Hi Tiffany,

I assume that the outside diameter of the tube is 42" and so the inside diameter is 42 - 2 3.5 = 42 - 7 = 35". Hence the radius is 17.5". Thus you have a cylinder of water that has a radius of 17.5" and a height of 108".

The volume of a cylinder is r² h where r is the radius and h is the height. Use the dimensions you have to find the volume. Since your dimensions are in inches the volume you obtain will be in cubic inches. There are 231 cubic inches in a US gallon so you can use this factor to convert the volume to gallons.

Penny