Well Pressure Tank Size Calculator

Size a well pressure tank so the pump runs at least 1 minute each cycle (2 minutes for pumps over 3/4 hp). A 10 gpm, 1 hp pump needs 20 gallons of drawdown. On a 40/60 switch a tank gives about 26% of its volume as drawdown, so you need about 78 gallons: an 86-gallon tank.

86-gallon tank

Drawdown needed: 20 gal
Usable drawdown at 40/60: 25.8% of tank volume
Minimum tank volume: 77.5 gal
This tank's drawdown about: 22.2 gal
Pre-charge (air) pressure: 38 psi (tank empty of water)

Show the math

drawdown needed = 10 gpm × 2 min = 20 gal

usable share at 40/60 = 25.8%

minimum tank = 20 ÷ 0.2579 = 77.5 gal

86-gallon tank gives 22.2 gal of drawdown

pre-charge = 40 - 2 = 38 psi

86-gallon tankBack to calculator

How to use this calculator

Enter your pump's flow rate in gallons per minute. It's on the pump paperwork or the well driller's report.
Pick the pump horsepower. Pumps over 3/4 hp should run at least 2 minutes per cycle.
Pick your pressure switch setting. It's printed inside the switch cover, for example 40/60.
Read the tank size and set the tank's air pre-charge to the number shown, with the water drained.

The formula

A pressure tank stores water above an air cushion. Drawdown is the water you can use between the pump shutting off and turning back on. The pump should run long enough each time to cool its motor, which means the tank's drawdown must be at least the pump's flow times the minimum run time.

Drawdown needed = pump gpm × minimum run time (1 minute up to 3/4 hp, 2 minutes above)

Tank size = drawdown needed ÷ that share

The share of the tank you can use depends on the switch setting. With the pre-charge 2 psi below the cut-in pressure, it's about 34% at 20/40, 30% at 30/50 and 26% at 40/60.

Example 1: 10 gpm, 1 hp, 40/60 switch. The pump needs 2 minutes, so 10 × 2 = 20 gallons of drawdown. 20 ÷ 0.258 = 77.5 gallons. The next standard size is 86 gallons, which gives about 22.2 gallons of drawdown. Set the pre-charge to 38 psi.

Example 2: 7 gpm, 1/2 hp, 30/50 switch. The pump needs 1 minute, so 7 gallons of drawdown. 7 ÷ 0.295 = 23.7 gallons, so a 26-gallon tank, giving about 7.7 gallons of drawdown. Pre-charge: 28 psi.

What this calculator doesn't cover

Constant-pressure (variable speed) pumps. They need much smaller tanks; follow the pump maker's chart.
Homes at high altitude. Air pressure is lower, which changes drawdown slightly.
Tanks piped together. Add their drawdowns.
Irrigation or livestock demand that runs the pump for long periods.
Brand-specific ratings. Manufacturers publish their own drawdown tables, which can differ by a few percent.

Drawdown by tank size and switch setting (gallons)

Tank (gal)	20/40	30/50	40/60
20	6.9	5.9	5.2
26	9.0	7.7	6.7
32	11.0	9.4	8.3
44	15.2	13.0	11.3
62	21.4	18.3	16.0
86	29.6	25.4	22.2
119	41.0	35.1	30.7

Calculated with Boyle's law, pre-charge 2 psi below cut-in, at sea level. Goulds publishes 5.3 gallons for its 19.9-gallon tank at 40/60 and 22.8 for its 84.9-gallon tank.

Minimum tank size for common pumps (1 minute up to 3/4 hp, 2 minutes above)

Pump	30/50	40/60
5 gpm, 1/2 hp	20	20
7 gpm, 1/2 hp	26	32
10 gpm, 3/4 hp	44	44
10 gpm, 1 hp	86	86
12 gpm, 1 hp	86	119
15 gpm, 1 1/2 hp	119	119
20 gpm, 2 hp	2 × 119	2 × 119

Tips

Check the pre-charge with the pump off and the tank drained, using a tire gauge on the air valve.
A waterlogged tank makes the pump click on and off every few seconds. That short-cycling wears out the pump and the switch.
Bigger is fine. A larger tank only lengthens each cycle and the time between starts.
If you raise the switch from 30/50 to 40/60, raise the pre-charge too, to 38 psi.

FAQ

What size pressure tank do I need for my well?

Multiply your pump's gpm by its minimum run time (1 minute up to 3/4 hp, 2 minutes above) to get the drawdown you need. Then divide by the usable share at your switch setting: about 0.30 at 30/50 or 0.26 at 40/60. A 10 gpm, 1 hp pump on 40/60 needs an 86-gallon tank.

What is pressure tank drawdown?

It's the amount of water the tank delivers between the pump shutting off at cut-out pressure and turning back on at cut-in. It's always much smaller than the tank's total size: about 26% of the volume at a 40/60 setting.

What should the air pressure be in my well pressure tank?

Set it 2 psi below the pump's cut-in pressure with the tank drained of water. That's 28 psi for a 30/50 switch and 38 psi for 40/60.

Is a bigger pressure tank better?

Usually. A bigger tank means longer, fewer pump cycles, which is easier on the motor. It costs more and takes more space, but it doesn't lower your water pressure.

What happens if my pressure tank is too small?

The pump starts and stops too often, called short-cycling. Each start heats the motor and wears the switch, so pumps on undersized tanks fail sooner.

How long should a well pump run each cycle?

At least 1 minute for pumps up to 3/4 hp and at least 2 minutes for larger pumps. Goulds calls 1 minute adequate and 2 minutes recommended.