Well Pressure-Tank Sizing Calculator
Size a well pressure tank so the pump runs long enough on each cycle to avoid short-cycling. Enter a target minimum run time, the pump’s GPM and your pressure switch — you get the required drawdown and the total tank size.
Calculator
For a 1.5-minute minimum run at 10.0 gpm you need about 15.0 gal of drawdown; with a 30-50 switch (acceptance ~0.30) that is a 50-gallon total tank. A bigger tank means fewer pump starts and a longer pump life.
“What size pressure tank for a 10 GPM pump?” is really a question about run time. A pressure tank is not there to store much water — it is there to keep the pump from starting too often, which is what wears a submersible pump out. This calculator turns a target minimum run time and your pump’s flow into the drawdown you need, then converts that to a total tank size using your pressure switch.
Formula
Size the tank so each pump cycle delivers a target amount of stored water (the drawdown) before the pump has to restart:
drawdown gal = run time min × pump GPMtank size gal = drawdown ÷ acceptance factor
The acceptance factor is the fraction of the tank’s total volume that is actually usable drawdown between the cut-in and cut-out pressures. A wider or higher pressure band stores more usable water per gallon of tank, so the factor rises with the switch setting: about 0.20 at 20/40, 0.30 at 30/50, and 0.40 at 40/60 psi.
Worked example
You want the pump to run at least 1.5 minutes per cycle, it delivers 10 GPM, and it uses a 30/50 psi switch (acceptance ~0.30):
drawdown = 1.5 × 10 = 15 galtank size = 15 ÷ 0.30 = 50 gal
So a 50-gallon total pressure tank gives the required 15 gallons of drawdown. A tank rated by total volume of 50 gallons is a common size for this duty. Sizing up to the next tank means fewer starts and a longer pump life, at the cost of a larger footprint.
Why the tank exists: stopping short-cycling
The point of a pressure tank is not really to store much water — it is to keep the pump from short-cycling. Every time a pump starts, the inrush current and mechanical stress are hard on it; a submersible motor that starts too often fails early. The tank absorbs each cycle’s demand so the pump runs a sensible minimum time (often one to two minutes) and starts fewer times per day.
That is why the math is built around a target run time rather than a household’s gallons. Pick a minimum run time, multiply by the pump’s flow to get the drawdown you need each cycle, then divide by the acceptance factor to convert usable drawdown into a total tank size. A modern bladder/diaphragm tank keeps air and water separate, so it holds its acceptance factor over time; an old plain steel tank can become waterlogged and lose almost all its drawdown.
Set the tank’s air pre-charge to about 2 psi below the pump’s cut-in pressure — roughly 28 psi for a 30/50 switch. Too little or too much pre-charge collapses the usable drawdown no matter how big the tank is. When in doubt, choose the larger of two candidate tanks: bigger tanks reduce starts, run cooler and quieter, and rarely cause problems.
Reference table
Drawdown acceptance factor by pressure-switch setting (labeled planning figures). A wider or higher band stores more usable water per gallon of tank.
| Cut-in / cut-out (psi) | Acceptance factor |
|---|---|
| 20-40 | ~0.20 |
| 30-50 | ~0.30 |
| 40-60 | ~0.40 |
Frequently asked questions
What size pressure tank for a 10 GPM pump?
Decide a minimum run time first. For a 1.5-minute run at 10 GPM you need 15 gallons of drawdown; with a 30/50 switch (acceptance ~0.30) that is a 50-gallon total tank. Target a longer run time or a wider switch band and the tank gets larger.
What is the acceptance factor?
It is the fraction of a tank’s total volume that is usable drawdown between cut-in and cut-out. It is not 100% because the tank always holds some reserve. It rises with the pressure band: about 0.20 at 20/40, 0.30 at 30/50, and 0.40 at 40/60 psi.
Why does short-cycling matter?
Each pump start stresses the motor with inrush current and heat. A pump that starts too often wears out early. A correctly sized tank gives the pump a minimum run time and cuts the number of daily starts, which is the single biggest factor in pump life.
What air pre-charge should the tank have?
Set the air pre-charge about 2 psi below the cut-in pressure — roughly 28 psi for a 30/50 switch. An incorrect pre-charge destroys the usable drawdown regardless of tank size, so check it with a gauge when the system is depressurized.