Pressure Tank Sizing Calculator

Engineering-Led Sizing · Watermain Supply · Houston, TX

Pressure Tank
Sizing Calculator

Jet, submersible or surface booster. Constant speed or variable speed. Enter the pump and the pressure settings and get the drawdown you need, the tank size that delivers it, and the precharge to set it at.

÷ 4The Formula Most People Miss
3Pump Types
2 PSIBelow Cut-In, Every Time
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The Short Version

  1. The tank exists to stop the pump short-cycling. It is not storage. Its whole job is to hold enough water that the motor gets a decent run and a decent rest.
  2. Drawdown is not tank size. A 44 gallon tank holds nowhere near 44 gallons of usable water. At household pressures it delivers roughly a quarter of its volume; at commercial pressures, closer to a tenth.
  3. Constant speed: drawdown = pump GPM × cycle minutes ÷ 4. The divide-by-four comes from the worst case, when demand is exactly half the pump's capacity. That is when cycling is fastest.
  4. Variable speed changes the question entirely. The tank is taken from the pump manufacturer's published requirement for that pump, not from a drawdown formula. Below about 80 GPM most VFD pumps ship with an integrated tank and a separate one is optional.
  5. Precharge is 2 PSI below cut-in, set with the system drained. Get this wrong and a correctly sized tank behaves like a small one.

The Calculator

Estimating tool. It produces a target to check a selection against. Confirm the acceptance volume and pressure rating on the tank data sheet before ordering.

Step 1 · The Pump

Type sets the default start limit. Motor size adjusts it.

Leave this alone unless you know better. It fills in automatically from the pump type and motor size above.
What “starts per hour” means, and whether to touch itIt is how many times an hour the motor is allowed to switch on. Every start draws a surge of current, heats the windings and loads the bearings, so manufacturers cap it — and that cap is what decides the tank size. Fewer starts allowed means a longer run needed, which means more water in the tank.

The box fills in for you when you pick the pump type and motor size. Small jet pumps tolerate about 20 an hour; submersibles fewer, because a short run may not move enough water past the motor to cool it; large commercial motors fewer still, 6 to 10.

Change it only if the motor data sheet gives a number, or you deliberately want a larger tank and longer pump life — lowering it to 6 will roughly double the tank and is money well spent on a pump that is expensive to pull.

Step 2 · Constant Speed Settings

Pump capacity is what the pump actually delivers at your operating pressure, read off its curve — not the headline maximum.

Where the pressure switch closes
Where it opens

Step 2 · Variable Speed Settings

On a VFD the drive is the pressure control, not the tank. The tank size is taken from the pump manufacturer's published requirement for the pump's flow, not from a drawdown formula. Enter the pump's rated flow.

The pump's capacity from its nameplate or spec sheet, not building demand
What the drive holds. Used for precharge.
How far pressure falls before it wakes. Sets the precharge.

Step 3 · The System

Stamped on the brass relief valve itself, usually at the water heater or on the pump discharge. 75, 100 and 150 PSI are the common ones. If there is no relief valve, leave 100 and tell us.
Drawdown Needed—Gallons
Minimum Nominal Volume—Gallons
Nearest Standard Tank—Gallons
Precharge—PSI
Working Value Where it comes from

Why Divide by Four

The one piece of arithmetic that decides every pressure tank, and the one most rules of thumb get wrong.

A pump on a pressure switch fills the tank faster than the building empties it. The pump runs until cut-out, stops, and the tank supplies the building until pressure falls to cut-in. One run plus one rest is a cycle, and the motor only tolerates so many cycles an hour.

Call the pump's capacity Q and the building's demand D. While the pump runs, the tank fills at Q − D. While it is off, the tank drains at D. So for a drawdown of V:

Cycle timeT = V ÷ (Q − D) + V ÷ D

That is shortest when demand is exactly half the pump's capacity — the pump gains the least while running and the tank empties fastest while off. Substitute D = Q÷2 and the whole thing collapses to

T = 4V ÷ Q, which rearranges to V = Q × T ÷ 4.

So the drawdown you need is the pump's capacity multiplied by the cycle time you want, divided by four. Nothing about the building's actual demand enters it, because you are sizing for the worst case rather than the average.

