Well pressure tank size calculator

A pressure tank has one job that matters to the pump: hold enough water between the pump stopping and starting again that the motor runs long enough each time to shed the heat of starting. Too small a tank and the pump starts over and over in short bursts, which wears out motors, control boxes and pressure switches. This calculator works it out from three numbers you can get off the pump and the switch, and it can also check the tank already in your basement or well house.

The math, step by step

1. Drawdown you need = pump flow × minimum run time. Franklin Electric's application, installation and maintenance (AIM) manual for submersible motors tells installers to run motors a minimum of one minute to dissipate the heat built up by the starting current, and to choose the pump, tank and controls to keep starts per day as low as possible. Amtrol's sizing chart for its Well-X-Trol tanks goes further: it assumes one minute for pumps up to 3/4 horsepower and two minutes for anything larger. A 10 gpm pump held to one minute needs 10 gallons of drawdown; at two minutes, 20.

2. Usable share of the tank comes from Boyle's law. The air in a captive-air tank is squeezed as water comes in, so pressure and air volume move opposite each other. In a 2011 article for The Driller, Bob Pelikan writes the share of the tank you can draw out as: precharge ÷ cut-in − precharge ÷ cut-out, with every pressure in absolute terms (add 14.7 psi at sea level). His worked example is a 30/50 switch with a 28 psi precharge, which gives about 0.295, so an 85-gallon tank yields roughly 25 gallons per cycle. The calculator uses the same formula, with the precharge set 2 psi below cut-in as Pelikan and the tank makers advise.

3. Tank size = drawdown needed ÷ usable share. Round up to the next size actually sold.

Quick table for common pumps

Total tank gallons needed for one minute of run time, precharge 2 psi under cut-in, sea-level pressure. Double the figures for a pump over 3/4 hp if you follow Amtrol's two-minute rule.

Two patterns stand out. A higher switch setting needs a bigger tank for the same pump, because compressed air at 40/60 has less room left to expand. And the jump from one to two minutes of run time doubles everything, which is why bigger pumps so often end up with two tanks teed together.

Why starts matter so much

Franklin's manual lists the most starts per 24 hours it recommends for its motors. For single-phase motors that is 300 a day up to 3/4 hp and 100 a day from 1 to 5.5 hp. A family running showers, laundry and a dishwasher piles up starts quickly when every cycle lasts only a few seconds, and on a deep Ozarks well the motor that pays the price sits hundreds of feet down. Swapping a tank is a surface job; pulling a burnt-out motor is a truck job.

Where the answer can be off

• Real tanks vary a little. Manufacturers publish their own drawdown figures, and they don't always match the textbook formula exactly. When the tank's label or spec sheet gives drawdown at your switch setting, go by that.
• Elevation helps slightly. Pelikan shows that lower air pressure at altitude raises drawdown a little. The calculator uses sea-level pressure, so it errs toward a larger tank.
• The precharge has to be right. Amtrol ships Well-X-Trol tanks precharged to 38 psi, the match for a 40/60 switch. On a 30/50 or 20/40 switch it must be let down, with the pump off and the tank empty, or you won't get the drawdown shown here.
• Constant-pressure pumps are different. Franklin says variable-speed drive systems need only a small tank, sized to the drive maker's instructions.
• Know your real flow. The pump's rated flow at your well's lift may be well below the number on the box. A pump installer can measure it at the tank.

Not sure the tank is the problem at all? The pressure tank replacement page covers the air-charge check and the signs of a ruptured bladder. When it's time for a new tank, call and we'll connect you with a licensed local pump installer.

Sources: Franklin Electric, AIM Manual, Submersible Systems, 60 Hz (2026), Frequency of Starts; Amtrol, Well-X-Trol brochure, ESP sizing table and precharge; Bob Pelikan, How to Calculate and Control Pressure-tank Drawdown, The Driller (2011).