Well Pressure Tank Calculator — Drawdown & Tank Size
A pressure tank is an air spring, not a bucket. On a 30/50 switch it gives up only 29.5% of its own volume before the pump has to start again — so a 20-gallon tank delivers about 5.9 gallons, not twenty. This works backwards from the run time your pump motor needs to the shell size that buys it, and shows what the pre-charge is really worth.
Size the pressure tank
The pump's delivery at your working pressure, not its nameplate maximum.
Picking a switch fills both settings below and re-derives the pre-charge.
Where the pump starts.
Where it stops.
2 psi under cut-in. Measure it drained and isolated.
Convention, not code. Longer is kinder.
Tank to fit
Smallest stocked shell covering the 33.9 gallon minimum. It delivers 13.0 gallons a cycle — 29.5% of its own volume.
Drawdown needed
10.0 gal
Drawdown fraction
29.5%
Check this before you buy
See the breakdown
Theoretical drawdown from Boyle's law. Real diaphragm tanks come in slightly under their published figure, so check the manufacturer's own drawdown table before ordering. Tank sizing is pump-industry practice, not plumbing code.
The formula, explained in plain English
How much water the pump has to deliver in one run, divided by the fraction of a shell that is actually water.
Absolute, not gauge
Boyle's law works on absolute pressure, so 14.7 psi is added to all three figures before any ratio is taken. Working in gauge pressure here overstates the drawdown badly, and worst at low pressures — exactly where wells operate.
It is a ratio, not a difference
This is why a higher pressure setting gives less drawdown for the same span. 20/40 and 40/60 are both 20 psi wide, but the first delivers 34.5% and the second only 25.8%.
Starts kill pumps, not hours
Every start draws locked-rotor current through a motor that cannot cool itself while stopped. A tank that buys one extra minute per cycle can cut daily starts by an order of magnitude, and that is the whole return on buying a larger shell.
Set the charge dry
Check the pre-charge with the tank isolated and drained. A gauge on a Schrader valve under system pressure reads the system, not the charge — the same trap as the domestic thermal expansion tank.
Worked examples
The standard house, the pre-charge fault that mimics a dead tank, and the case where widening the band is cheaper than buying a bigger one.
A 10 gpm submersible on a 30/50 switch
fraction at 28 psi pre-charge = 29.53%
shell = 10 ÷ 0.2953 = 33.87 gal
→ 44 gallon tank, delivering 12.99 gal = 1.30 min
Result: the tank people expect to be "about 20 gallons" is really a 44. The 20-gallon tank on the shelf next to it delivers 5.9 gallons — only 0.59 minutes of run.
"New tank, still short-cycling"
Same 20-gallon shell, same 30/50 switch, only the pre-charge changing. Nothing else on the system is different.
40 psi — over-charged: 15.46% → 3.09 gal
45 psi — badly over-charged: 7.73% → 1.55 gal
Result: a charge sitting 15 psi high cuts the delivery to 26% of what it should be, and the symptom is indistinguishable from a waterlogged tank. Check the Schrader valve before condemning the tank — it is a two-minute test.
Widening the band instead of buying a bigger tank
30/70 (40 psi span): 45.11% → shell 22.2 gal → 26 gal tank
Result: the same pump and the same one-minute run drop 44 gallons of shell to 26 purely by giving the switch more room to work in. The trade is real though: the fixtures now see 70 psi at cut-out, which is close enough to the 80 psi IPC 604.8 threshold to be worth checking, and everything downstream cycles over a wider pressure swing.
What the pressure switch is worth
Drawdown as a fraction of the shell, with the pre-charge set 2 psi under cut-in each time. The last column is what a 20-gallon tank actually gives you. Note that the three 20 psi bands are not equal.
| Switch | Span | Drawdown fraction | 20 gal tank gives |
|---|---|---|---|
| 20 / 40 psi | 20 psi | 34.46% | 6.89 gal |
| 30 / 50 psi | 20 psi | 29.53% | 5.91 gal |
| 40 / 60 psi | 20 psi | 25.79% | 5.16 gal |
| 50 / 70 psi | 20 psi | 22.88% | 4.58 gal |
| 30 / 70 psi | 40 psi | 45.11% | 9.02 gal |
| 40 / 80 psi | 40 psi | 40.69% | 8.14 gal |
What the pre-charge is worth
A 20-gallon tank on a 30/50 switch, with only the Schrader valve changing. The peak is at cut-in; the 2 psi margin below it costs about 1.4 of a percentage point and saves the diaphragm from bottoming out on every cycle.
| Pre-charge | Drawdown fraction | 20 gal tank gives | Verdict |
|---|---|---|---|
| 20 psi | 24.00% | 4.80 gal | Low — diaphragm sits compressed |
| 25 psi | 27.45% | 5.49 gal | Low — diaphragm sits compressed |
| 28 psi | 29.53% | 5.91 gal | Correct |
| 30 psi | 30.91% | 6.18 gal | At cut-in — best on paper, hard on the bag |
| 35 psi | 23.18% | 4.64 gal | Over-charged — short-cycles |
| 40 psi | 15.46% | 3.09 gal | Over-charged — short-cycles |
| 45 psi | 7.73% | 1.55 gal | Over-charged — short-cycles |
Tank by pump size
On a 30/50 switch, with the conventional minimum run time for each pump. The jump at 12 gpm is the run-time rule stepping up, not the pump.
