This page is intended to help your department calculate either the capacity of an existing tank or to calculate the size of a tank that will need to be built.  All calculations are built around polypropylene tanks and take material thickness into consideration.  To use these figures with steel, simply leave out the "-1" subtraction that is used against the primary dimensions.

If you are needing a quick, down and dirty estimate of weight, use 9 pounds per gallon with a poly tank and 10 pounds per gallon with steel construction.

Simple Calculation, Rectangular Tank

All calculations are based on the fact that there are 231 cubic inches in a gallon of water.

If you know the full outside dimensions you will have and only want to know how many gallons it will hold use this formula: 

((Length – 1) X (Width –1) X (Height-1)) / 231 = Total Gallons


We have a tank measuring 100” long, 48” wide, and 50” high.  The calculations would look like this:

((100-1) X (48-1) X (50-1))  / 231 = 987 Gallons
(99 X 47 X 49) / 231 = 987 Gallons
227,997 / 231 = 987


What If? Calculation

When you know two of the variables but not the last this method comes in handy.  For instance, we know the tank can be 48” wide and 100” long but need to know how high the top will have to be to hold 1000 gallons.  First we figure the cross section of the tank, less the thickness of the sides and how much that shape will hold per 1” of depth by using this formula:

((Length-1) X (Width-1)) /231 =  Gallons per inch of height


((100-1) X (48-1)) /231 = 20.143    (Carry out to three decimal places minimum)
(99 x 47) /231 = 20.143
4,653 / 231 = 20.143

Then, simply divide the gallons needed by the above result and add 1” for material thickness.

(1000 / 20.143) + 1 =  50.645  (Required tank height for 1000 gallons)

Or, 51” for all practical purposes (1027 Gallons using the formula at the top of the page.)  Every tank should have a slight reserve.

Tee Tanks

To calculate the size of a tee tank, simply look at it from the end.  It is two different size rectangles, one above the other.  Calculate the size of each rectangle and simply add them together.  

Please Note:  These formulas are “rule of thumb” calculations designed to get “into the ballpark”.  Final capacity can vary depending on baffles, pass through tubes etc….

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Last modified: 03/27/12