total tank size; round up to the next standard size on the manufacturer's chart.
The results also cover the maintenance side: the pre-charge should sit about 2 psi below cut-in, checked with the pump off and a faucet open, because a lost air charge turns a correctly sized tank into a short-cycling one. Reference math only — manufacturer drawdown charts for your exact model govern, and this assumes sea-level atmospheric pressure. Free, no signup.
Well Pressure Tank Calculator
What size pressure tank stops your pump from short-cycling? Enter the pump GPM and switch setting — only about a quarter to a third of a tank is usable water, and this runs the real Boyle's-Law math instead of a guess. Free, no signup.
Pump & pressure switch
Leave pre-charge blank to use the standard setting — 2 psi below cut-in. One minute is the accepted minimum run time to protect the pump motor.
Calculation Formulas
P1 = pre-charge, P2 = cut-in, P3 = cut-out — all gauge + 14.7 psi. The fraction of total tank volume that is usable water between pump cycles.
Example:
38/40/60 psi gauge → ≈0.26; only about a quarter of the tank is water you can use.
The rule of thumb: 1 gallon of usable drawdown per 1 GPM of pump output, guaranteeing at least a 1-minute run.
Example:
10 GPM pump → 10 gallons of drawdown.
Then round UP to the next standard size on the manufacturer chart.
Example:
10 gal ÷ ~0.26 ≈ 39 gal → next standard size up.
Checked with the pump off and the tank drained. A lost air charge is the top cause of short-cycling.
Example:
40/60 switch → 38 psi pre-charge.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Atmospheric pressure | 14.7 psi | Sea-level constant for gauge→absolute conversion; the math shifts slightly at altitude. |
| 40/60 usable fraction | ≈26–28% | Research reference: ~28%; exact value depends on pre-charge. |
| 30/50 usable fraction | ≈31% | Lower switch settings make more of the tank usable. |
| Minimum run time | ≥1 minute | Protects the pump motor from short-cycle heating. |
| Worked example | 10 GPM, 40/60 → ~36–39 gal | The research anchor this calculator reproduces. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
Manufacturer drawdown charts(Amtrol Well-X-Trol (and equivalents))
View StandardPublished drawdown per model at each standard switch setting — the authoritative check on this screening math.
Key Requirements:
- •Confirm model drawdown ≥ required drawdown at YOUR switch setting
Boyle's Law(P1V1 = P2V2 (isothermal))
View StandardThe physics of the captive air cushion — why usable volume is a fraction of tank size and why pressures must be absolute.
Key Requirements:
- •Absolute pressures (gauge + 14.7)
Pump protection practice(SSPMA / pump manufacturer guidance)
View StandardMinimum-run-time and cycle-rate limits that motivate the 1 gal per GPM rule.
Key Requirements:
- •≥1 min run
- •Avoid short-cycling
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Altitude
The 14.7 assumption
Gauge→absolute conversion uses sea-level atmospheric pressure. At high elevation, atmospheric pressure drops (~12.2 psi at 5,000 ft), shifting acceptance slightly — the direction is conservative for this sizing.
Regional Examples:
Low-yield wells
Bigger tank as buffer
On wells that recover slowly, installers deliberately oversize the tank well past the minimum so household draw leans on stored water instead of the well.
Regional Examples:
Permits & scope
Who can do the work
Tank swaps involve plumbing and often electrical (pressure switch) — permit treatment varies; well work in some states must go through licensed well contractors.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Plan disposal before you start
Smaller jobs still produce more debris than a few trash bags can hold. Check what's allowed in a dumpster and which disposal option fits the scope.
See disposal options →
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How to Use This Calculator
- Enter your pump output in GPM — it's on the pump nameplate or spec sheet, or measure it with a bucket test at the tank tee.
- Pick your pressure-switch setting: 30/50 and 40/60 are the standards (printed inside the switch cover), or enter custom cut-in/cut-out pressures.
- Leave pre-charge blank to use the standard setting — 2 psi below cut-in — or enter your measured value.
- Keep the 1-minute minimum run time unless your pump manufacturer specifies longer.
- Click Calculate for the required drawdown, the acceptance factor (what fraction of the tank is usable water), and the minimum total tank size to buy.
Already short-cycling?
Before buying a bigger tank, check the pre-charge: pump off, drain the tank, and read the air valve with a tire gauge. It should be about 2 psi below your cut-in pressure. A waterlogged tank (failed bladder) or lost air charge causes short-cycling even when the size is right — and either way, this calculator tells you what the correctly working setup should be.
Frequently Asked Questions
What size pressure tank do I need for a 10 GPM pump?
Using the standard sizing rule — 1 gallon of drawdown per GPM for a 1-minute minimum run — a 10 GPM pump needs 10 gallons of usable drawdown. On a 40/60 switch, only about 26–28% of a tank's volume is usable water, so that's roughly a 36-to-39-gallon total tank minimum; round up to the next standard size on the manufacturer's chart. On a 30/50 switch the usable fraction improves to about 31%, shrinking the required tank slightly.
Why is only a quarter of my pressure tank usable water?
Boyle's Law. The tank holds a captive air cushion that compresses as the pump fills the bladder and expands to push water out as you draw. Between a 40 psi cut-in and 60 psi cut-out, the air can only expand enough to expel about a quarter of the tank's volume — computed with absolute pressures (gauge + 14.7 psi), which is the detail casual explanations miss. Wider pressure bands and lower settings increase the usable fraction; the physics can't be cheated, only sized around.
What should my pressure tank's pre-charge be?
About 2 psi below the pump's cut-in pressure — 38 psi for a 40/60 switch, 28 psi for a 30/50. Check it with the pump off and the tank fully drained (open a faucet), using a tire-style gauge on the air valve. A pre-charge that's leaked down is the number-one cause of short-cycling: the tank holds plenty of water but almost none of it is deliverable between switch trips. Recheck annually; bladder tanks lose a little air over time.
What happens if my pressure tank is too small?
Short-cycling — the pump starts and stops every few seconds during any sustained draw. Each start spikes motor current and heat; pump motors are designed around minimum run times of about a minute, and rapid cycling dramatically shortens pump life (and burns out pressure switches). A too-small tank is the classic cause, alongside a lost air charge or failed bladder. Oversizing, by contrast, is harmless to the pump — it just costs more and takes up floor space.
Is a bigger pressure tank better for a low-yield well?
Often, yes — deliberately. On wells that recover slowly, installers oversize the tank well past the calculated minimum so that household demand draws from stored water instead of forcing the pump to chase a weak well. This calculator returns the floor that protects the pump; low-yield situations size above it, and severe cases add a separate storage tank with a booster pump. Your well's tested yield (GPM recovery) decides which regime you're in.