| Pump | Min run | Drawdown needed | Shell | Tank |
|---|---|---|---|---|
| 5 gpm | 1.00 min | 5.0 gal | 16.9 gal | 20 gal |
| 8 gpm | 1.00 min | 8.0 gal | 27.1 gal | 32 gal |
| 10 gpm | 1.00 min | 10.0 gal | 33.9 gal | 44 gal |
| 12 gpm | 1.50 min | 18.0 gal | 61.0 gal | 62 gal |
| 15 gpm | 1.50 min | 22.5 gal | 76.2 gal | 86 gal |
| 20 gpm | 1.50 min | 30.0 gal | 101.6 gal | 119 gal |
| 25 gpm | 2.00 min | 50.0 gal | 169.3 gal | Two in parallel |
What each stocked tank really delivers
Every one of these is sold by its shell volume. At 30/50 with a correct pre-charge, this is the water you get — and the run time it buys a 10 gpm pump.
| Tank sold as | Drawdown at 30/50 | Run time at 10 gpm |
|---|---|---|
| 6 gal | 1.77 gal | 0.18 min |
| 14 gal | 4.13 gal | 0.41 min |
| 20 gal | 5.91 gal | 0.59 min |
| 26 gal | 7.68 gal | 0.77 min |
| 32 gal | 9.45 gal | 0.94 min |
| 44 gal | 12.99 gal | 1.30 min |
| 62 gal | 18.31 gal | 1.83 min |
| 86 gal | 25.39 gal | 2.54 min |
| 119 gal | 35.14 gal | 3.51 min |
Sources & standards: the drawdown equation is Boyle's law applied to the pre-charged air volume, computed here on absolute pressures with 14.7 psi of atmosphere added. Shell sizes are common stocked capacities and the minimum run times are pump-industry convention, not code — the IPC governs the potable system downstream of the tank, not how the well equipment is proportioned. Real diaphragm tanks deliver slightly under the theoretical figure, so confirm against the manufacturer's published drawdown table. Constant-pressure and variable-speed systems are sized differently.
Frequently asked questions
Common questions about drawdown, pre-charge pressure, and pump cycling.
What size pressure tank do I need for my well pump?
For a 10 gpm pump on a 30/50 psi switch, you need 10 gallons of drawdown — one full minute of pump run. Because a 30/50 tank only gives up 29.5% of its own volume, that takes a 33.9 gallon shell, so the 44-gallon tank. It actually delivers 13.0 gallons a cycle.
Why does a 20-gallon tank only hold about 6 gallons of water?
Because a pressure tank is an air spring, not a bucket. The air charge is compressed from the pre-charge up to cut-out, and only the volume swept between cut-out and cut-in is water you get without the pump running. At 30/50 that swept fraction is 29.5%, so a 20-gallon shell delivers about 5.9 gallons. The rest of the shell is air doing the work of holding pressure.
Does a higher pressure setting give more drawdown?
No — the opposite, and this surprises people. Drawdown is a ratio of absolute pressures, not a difference, so the same 20 psi span delivers more at a lower band. A 20-gallon tank gives 6.89 gallons at 20/40 but only 5.16 at 40/60. What genuinely helps is a wider band: 30/70 gives 45.1% and drops the required shell from 33.9 to 22.2 gallons.
What should the pre-charge be set to?
About 2 psi below cut-in — 28 psi on a 30/50 switch — measured with the tank isolated and drained, not while it is under system pressure. Setting it level with cut-in squeezes out a fraction more drawdown (30.9% against 29.5%), but leaves the diaphragm slapping the bottom of the shell on every cycle. The margin buys diaphragm life for about a point of performance.
What happens if the pre-charge is too high?
The drawdown collapses. At 30/50 a correct 28 psi charge gives 29.5%, but 45 psi gives 7.7% — the tank empties before the pump even restarts, so it short-cycles as if there were no tank at all. It is the commonest fault on a system that "has a new tank and still hammers", and it costs nothing to check.
Why does minimum run time matter more than tank size?
Because pump motors are killed by starts, not by running. Each start draws locked-rotor current and heats the windings, and a submersible cannot shed that heat while stopped. The convention is at least one minute of run per cycle up to 10 gpm, 1.5 minutes to 20 gpm, and 2 minutes above that. The tank exists to buy that minute — which is why the calculation starts from run time and works backwards to a shell size.
Is any of this in the plumbing code?
No. Tank sizing and minimum run time are pump-industry practice, not IPC requirements — the code governs the potable system downstream, not how the well equipment is proportioned. That is why every input here is editable. Manufacturers also publish per-model drawdown tables that account for real diaphragm geometry, and those figures usually land slightly under the theoretical Boyle's-law number below.
What about a constant-pressure or variable-speed pump?
Different problem entirely. A variable-speed pump modulates to match demand instead of cycling between two pressures, so it needs only a small tank — often 2 to 20 gallons — to absorb transients rather than to store a minute of flow. This calculator models a conventional on/off pump with a pressure switch. If the system is constant-pressure, size to the pump manufacturer's spec, not to a run-time rule.
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