Pressure tank sizing calculator
Enter the pump's flow and the pressure switch setting. You get the drawdown the tank needs so the pump runs at least a minute, the total tank volume that gives it, the precharge to set, and the most times an hour the pump can start.
Inputs
Result
Common tank sizes at your setting
Total tank volume, the drawdown it gives at your switch setting and precharge, and the most starts an hour (when the house uses half the pump’s flow).
| Tank total volume | Drawdown | Pump runs at least | Most starts an hour |
|---|
How a pressure tank is sized
A well pressure tank holds a cushion of air above a bladder or diaphragm. The pump fills it to the cut-out pressure and stops; the house draws water until the pressure falls to the cut-in and the pump starts again. The water you get between those two pressures is the drawdown, and it is what decides how often the pump starts.
Each start heats the motor, so the tank is sized to make the pump run for at least a minute each time. That means at least one gallon of drawdown for every gpm of pump flow.
The factor is Boyle’s law on the air charge, with 14.7 psi added to turn gauge pressure into absolute. With the precharge 2 psi below the cut-in, it works out at 34% of the tank on a 20/40 switch, 30% on 30/50, 26% on 40/60 and 23% on 50/70. Higher pressures squeeze the air harder, so the same tank holds less usable water. As a check, the BC plumbing apprenticeship textbook works a 15 gallon tank to 5.48 gallons at 20/40 and 4.64 at 30/50 with the precharge at the cut-in; the formula gives 5.48 and 4.64.
Worked example
A 10 gpm pump on a 40/60 switch, precharged to 38 psi, needs 10 gallons of drawdown. The factor is 0.258, so the tank must be at least 10 ÷ 0.258 = 39 gallons total; the next common size is 44 gallons. On a 30/50 switch the same pump needs 34 gallons, so a 44 gallon tank.
Pressure tank drawdown chart
Gallons of drawdown by total tank volume and switch setting, precharge 2 psi below cut-in.
| Tank total volume | 20/40 | 30/50 | 40/60 | 50/70 |
|---|---|---|---|---|
| 14 gal | 4.8 | 4.1 | 3.6 | 3.2 |
| 20 gal | 6.9 | 5.9 | 5.2 | 4.6 |
| 26 gal | 9.0 | 7.7 | 6.7 | 5.9 |
| 32 gal | 11.0 | 9.4 | 8.3 | 7.3 |
| 44 gal | 15.2 | 13.0 | 11.3 | 10.1 |
| 62 gal | 21.4 | 18.3 | 16.0 | 14.2 |
| 86 gal | 29.6 | 25.4 | 22.2 | 19.7 |
| 119 gal | 41.0 | 35.1 | 30.7 | 27.2 |
Pressure tank size by pump flow
Minimum total volume for a one-minute run, and the common size to buy, by pump flow and switch setting.
| Pump | 20/40 | 30/50 | 40/60 | 50/70 |
|---|---|---|---|---|
| 5 gpm | 15 → 20 | 17 → 20 | 19 → 20 | 22 → 26 |
| 7 gpm | 20 → 26 | 24 → 26 | 27 → 32 | 31 → 32 |
| 8 gpm | 23 → 26 | 27 → 32 | 31 → 32 | 35 → 44 |
| 10 gpm | 29 → 32 | 34 → 44 | 39 → 44 | 44 → 44 |
| 12 gpm | 35 → 44 | 41 → 44 | 47 → 62 | 52 → 62 |
| 15 gpm | 44 → 44 | 51 → 62 | 58 → 62 | 66 → 86 |
| 20 gpm | 58 → 62 | 68 → 86 | 78 → 86 | 87 → 119 |
Setting the precharge
Set the air precharge 2 psi below the cut-in pressure: 18 psi for a 20/40 switch, 28 for 30/50, 38 for 40/60, 48 for 50/70. Measure it at the air valve with the pump off and the tank drained, because any water in the tank raises the reading. A precharge set too high makes the pump short-cycle; too low wastes drawdown.
How often the pump starts
The pump cycles most when the house draws exactly half its flow. Then each cycle takes four times the drawdown divided by the pump’s flow, so the most starts an hour is 15 x pump gpm ÷ drawdown. A tank sized at one gallon per gpm caps it at 15 starts an hour; a bigger tank brings that down.
Questions
What size pressure tank do I need?
Enough drawdown for the pump to run at least a minute: one gallon per gpm. A 10 gpm pump on a 40/60 switch needs a tank of at least 39 gallons total, so 44 gallons; on a 30/50 switch, 44 gallons.
What size pressure tank for a family of 4?
The pump decides it, not the family. A three-bedroom, two-bath house needs about 10 to 12 gpm (1 gpm per fixture, or 12 gpm in the BC peak demand table), so 10 to 12 gallons of drawdown: a 44 gallon tank on a 30/50 switch, or 44 to 62 gallons on 40/60.
Can a pressure tank be too big?
Not for the pump: a bigger tank means fewer, longer runs, which is easier on the motor. The limits are price and floor space.
What should the precharge be on a well pressure tank?
2 psi below the cut-in: 28 psi for a 30/50 switch, 38 psi for 40/60. Set it with the pump off and the tank empty of water.
How much drawdown does a 20 gallon pressure tank have?
About 5.9 gallons on a 30/50 switch and 5.2 gallons on 40/60, with the precharge 2 psi below cut-in.
Does a 40/60 switch need a bigger tank than 30/50?
Yes. At higher pressure the air compresses less per psi, so drawdown falls from 30% of the tank on 30/50 to 26% on 40/60. The same pump needs about 14% more tank.
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