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Water Tank Size, Volume, & Requirements InspectAPedia®  -     Water Tank Size - how much water is in the water pressure tank? How much water volume do we need to avoid short cycling the water pump? How to determine the volume of water provided by a water pressure tank Water Pump Short Cycling & Water Tank Air - How & Why to Add Air Water Tank Repair or Replacement

Here we discuss Water Tank Size - how much water is in the water pressure tank? and we address the question of how much water volume we need to avoid short cycling the water pump? This website answers just about any question you may have about pumps, wells, and drinking water. Even before performing water quantity, quality, equipment function tests, there is an enormous amount we can determine about a building's water supply just by looking at the equipment. Articles here provide details on water pumps, tanks, controls, and wells and water supply inspection, diagnosis, and repair. The photograph shows a one-line jet pump, the water pressure tank, and a water softener. We know from the fact that this is a single line jet well pump that the well is a shallow one, probably less than 27' deep. Well depth may have implications for water quantity and quality and vulnerability to surface water contamination. The page top photo shows our client holding the open top of his well casing along with some unusual well casing exit plumbing at a drilled well with a modern steel casing. Finding the location of your well and inspecting the condition of the well piping and equipment are an important first step to assure a functional and potable drinking water supply - that is, having enough water supply and having water that is safe to drink. The articles listed below provide detailed advice on diagnosing and repairing problems with water pumps, water tanks, wells, and other water supply equipment.

How to Find Out How Much Water is in the Water Tank, Considering the Space Taken up by Air in a Water Pressure Tank

It's not how much water the tank holds that is useful to know. It's how much water we can get out of the water tank before the pump has to turn on. This is the "draw down" volume, which we can measure or calculate, or we can focus instead on how long (in time) we can run the water before the pump has to turn on. (See our discussion of "short cycling" water pumps at Water Tank Repairs: Diagnose "Water Pump Short Cycling" & Restore Air in a Building Water Tank. If your pump is short cycling you want to fix it, as we explain in that article.

How to calculate the volume of water in a water tank

IF we just wanted to calculate the volume of water inside of a round water tank (photo at left), for simplicity ignoring a concave or convex (domed) water tank top and maybe bottom, we can use the formula to calculate the volume of a cylinder. Just measure the height and circumference of the water tank. Reader Craig Revill send us this photograph of a large water storage tank in an 80 to 100 year old home, along with the tank dimensions: 36" in diameter and 99" in length. You can use the following formula to calculate the number of gallons of water (or any other liquid) in a storage tank: pi * radius2 * height (pi is 3.1416) Gallons of water in a tank: one gallon = 231 cu.in. Reader Roger Davis reminded us to make clear that in the formula above, the radius (which is half of the tank diameter) should be squared. That is, divide the diameter in half to obtain the radius, and multiply r x r to obtain r2.

For the 36" diameter x 99" long water tank above, we calculated the water tank volume as follows:

[3.1416 x (18")² x 99" = 100,782 cu.in.] / 231 = 436 gallons in this water tank.

That's a big tank storing a lot of water, probably indicating a well with a very low flow rate at the home where this water tank was installed. Or someone was planning to survive a long dry period. The small control connection shown at the 11 o'clock position on the end of this water tank was an air volume control.

Domed water tank volume: If we needed to be precise and if the bottom and top of a water tank are domed at the top and convex at the bottom (usually) we can measure these areas and calculate their volume using the formula for the volume of a sphere (or part of one). But we suggest skipping this detail. Probably the spherical volume lost from the convex tank bottom is about equal to the spherical volume of the tank top, so it's a wash and we can just use the tank's overall height and diameter.

A 30 gallon water tank does not give you 30 gallons before the pump turns on

Remember that a "30 gallon water tank" used to control water pressure and pump cycling in a building does not hold 30 gallons of water, but something less than that (say 20 gallons of water max and 10 gallons of air at the point of pump cut-off). Then as you draw water out (and the in-tank pressure falls down to the pump cut-on point) the pump is going to come on before all of the water leaves the tank.

So the maximum actual water you get out of the tank is less than the tank size, maybe 15 to 20 gallons max. The bladder-type pressure tank manufacturers cite an "equivalent" draw-down water volume as that provided by the older bladderless tanks.

How to measure the draw-down water volume provided by a water tank

• After the water pump has just shut off at full cut-out pressure (the water tank is now as full as it's going to get), put a 5 gallon bucket under a bath tub spout or other convenient faucet or hose.
• Then turn on the water and let it run into your test bucket.
• Listen for when the pump cuts on, or if your pump is submersible (and you can't hear the pump) station an assistant next to the pump control and have them listen for the click that indicates that the pump relay has turned "ON" - they'll also see the pump pressure gauge start to move back up.
• When the assistant sees or hears that condition, turn OFF the water at your test fixture.
• Measure the volume of water in your bucket.
• That's the draw down volume.

What are some typical water tank draw down volumes?

A 10 gallon water pressure tank that starts fully empty and is pumped up to about 50 psi will contain about 3 gallons of air and 7 gallons of water. The water tank in normal operation does not draw down to 0 gauge pressure before the pump comes on.

The water tank provides out flowing water down to 20 psi (on a 20-40 psi system or down to 30 at a 30-50 psi system) when the pump comes on.

A water pressure tank with a total volume of 10 gallons and operating between 20 psi and 50 psi of pressure will have a draw down water volume of just 4.35 gallons of water.

A typical kitchen water faucet runs between 3 gpm and 5 gpm (varying as the water pressure in the system varies as the water pump cycles on and off), so we can expect to run the water at the tap for about a minute before the pump will come on with this theoretical water tank. Because the pressure drops as the water tank empties and then increases as the water pump comes back on, the water pressure at a faucet or other plumbing fixture will vary between the pump cut-in pressure (typically 20 psi or 30 psi) and the pump cut out cycle (typically 40 psi to 50 psi). quoting from Water Tank Pressure, Temperature, and Air Volume Calculations.

The above-cited article, which we admit is a bit unnecessarily complex (I was answering someone else's query) has the math you need to calculate the actual draw-down volume of water you get with a given sized tank, with a given in-tank water volume when the pump has reached its shutoff point.

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WATER TANK AIR, HOW TO ADD
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WATER TANK REPLACEMENT
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WATER TANK SIZE & VOLUME
WATER TANKS HOW THEY WORK
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• Roger Davis thoughtfully reminded us to show the correct and clear formula for calculating the volume of a tank. Mr. Davis is a professional engineer specializing in mechanical engineering with the Warren Group, Inc., forensic engineers and consultants in Irmo SC. Mr. Davis or his firm can be reached at 803-732-6600, or 888-827-7823, www.warren-group.com. email: roger@warren-group.com
• Craig Revill provided the photo of a large water storage tank and air volume control found in an house dating ca 1900.

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