Starts per hour Cycle time Drawdown per GPM of pump For a 100 GPM pump
4 15 min 3.75 gal 375 gal
6 10 min 2.5 gal 250 gal
10 6 min 1.5 gal 150 gal
15 4 min 1.0 gal 100 gal
20 3 min 0.75 gal 75 gal
Where the old rule comes fromWell installers have used “one gallon of drawdown per GPM of pump” for decades. Read the table: that is exactly the 15-starts-per-hour row. The rule of thumb and the physics are the same thing, and the rule works because small well motors genuinely tolerate 15 starts an hour.

From drawdown to tank size

Drawdown is the water. Tank size is the vessel, and the two are not close. The air in a bladder tank is what pushes the water out, and how much water a given pressure swing moves depends on the absolute pressures involved:

Boyle’s law, appliedUsable fraction = Pprecharge × ( 1 ÷ Pcut-in − 1 ÷ Pcut-out )

All pressures absolute — add 14.7 to each gauge reading.

On a 30/50 switch a tank gives up about 26% of its volume. On 60/80 it gives about 15%. On a commercial booster at 100/120, closer to 10%. Same tank, less than half the water, purely because the air starts more compressed.

Which is why raising a system's pressure without resizing the tank causes short-cycling that looks like a pump fault and is not.

Constant Speed and Variable Speed Are Different Problems

Same tank, two entirely different sizing questions. Getting this backwards is the most expensive mistake on the page.

Constant speed Variable speed
What the tank does Carries the whole cycle. It is the pressure control. Rides out brief demand so the drive can sleep. The drive is the pressure control.
Sized against Pump capacity The pump manufacturer's published tank requirement
Formula Q × T ÷ 4 Table lookup by pump flow — no drawdown formula
Scales with pump size Yes, directly Steps up with flow, far more gently
Typical result Tens to hundreds of gallons Integrated tank below ~80 GPM; 32–86 gal above
Why a variable speed pump still needs oneA drive cannot ramp to zero. Below roughly 25 to 30 Hz the pump makes no useful head and simply recirculates and heats. At trickle demand — one running toilet, overnight leakage — the pump would start and stop endlessly. The tank gives it somewhere to park that water so it can sleep. Most VFD pumps include a small integrated tank for exactly this; a supplemental tank extends it.
Below 80 GPM the tank is optionalIn the residential and residential-ish range the integrated tank usually covers the sleep demand on its own. A small supplemental diaphragm tank still helps — it lengthens runs and extends pump life — but it is a recommendation, not a requirement, and it comes down to whether there is space for it.
What the drive buys youA 100 GPM constant speed booster at 6 starts an hour needs about 250 gallons of drawdown — a hydropneumatic tank of a thousand gallons or more at commercial pressure. The same duty on a drive follows the manufacturer's table, which lands in the tens of gallons. That is the difference between a tank that needs its own floor space and one that hangs on a wall.
The number to enter is the pump's rated flowThe manufacturer indexes its tank requirement on the pump's capacity — the flow on the nameplate — not on building demand. Enter that figure and the tool returns the published minimum for that pump, rounded up to a stock tank.

By Pump Type

On constant speed the formula does not change; what changes is how many starts the motor will tolerate, and that drives everything. On a drive the tank follows the manufacturer's table instead.

Jet pumps

Surface-mounted, air-cooled, easy to inspect and cheap to replace. They tolerate the most cycling of the three — 20 starts an hour is workable on a fractional-horsepower unit. That keeps tanks small: a 10 GPM jet on a 30/50 switch needs only about 8 gallons of drawdown, which a 26 gallon tank delivers. Cut-in and cut-out are usually 30/50 or 40/60.

Shop Jet Pumps

Submersible well pumps

Down the well, cooled by the water flowing past the motor, and expensive to pull. Every start heats the motor and loads the thrust bearing, and a short run may not move enough water past the motor to cool it. Manufacturers publish a daily start limit as well as an hourly one, and a one-minute minimum run time is the usual floor. Size generously: the tank is the cheapest part of the system and the pump is the most expensive to replace.

Shop Submersible Pumps

Surface boosters, constant speed

Commercial duty, larger motors, and the tightest start limits — 6 to 10 an hour above 10 HP. Combine that with commercial pressures, where a tank gives up only a tenth of its volume, and the numbers get large fast. This is why hydropneumatic tanks on constant speed booster systems run to hundreds or thousands of gallons, and why so many of these systems get replaced with variable speed.

