



 
Hi Steve. I did a similar calculation a couple of years ago. You can see the mathematical derivation here. You appear to have a radius of 13.5 inches and a length of 69 inches. Since you are writing from the United States, I presume you know that 231 cubic inches equals a US gallon. So the volume of your tank is πr^{2}L cubic inches. Thus according to the dimensions you provided, that is 3.14159(13.5)^{2}(69) = 39506 cubic inches. That's 39506/231 = 171 gallons, not the 140 gallons you indicated. I think this discrepancy may be due to external dimensions compared to internal dimensions, or perhaps imprecise measurements. The computations I did for Andy can be represented as a "percentage full" based on the inch level. I derived the following for your situation (using r as the radius and d as the central depth of liquid): I punched this into a spreadsheet using r = 13.5 and d going from 0 to 27. Then I used the two volumes (my calculation of 171 based on your measurements; and your given volume of 140 gallons). So here is a depth to volume table: Hope this helps,  


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