The short answer
A 10 GPM well pump on a 30/50 pressure switch needs a pressure tank of at least 34 gallons total volume. The pump should run 1 minute or more each start, so the tank must deliver 10 gallons between starts, and only about 29.53% of a tank's volume comes out at 30/50 psi. Set the air charge to 28 psi.
Tank size by pump flow and switch setting
Smallest tank, total gallons, for a 1 minute minimum run time.
| Pump | 20/40 psi | 30/50 psi | 40/60 psi |
|---|---|---|---|
| 5 GPM | 15 gal | 17 gal | 20 gal |
| 7 GPM | 21 gal | 24 gal | 28 gal |
| 10 GPM | 30 gal | 34 gal | 39 gal |
| 12 GPM | 35 gal | 41 gal | 47 gal |
| 15 GPM | 44 gal | 51 gal | 59 gal |
| 20 GPM | 59 gal | 68 gal | 78 gal |
| 25 GPM | 73 gal | 85 gal | 97 gal |
How it works
The formula
The tank has to give the pump something to fill: at least its flow times the minimum run time. A diaphragm tank only delivers part of its volume, set by how far the air inside compresses between the two switch pressures (Boyle's law).
usable share = (air charge + 14.7) × (1 ÷ (cut-in + 14.7) − 1 ÷ (cut-out + 14.7))
tank = drawdown ÷ usable share
Assumptions
- The formula and the 1 minute minimum run time follow Pentair's WellMate pressure tank sizing guide. Our usable shares match its drawdown chart: 20/40 psi 0.34, 30/50 0.3, 40/60 0.26.
- The air charge is 2 psi below cut-in, the usual setting; 14.7 psi turns gauge pressure into absolute.
- The tank size is total volume; tank labels also list the drawdown at each setting, which you can compare with the drawdown here.
Worked example: 10 GPM pump, 30/50 switch
- Drawdown: 10 GPM × 1 min = 10 gal
- Usable share: (28 + 14.7) × (1 ÷ 44.7 − 1 ÷ 64.7) = 0.295
- 10 ÷ 0.295 = 33.9 → at least 34 gallons
Too big or too small
- Too small: short cycling. The pump starts every few seconds of water use, wearing out the motor, the switch and the check valves.
- Waterlogged tank: same as too small. If the air charge is gone, the tank holds little water; check it once a year with the pump off.
- Raise the switch, raise the tank. At 40/60 a tank delivers less of its volume than at 30/50, so it has to be bigger.
Questions people ask
What size pressure tank do I need for a 10 GPM pump?
At least 34 gallons of total tank volume on a 30/50 switch. The pump should run at least 1 minute each start, so the tank must hold 10 gallons of drawdown, and at 30/50 only about 29.53% of a tank's volume comes out between pump starts. On a 40/60 switch it's 39 gallons.
What is drawdown?
The water a pressure tank delivers between the pump shutting off and turning back on. It is only part of the tank: the rest is the compressed air that pushes the water out. Tank labels list drawdown at 20/40, 30/50 and 40/60 psi.
What should the air pressure in my tank be?
2 psi below the pump's cut-in pressure, checked with the pump off and the tank empty: 28 psi for a 30/50 switch, 38 psi for 40/60.
Is a bigger pressure tank better?
For the pump, yes: fewer, longer runs mean less wear and heat in the motor. A bigger tank also stores more water during a power outage. The limits are space, cost and the floor load of a full tank.
Why does the minimum run time matter?
A well pump motor heats up at every start and needs time running to cool and to carry the heat away. Pentair's WellMate guide asks for at least 1 minute; check your pump's manual, as some larger motors need more.