Shop Booster Pumps

Any of the three on a drive

Sizing switches to the manufacturer's published tank for the pump's flow, and the tank shrinks dramatically. What does not change is that you still need one. A variable speed pump with no tank at all short-cycles on the smallest leak, and the drive cannot protect it from that.

Size the Pump First

The capacity figure people get wrongPump capacity in the constant-speed formula means what the pump delivers at your operating pressure, read off its curve at cut-out. Not the headline maximum, which happens at almost no pressure. Use the catalogue number and you will oversize the tank, which is harmless, or undersize the run time expectation, which is not.

Which Tank Is Which

Four things get called “a tank” and they are not interchangeable. This is the short version of what each one does.

Diaphragm pressure tank

The one this calculator sizes. Goes on the pump discharge and stores usable water so the pump gets a decent run and a decent rest. A rubber diaphragm keeps the air and the water apart, so the air charge cannot dissolve away the way it did in old galvanised tanks. In-line, horizontal or vertical mounting, 2 to 119 gallons.

Shop Diaphragm Pressure Tanks

Thermal expansion tank, potable

Goes on the cold supply at the water heater, not on the pump. Absorbs the extra volume created when water is heated on a closed system, so pressure does not climb until the relief valve weeps. Small — 2 to 5 gallons covers most domestic heaters. Rated for drinking water contact, which the hydronic version is not.

Shop Potable Expansion Tanks

Hydronic expansion tank

Same idea, closed heating loop instead of drinking water. Takes up the expansion in a boiler or radiant system as the loop comes up to temperature. Not for potable water — the bladder is not certified for drinking water contact and it must never be fitted to a domestic supply.

Shop Hydronic Expansion Tanks

Mounting bracket

Carries a small expansion tank off the pipe rather than hanging its weight on the fitting. A full 5 gallon tank is around 45 lb and a copper stub was never meant to hold that. Cheap, and it stops the joint it is fitted to becoming the next callback.

Shop Mounting Brackets

The mistake that costs a callbackA pressure tank and an expansion tank look identical on the shelf. Fit an expansion tank where a pressure tank belongs and the pump short-cycles from day one, because 2 gallons of vessel is nowhere near the drawdown any pump needs. Fit a pressure tank at the water heater and it is precharged to the wrong pressure and usually on the wrong side of a check valve. A system with a booster and a storage water heater needs one of each.

Precharge, and Why Good Tanks Fail Early

The single most common field fault on pressure tanks is not a defective tank. It is air.

The ruleConstant speed: precharge 2 PSI below cut-in. Variable speed: about 5 PSI below the restart pressure. Either way, set it with the system drained and no water pressure on the tank, and check it with a tire gauge at the Schrader valve.

Two PSI below rather than exactly at cut-in, so the bladder is just barely empty of water when the pump starts. Set it above cut-in and the tank runs dry before the pump comes on, and the drawdown you calculated never arrives.

Set it far below and the bladder never fully empties, so part of the tank you paid for holds water permanently. Charge a tank to a residential 38 PSI in an 80 PSI system and a 44 gallon tank behaves like a 15 gallon one.

WaterloggingA tank that has lost its air charge, or whose bladder has failed, has no drawdown at all. The pump then cycles on every cup of water drawn and can rack up thousands of starts in a day. It is the leading cause of premature pump failure and it is invisible until you check the precharge.

Check it annually, and always when investigating short-cycling. On a bladder tank, press the Schrader valve with the system drained: air means the bladder is intact, water means it has failed and the tank is finished.
Two more things to confirmThe tank’s pressure rating must exceed the relief valve setting, not just the cut-out. And on a potable system the tank must be rated for drinking water contact — NSF/ANSI 61 and 372.

Pressure Tanks and Expansion Tanks Are Not the Same Thing

They look alike, they mount alike, and they do opposite jobs. Fitting one where the other belongs is common and neither problem gets solved.

