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Water tank sizing charts, formulas, advice:
This article explains Water Tank Size - how much water is in the water pressure tank? We address the question of how much water volume we need to avoid short cycling the water pump?
This article series answers just about any question you may have about pumps, wells, and drinking water.
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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 [above] 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.
How much water the tank holds or the physical volume of the water tank that is useful to know but iIt's how much water we can get out of the water tank before the pump has to turn on that is important when sizing or adjusting water supply systems and pumps. 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.
If your water pump turns on and off to frequently it can overheat or damage the pump or its controls. Usually this short cycling of the pump is not because the water tank is too small, it's because the tank has lost its air charge. See our discussion of "short cycling" water pumps at WATER PUMP SHORT CYCLING - home . If your pump is short cycling you want to fix it, as we explain in that article.
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.
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.
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.
How much water should be in the water pressure tank internal bladder? The water tanks are rated as 85 gallons. When the system cut-out water pressure is reached there is only about 5gal in tanks? - Stephen
Modern internal bladder type water pressure tanks give an "equivalent draw down" cycle to the actual "gallons" number on the equipment, but the actual physical volume of water may be significantly less.
5 Gallons does not sound reasonable. Typically on an "85 gallon" sized water tank, the actual water draw-down volume will be about 25 gallons. Below we include a table of rough estimates of actual water draw down volumes for different internal bladder water pressure tanks.
Check your water pressure tank air pre-charge. When the water tank is completely empty the air pre-charge should be 2 psi below the pump control's cut-in pressure. If the pre-charge pressure is too high that will reduce the volume of water held in your water tank.
Table of Typical Water Tank Draw-Down vs Rated Size
|Water Tank Rated-Size (approximate gallons)||Actual Volume of Water Draw Down (approximate gallons)|
Notes to the water tank draw-down volume table above:
Detailed Chart of Water Pressure Tank Sizing, Water Tank Volume, Water Drawdown Volume in Gallons
|Well-Rite Water Tank Model||Total Water Tank Volume||Equivalent
Water Tank Size
|Drawdown Water Volume at Various Pressure Control Settings|
Notes to the Wel-Rite Water Tank Sizing Chart
Data based on a Well-Rite® water pressure tank 
When choosing an internal-bladder type water pressure tank, take note of the operating pressure range of your well pump equipment. You will see in the table above that the higher you set the water pressure cut-in and cut-out, the smaller will be the draw-down volume provided by a particular water pressure tank.
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.
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.
Continue reading at LARGE CAPACITY WATER STORAGE TANKS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see EXPANSION TANK SIZING GUIDE - tank sizing tables for expansion tanks used on hot water heating systems
Or see ROOFTOP WATER TANKS
Or see PLASTIC CONTAINERS, TANKS, TYPES where we describe health and other concerns involving plastic tanks and other containers used for water storage.
Or see WATER QUANTITY IMPROVEMENT
Or see WELL YIELD IMPROVEMENT
Or see WATER TANK SIZE & VOLUME FAQs - questions and answers about choosing the right size water tank
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Please see see WATER TANK SIZE & VOLUME FAQs - questions and answers about choosing the right size water tank
Or see WATER TANK DIAGNOSTIC FAQs
Or see WATER TANK BLADDER DIAGNOSTIC FAQs - for tanks that use an internal bladder
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