Water Tank Size & Volume FAQs

Water Tank Size / Capacity Calculation

How can I calculate water tank volume in gallons

How much water does a 56-inch by 26-inch by 17-inch water tank actually hold?

Reply: here's the procedure

Anon:

I'll be glad to answer this but I have to choose just two of your three numbers since for a cylinder or round water tank all we can use is height and diameter. (I'm not sure what your third number is.)

Assuming that your water tank is round and has a flat top and bottom, it's easy because in that case we're just computing the volume of a cylinder.

1. Calculate the water tank volume in cubic inches. Here's the formula:
   
   

   The volume of a cylinder V = pi x r2 x h 

   where pi = 3.1416,

   r = cylinder radius (1/2 the diameter)

   h = the cylinder height or length of pipe in our case and

   G = the volume of water in U.S. gallons = 0.004329 gallons per cubic inch or more conveniently 1 U.S. gallon contains 231 cubic inches of liquid.

   For U.K. readers,

   GU.K. = the volume of an imperial gallon is 277.419 cubic inches.

   So for your water tank we calculate Volume V(in cubic inches) = 3.1416 x (1/2 x 26)² x 56 = 3.1416 x 169 x 56 = 29,732 cubic inches

2. Multiply the volume in cubic inches by 0.004329 to get the volume in U.S. gallons
   
   So So for your water tank we calculate Volume G (in U.S. Gallons) = 0.004329 x 29,732 = 130.5 US Gallons

We show this procedure in detail at WELL WATER VOLUME CALCULATION - calculate the volume of a cylinder, gallons per foot of well depth - Daniel F.

Water Pressure Tank Drawdown Quantity

What are Normal Water Tank Drawdown Amounts

How many gallons of water should be used before the well pump starts again if we have an 86 gallon tank?

My 86 gallon tank would re-pump after 9 gallon water usage, does that mean it is short cycled? (July 30, 2015) Southern Girl

Reply:

Typically you should see 30 seconds to a minute of running water at a single sink tap before the pump turns on.

A typical 86 gallon water pressure tank, when the pump turns off at the end of a PUMP-ON cycle, will be about 75% to 80% full of water. The top 25% or so of the tank contains the air charge (that's necessary to prevent the water pump from cycling on and off rapidly when you're using water).

So you might get as much as 60 gallons of water out of the tank before the pump turns on.

And yes, if your pump turns on after just 9 gallons, the pump is short cycling and I suspect your tank is waterlogged.

Read WATER TANK AIR, HOW TO ADD for details of how to fix this problem.

The volume of water drawn down will vary depending on the system operating pressure range, pipe sizes, and plumbing fixture flow rates. Typically at a kitchen sink tap I might see 3-7 gpm, so you should certainly see several minutes of draw-down before the pump cuts on IF you start with the tank full - that is, right at the end of a pump cycle when the pump has just shut off.

Other causes of short cycling are at WATER PUMP SHORT CYCLING

Also see AIR VOLUME CONTROLS, WATER TANK

Could the pressure tank be defective? We only get 5 gallons out of the tank.

I have a new 32 gallon pressure tank that puts out 5.5 gallons of water before it's empty. That seems a little short. The manufacturer tells me that is correct. We shut the pump off at full 60 pounds (40/60 switch) then turned an outside spicket on to fill a bucket. It quit at 5.5 gallons.

If what they say is true, 2/3rds air and 1/3rd water in the tank, shouldn't I be getting about 10 - 11 gallons before empty?

The reason it got my attention is the fact that the pump seems to run a lot when any water is being used in the house - not cycling mind you, but running steady such as when someone is in the shower.

Do you think it's OK? It's only 1 year old, but maybe defective?

It's like most of the pressure building is in the plumbing and not much going into the tank. In fact you can grab the top of the tank and it moves around like it feels very light as in no water in it. Also the gauge in line says 60 pounds at shut off as it should but the air pressure in the top of the tank is 44 pounds.

My well pump guy said they should be equal which makes sense to me too. By the way the pressure in the tank at pump kick on is 38 pounds as it should be.

Oh, and the manufacturer tells me, "my tire gauge must be wrong". I don't think so. (May 2, 2014) Larry

Diagnostic suggestion by mod: Check the condensation or water level in the tank for a clue to how much water it is holding

Take a look at the condensation line on your water tank and you'll see the level to which it's actually usually filled when in service - at the point of pump shut-off. If the "sweat line" is near the top of the tank it may be close to water-logged.

I agree that you ought to get a minute or more of running water at a sink or about 10 gallons or a bit more before the pump has to come on.

Check the water tank air pre-charge: is it set correctly at 2 psi below the pump control switch CUT-IN pressure?

Really? Keep in mind that the higher you set your water pressure the more-quickly you'll exhaust the water charge in the water pressure tank. Typical residential water pressure switch CUT-IN/CUT-OUT pressures are 20/40 psi or 30/50 psi. If you're running above that it could be a contributor to your issue. - Dan

How can I improve the drawdown cycle from my Water Tank & Pump

I have a 33 gal Red lion water tank & a 3/4 hp red lion submersible pump water cut in 42 psi out 62 psi
Tank air is 40 psi
My draw down between cycles is only 5 gallons.
How do I improve it ? (May 29, 2016) Dave

Reply:

If I use a rough rule of thumb that your water tank, when full and when the pump turns off at 40 psi, you might have .75 x 40 = 30 gallons of water that you can run before the pump turns on.

If you're only getting 5 gallons of water before the pump turns on then your water tank is waterlogged.

You need to put a correct air pre-charge into the tank. That will give the maximum water tank draw-down before the pump has to start.

See WATER TANK AIR, HOW TO ADD for details of exactly how to do that. - Daniel

Do I Really Need a Big Water Tank?

There's a big water tank in my closet: can I remove it or do I need it?

I moved into a house with septic and we'll, house built in 2004 or 2009. The garage was converted to a master bedroom.

There is a large 81 gal water tank in closet area, do I need that? Can I take it out? If I do need it, is it expensive to move to another area in the house? On 2017-10-20 by Terri

by (mod) - you need it but if that's a water heater (not just water storage) it may be unsafe and illegal

...

Terri,

Watch out: If the water tank is for hot water, and if it is gas or oil heated (see the photos I include above), having it in a bedroom closet is unsafe and illegal.

Compare those photos with the water pressure tank I show at the top of this page.

If it is a water pressure tank, perhaps looking like the blue water tank we show below, then yes you need the water tank as part of the water delivery system.

WATER TANK vs WATER PRESSURE - explains how the presence of a pressure tank or storage tank affects building water flow.

 

Do I need a small water pressure tank in addition to a very large water storage tank?

I'm forced to drill a well to replace my water delivery system PG&E just yanked out from under us with no warning--but that's a different story.

I want to fill my existing 2,500 gal storage tank from the well, continue to gravity feed this to my home and continue with the jet pump/bladder tank pressure system I have in my home.

Do I need another bladder/pressure tank between the submerged well pump and storage tanks? (July 10, 2015) DavidCoffee

Reply: include a pressure tank

David if you are using a pressure booster pump

EITHER you'll need a water pressure tank as well

OR you'll need a tankless pressure booster pump that works without its own pressure tank.

If the primary large water storate tank location is high enough, a gravity feed can work.

Watch out: If you try to fill the tank too rapidly and your well flow rate is not able to support that flow, either you'll need a pump protection device ( a tailpiece or a pump protection switch) or you'll be sorry as you'll risk burning up a pump.