How much water does your well tank really give?

A pressure tank is not a water tank in the usual sense: most of its volume is air. How much water you can take from it between two pump starts, the drawdown, decides how often the pump cycles and therefore how long the pump and the pressure switch last. This calculator works out the drawdown from the tank, the switch and the air charge, and the tank a pump needs.

What the number means

A well pump is switched by a pressure switch: it starts when the pressure falls to the cut-in value and stops at the cut-out value. The tank sits between the pump and the house, so the house can draw water without starting the pump every time a tap opens. The usable water is the drawdown. A small drawdown means the pump starts often, and every start is a hard load for the motor and the switch.

Method and variables

With the water chamber empty, the air in the tank is at the pre-charge pressure P1. As water enters the tank the air is compressed (Boyle's law, P × V constant). At cut-in the air occupies V × P1 ÷ P2, and at cut-out V × P1 ÷ P3. The water in between is the drawdown: V × (P1 ÷ P2 − P1 ÷ P3), where V is the tank's total volume and every pressure is absolute, gauge plus 14.7 psi at sea level. If the pre-charge is higher than the cut-in pressure, the first term becomes 1 and the pre-charge no longer helps. Size mode inverts the formula: the tank volume is the required drawdown (pump gpm × minutes) divided by the acceptance factor.

Worked example: 10 gpm pump, 40/60 switch

The pump must run at least one minute, so the drawdown must be at least 10 gallons. With a pre-charge of 38 psi, P1 = 52.7, P2 = 54.7 and P3 = 74.7 psi absolute, so the factor is 52.7 ÷ 54.7 − 52.7 ÷ 74.7 = 0.258 and the tank must be 38.8 gallons or more. If you choose a catalog tank of 44 gallons, its drawdown is 11.3 gallons, the pump runs 1.1 minutes when nothing else draws water, and at worst it cycles every 4.5 minutes.

Common mistakes and limits

The most common mistake is to read the tank's size as its drawdown. The second is to set the pre-charge with water in the tank: the gauge then shows the water pressure, not the air. Drain the tank, set the air 2 psi below cut-in, and only then let the pump refill it. This calculator assumes an ideal gas at a constant temperature and a tank without losses, so the maker's published drawdown for your model is always the better number. The one-minute rule is Franklin Electric's minimum for submersible motors; vendors also quote longer minimums for larger motors, which we could not trace to a primary document and do not use. Plan for a bigger tank if the well yields little and you want a reserve: the drawdown here is for cycling, not for storage.

What is drawdown in a pressure tank?

Drawdown is the amount of water the tank hands out between the moment the pump switches off at the cut-out pressure and the moment it switches on again at the cut-in pressure. It is much less than the tank's size, because a bladder or diaphragm tank is mostly a cushion of compressed air. An 85 gallon tank on a 30/50 switch with its air set to 28 psi gives only 25.1 gallons, about 30% of its volume.

What should the pre-charge be?

Two psi below the cut-in pressure, measured with the water drained out of the tank (a 28 psi charge for a 30/50 switch, 38 psi for a 40/60). The Driller calls this a minimum margin so that the system pressure cannot drop abruptly to zero before the pump starts, and Amtrol's Well-X-Trol instructions give the same rule with 38 psig as the factory setting. Without any air the same 85 gallon tank gives just 8.6 gallons: the pre-charge is what makes the tank useful.

How big a tank do I need for my pump?

Work backward from how long you want the pump to run. Franklin Electric's installation manual says to run submersible motors a minimum of one minute per start, to shed the heat of the starting current. A 10 gpm pump therefore needs 10.0 gallons of drawdown. On a 40/60 switch with a 38 psi charge a tank gives 0.258 of its volume, so the tank must be 38.8 gallons or more. On a 30/50 switch the factor is 0.295 and the tank 33.9 gallons.

Does a bigger gap between cut-in and cut-out help?

Yes, up to a point. A 30/60 switch with 28 psi in the same 85 gallon tank gives 32.6 gallons against 25.1 on a 30/50, but the pressure at the taps swings by 30 psi instead of 20. The Driller advises checking with the tank maker before using a gap wider than 20 psi, because the bladder may over-expand. The calculator warns you when the gap is wider.

Why does altitude matter?

The formula works on absolute pressure, gauge pressure plus the atmosphere. At sea level the atmosphere is 14.7 psi; the Driller gives 11.8 psi at 6,000 feet. For the 85 gallon, 30/50 example that moves the factor from 0.295 to 0.308. The article prints 0.307 for the high-altitude case; the formula gives the value shown here, a rounding difference of about a tenth of a gallon. Change the "air pressure" field to your own elevation.

How many times will my pump start per day?

The shortest cycle happens when the house draws half of the pump's flow: the pump then fills the tank in the same time that the house empties it, and a full cycle takes 4 × drawdown ÷ pump flow. A 44 gallon tank on 40/60 gives 11.3 gallons of drawdown, so a 10 gpm pump runs 1.1 minutes with no draw and cycles at worst every 4.5 minutes, up to 13 starts an hour while water is running. Franklin lists a maximum number of starts per 24 hours by motor size in its manual; look the figure up for your motor.

My tank says 20 gallons but I get far less water. Is it faulty?

Probably not. The size on a bladder tank is its total volume, and the water you can draw is a fraction of that, set by your pressure switch and the air charge. A waterlogged tank, one that has lost its air or has a ruptured bladder, shows as a pump that cycles fast. Check the charge with the water drained and the power off.

Sources: The Driller, “How to Calculate and Control Pressure Tank Drawdown” (thedriller.com, article 84709); Amtrol, Well-X-Trol installation and operation manual, MC10185 (amtrol.com); Franklin Electric, AIM Manual, 60 Hz Submersible Systems, 2023 edition, section “Frequency of Starts”, all read October 27, 2026. The Water Systems Council guidance was not opened. Related: pipe size calculator, friction loss calculator, expansion tank calculator, sump pump size calculator. How we check our calculators.