Pressure tank Thermal expansion tank
Job Stops the pump short-cycling by storing usable water Absorbs the extra volume created when water is heated
Where On the pump discharge On the cold supply at the water heater
Sized by Pump capacity or trickle flow, and the pressure band Water heater volume, temperature rise and system pressure
Typical size Tens to hundreds of gallons 2 to 5 gallons on a domestic heater
Precharge 2 PSI below cut-in Matched to static system pressure

Water expands roughly two percent going from 50°F to 140°F. On an open system it pushes harmlessly back into the street main. Add a backflow preventer, a check valve or a booster and the system is closed — that expansion has nowhere to go, so pressure climbs until the heater’s relief valve weeps. A relief valve that drips after a hot shower is almost always this, not a faulty valve.

If you have both, you need bothA pressure tank on the pump discharge does not substitute for an expansion tank at the heater, even though it contains air and sits on the same piping. It is precharged to a different pressure, it is usually the wrong side of a check valve, and it is not sized for the job. Codes generally require thermal expansion control on any closed system with a storage water heater.

Expansion tank sizing is a different calculation from anything on this page and depends on heater volume, incoming and set temperatures, and static pressure. Call us with those four numbers and we will size it.

Checked Against the Manufacturer’s Own Method

The constant-speed side does not invent a sizing method. It is the same arithmetic A.Y. McDonald publishes in its tank sizing charts, expressed in different units — so if you are holding the printed chart, the two agree.

The multiplier table is Boyle’s law

A.Y. McDonald’s Chart 2 gives a “drawdown volume multiplier” for each pressure range — 0.31 for 30/50, 0.27 for 40/60, and so on. That is the usable fraction this page calculates. Every published cell reproduces from the same equation:

Same equation, both sidesUsable fraction = Pprecharge × ( 1 ÷ Pcut-in − 1 ÷ Pcut-out ), all absolute.

30/50 → 0.309. Published: 0.31.
40/60 → 0.268. Published: 0.27.
60/80 → 0.211. Published: 0.21.
50/100 → 0.436. Published: 0.44.

Run time and starts per hour are the same input

The chart asks for a minimum run time. This page asks for maximum starts per hour. They convert exactly, because a run time measured with no draw becomes twice that run and four times that cycle once the building is actually drawing at the worst-case rate — half the pump’s capacity.

Chart asks for Equivalent cycle Enter here as Drawdown per GPM of pump
1 minute run 4 min 15 starts per hour 1.0 gal
1-1/2 minute run 6 min 10 starts per hour 1.5 gal
2 minute run 8 min 8 starts per hour 2.0 gal

Starts per hour = 15 ÷ run time in minutes. Both come from V = Q × T ÷ 4.

Where the two differ, and why

Precharge

The published multipliers assume the tank is precharged at cut-in. Every installation instruction — including the tank’s own — says 2 PSI below cut-in, so the bladder actually empties. This page uses 2 below, which makes the usable fraction about 5% smaller and the tank slightly larger. Conservative in the direction that protects the pump.

Tank ladder

A printed chart has to land on the models in that manufacturer’s range, so individual cells round up or down to the nearest one they build. This page returns the calculated volume first and then the nearest standard size, so you can see how much margin the rounding gave you.

Worked cross-checkThe chart’s own example: 10 GPM, one minute of run time, 30–50 PSI. Their formula gives 10 × 1 ÷ 0.31 = 32.3 gallons minimum, and they select a 34 gallon tank.

Enter 10 GPM, 30/50, 15 starts per hour here and you get 10 gallons of drawdown, 33.9 gallons minimum volume, and a 34 gallon tank. Same answer, arrived at from the other end.

Assumptions and Sources

Everything the calculator assumes, so the arithmetic can be audited rather than trusted.

