Well pressure tank size calculator

How the well tank size is worked out

This page sizes a precharged tank by Boyle's law and does not read a maker's table. The pump should run for a minimum time each cycle, so the tank has to hand out that much water between the cut-in and cut-out pressures. Drawdown needed is the pump flow in gallons per minute times the run time in minutes. The acceptance factor is the share of the tank volume that is water between the two pressures, worked from absolute pressures, and the minimum total tank volume is the drawdown needed divided by that factor. Unless you enter one, the precharge is the cut-in minus 2 psi.

Drawdown needed

drawdown needed (gal) = pump flow (gpm) x minimum run time (min)At 10 gpm for 1 minute the tank must give 10.0 gallons each cycle, and for 2 minutes 20.0 gallons. Two minutes is the figure an Amtrol article uses for a 1 hp pump, and one minute is the least a pump supplier's guide gives a residential pump.

Absolute pressure

P = gauge pressure + 14.7 psiBoyle's law works on absolute pressure, so each gauge reading gets 14.7 psi added, the atmosphere at sea level. For a 40/60 switch with a 38 psi precharge the three pressures are 52.7, 54.7 and 74.7 psia.

Acceptance factor

AF = P1/P2 - P1/P3P1 is the precharge, P2 the cut-in and P3 the cut-out, all absolute. For 40/60 with 38 psi that is 52.7/54.7 - 52.7/74.7 = 0.258, so 25.8 percent of the tank volume is water between the two pressures. Pelikan prints the same 0.258 for 40/60 and 0.295 for 30/50 in The Driller.

Minimum tank volume

total tank volume = drawdown needed / AF10.0 gallons divided by 0.258 is 38.8 gallons, so the tank is 39 gallons or larger. A tank of 39 gallons gives 10.1 gallons of drawdown. The 2 minute case takes 77.5 gallons, twice the 1 minute figure because the factor does not change.

Precharge

precharge = cut-in - 2 psiPelikan, Amtrol and a supplier's guide all set the precharge 2 psi below the cut-in, 38 psi on a 40/60 switch and 28 psi on 30/50. A precharge equal to the cut-in raises the factor of a 30/50 tank by 4.7 percent, which is the 5 percent overstatement Pelikan names, and it leaves no margin for a switch or gauge that reads a few psi off.

Worked examples from the formula

The default case is a 10 gpm pump on a 40/60 switch, run for at least 1 minute. It needs 10.0 gallons each cycle. With the 38 psi precharge the absolute pressures are 52.7, 54.7 and 74.7 psia, the acceptance factor is 0.258, and 10.0 gallons divided by 0.258 is 38.8 gallons. A 39 gallon tank gives 10.1 gallons of drawdown. A 2 minute run needs 78 gallons.

The same 10 gpm pump and 1 minute on the four common switches, at the usual precharge.

Amtrol prints the drawdown of its Well-X-Trol tanks. Boyle's law at sea level pressure gives a little less, so this page sizes a tank a little large.

Eight cases below run through the same functions the form uses. The Amtrol article sizes a 15 gpm pump run for 2 minutes on 40/60 at about 111 gallons with its factor of 0.27, and this page gives 117 gallons, since it works the factor at 0.258. The 10 gpm pump needs 34 gallons on 30/50 and 39 gallons on 40/60.

Standards and data sources

• Bob Pelikan, How to Calculate and Control Pressure-tank Drawdown, The Driller (2011), and What Boyle's Law Means to You (2002) - The formula of this page, drawdown = P1V/P2 - P1V/P3 with absolute pressures, the acceptance factor P1/P2 - P1/P3 and the precharge 2 psi below the cut-in. Worked examples for an 85 gallon tank, 25.1 gallons on 30/50 with 28 psi, 21.9 gallons on 40/60 with 38 psi and 32.6 gallons on 30/60, are the test values of the page. Both articles say to check with the tank maker before a differential above 20 psi.
• Amtrol, Tank Sizing Made Easy (Eliminating Guesswork), Water Conditioning and Purification (2000) - The pump cycle control example, a 1 hp pump at 15 gpm on 40/60 for 2 minutes, which stores 30 gallons and takes about 111 gallons at the 0.27 factor of the article. This page gives 117 gallons for the same case. The article names Boyle's law and a precharge 2 psi below the cut-in.
• Amtrol, Well-X-Trol specification sheet, WX-101 to WX-350 - The maker's drawdown at 30/50, 40/60 and 50/70 psig for each tank, for example 21.7 gallons at 40/60 for the 81 gallon WX-255, against 20.9 gallons from Boyle's law. It also gives the working pressure of the tanks, 150 psig and 100 psig for one model.
• Fresh Water Systems, How to Size a Well Pressure Tank - The rule that the minimum drawdown is the pump gpm times the minimum run time, one minute at least for a residential well pump, with a precharge about 2 psi below the cut-in, and the note that drawdown varies by maker and model.
• Manitoba Water Services Board, Pressure Tanks for Water Systems (Water Facts) - A government fact sheet that sets the precharge about 3 psi below the cut-in, puts the tank volume at three to four times the pump gpm, and tabulates drawdown of fixed diaphragm tanks, which run below the Boyle's law figure of this page.

Limitations of this calculator

• Captive-air tanks only, the precharged diaphragm or bladder kind. A galvanized tank with an air volume control, a saddle tank and an air-over-water tank hold their air by other means and are not sized here, and the page does not model how fully a bladder or diaphragm empties.
• Pressure at sea level, 14.7 psi, is assumed. Pelikan prints 26.1 gallons of drawdown at 6,000 ft against 25.1 gallons at sea level for an 85 gallon tank on 30/50, so a high site needs a slightly smaller tank than this page gives.
• Constant-pressure systems with a variable frequency drive hold the pressure by changing the pump speed, and their small tank is sized by the drive maker and not by a pump run time. They are not covered.
• Makers print a drawdown a few percent above the Boyle's law figure, 21.7 against 20.9 gallons for the 81 gallon WX-255 at 40/60, so this page sizes a tank a little large. The Manitoba fact sheet prints less, about 0.22 of the tank volume for its 330 L tank at 40 to 60 psi, so the tables differ in both directions. Check the table of the tank you buy.
• The pump curve, the well yield, the pumping level in the well and the pipe friction between the pump and the tank are not taken. Pump flow is the flow at the working pressure, and the pump maker's limit on starts per hour is not read.
• The run time is a choice of 1 or 2 minutes. Sources differ: a supplier's guide gives 1 minute as the least, and an Amtrol article uses 2 minutes for a 1 hp pump. Some tank makers pair 1 minute with pumps up to 3/4 hp and 2 minutes above that, so read the sheet of the tank you buy.
• The inputs are limited to 0.5 to 100 gpm, a cut-in of 10 to 80 psi and a differential of 10 to 40 psi, which are limits of this page and not of the sources. A differential above 20 psi needs the approval of the tank maker.
• The page gives a minimum volume. Tank makers also print a maximum acceptance factor for each model, from 0.35 up to 1.00 on one maker's sheet, and a wide pressure range can ask for more than a tank gives. The page does not choose a tank model, check the working pressure of a tank, or size tanks in parallel.
• This is a planning estimate, not a pump and well design. A licensed well driller, pump installer or engineer and the local rules decide the system and the installation.

Common well tank sizing mistakes

1. Reading the tank label as water. A 39 gallon tank is not 39 gallons of usable water. On 40/60 it gives 10.1 gallons between cut-in and cut-out, which is 25.8 percent of its volume.
2. Sizing the tank to the pump gpm. A 10 gpm pump on a 40/60 switch needs 10.0 gallons of drawdown for 1 minute, which takes a 39 gallon tank and not a 10 gallon one. Total volume runs about 3.9 times the gpm here, in line with the three to four times in the Manitoba fact sheet.
3. Setting the precharge at the cut-in. A precharge of 40 psi on 40/60 gives a factor of 0.268 against 0.258 at 38 psi, and the 10 gpm case falls from 39 to 38 gallons. The saving is not real, since a switch or gauge that reads a few psi off leaves the tank empty before the pump starts, and the sources set the precharge 2 psi below the cut-in.
4. Leaving the tank with no precharge. With no precharge a 30/50 tank gives a factor of 0.102, so the 10 gpm case needs 99 gallons against 34 gallons at 28 psi. A tank that has lost its air behaves like this, and the pump then short cycles.
5. Widening the differential without asking the maker. A 30/60 switch raises the factor to 0.384 and takes the 10 gpm case down to 27 gallons. Pelikan says to check with the tank maker above 20 psi, since a wide differential can over-expand the water chamber.
6. Using this for a closed heating loop. An expansion tank on a closed water heater or hydronic loop is sized from the thermal expansion of the water and not from a pump run time. The expansion tank size calculator covers that tank.

Common questions about well pressure tank size

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