Item What is used
Constant speed drawdown V = Q × T ÷ 4, derived from the cycle equation at its worst case, demand equal to half of pump capacity. T is 60 divided by the maximum starts per hour.
Variable speed tank Taken from A.Y. McDonald DuraMAC published tank data, indexed on the pump's rated flow (running maximum across all models to each flow), rounded up to the nearest stock tank. Below 80 GPM a tank is treated as optional because VFD pumps in that range ship with an integrated tank. No drawdown formula is applied on the VFD path.
Nominal tank volume (constant speed) Boyle’s law between cut-in and cut-out, at the stated precharge, all pressures absolute at 14.7 PSI atmospheric. Isothermal — error under one percent at these ratios.
Acceptance factor Not applied. The figure shown is theoretical nominal volume. Real bladder tanks accept slightly less because the bladder cannot expand into every corner, which is why the calculator recommends the next standard size up rather than the exact number.
Precharge 2 PSI below cut-in on constant speed, 5 PSI below restart on variable speed. Published manufacturer multiplier tables assume precharge at cut-in, which yields a fraction about 5% larger and a slightly smaller tank.
Validation The usable-fraction formula reproduces 23 of the 25 legible cells of A.Y. McDonald’s published drawdown multiplier chart. Constant-speed selections were cross-checked against the same manufacturer’s tank selection chart; variable-speed selections against its DuraMAC technical tables.
Default start limits (constant speed) Jet 20 per hour up to 2 HP, 15 above. Submersible 15 up to 2 HP, 12 at 3 to 5 HP, 10 above. Surface booster 15 up to 5 HP, 10 at 7-1/2 to 25 HP. Any pump above 25 HP, 6. Design targets in common use, not manufacturer data. The motor data sheet governs.
Standard sizes A ladder of common nominal volumes. Individual manufacturers differ; confirm the acceptance volume and pressure rating on the data sheet.
Not included Thermal expansion volume, altitude correction, hot water service, and any storage requirement beyond anti-cycling.

This tool estimates a design target. It is not a stamped design and does not replace the engineer of record.

Questions Buyers Ask

If the answer you need is not here, call. Sizing conversations are the part of this we are actually good at.

How much water does a 44 gallon tank actually hold?

Far less than 44 gallons. On a 30/50 switch, about 11. On 40/60, about 9. On a commercial 60/80, about 7. The rest is air, and the air is what does the work. Always size on drawdown, never on the number printed on the tank.

Can I use a bigger tank than the calculator says?

Yes, and it is usually money well spent. A larger tank means longer runs, fewer starts and a longer-lived pump. There is no cycling penalty for oversizing a pressure tank — only floor space and cost.

Do I need a tank with a variable speed pump?

Almost always, but a small one, and below about 80 GPM the pump's integrated tank usually covers it — a separate one is optional and mainly extends pump life. Above that, follow the manufacturer's published tank size for the pump's flow, which is why this tool asks for the pump's rated flow rather than a drawdown.

What precharge should I set?

Two PSI below cut-in on a pressure switch system, about five below the restart pressure on a variable speed one. Set it with the system drained so there is no water pressure on the tank, and check it with a tire gauge.

My pump short-cycles. Is the tank too small?

Check the precharge first. A waterlogged tank or a failed bladder gives no drawdown at all and looks exactly like an undersized tank. Drain the system and press the Schrader valve: air means the bladder is intact, water means the tank has failed.

Why did my tank stop working after we raised the pressure?

Because drawdown depends on absolute pressure. Moving from 30/50 to 60/80 cuts a tank’s usable water roughly in half even though the band is the same 20 PSI. The tank did not fail; it is simply too small for the new setting.

Is a bigger pressure band better?

For the tank, yes — a 60/80 band delivers more drawdown than 70/80. But a wide band means occupants feel the pressure swing, which is the thing a constant pressure system is bought to avoid. Widen the band to solve cycling only where nobody will notice.

Does an expansion tank count toward my drawdown?

No. It sits at the water heater, usually on the far side of a check valve, and is precharged to a different pressure. The two tanks do different jobs and a system with a booster and a storage water heater generally needs both.

Get It Sized by a Person

Send the pump model, the pressure settings and what the tank is feeding, and we will come back with a size, a precharge and the reasoning behind both. If the problem is the precharge rather than the tank, we will tell you that instead of selling you one.

Call 1-888.510.4036 Email a Sizing Request Shop Pressure Tanks Size the Pump First
All products are genuine, factory-new equipment in original manufacturer packaging, sold and supported by Watermain Supply, an independent industrial supplier. Engineering-led sizing, based in Houston and serving the Gulf Coast. 1-888.510.4036
Estimating figures on this page are subject to change and are not a substitute for the plumbing code as adopted by the authority having jurisdiction. Confirm all sizing against current manufacturer literature before specifying.
Watermain Supply is a DBA of E4 Industrial LLC, an independent industrial supplier based in Houston, Texas.