



 
Ray, Your copper pipe is a circular cylinder and the volume is given by V = π r^{2} h where r is the radius and h is the height. You want the volume to be 3 gallons and there are 231 cubic inches in a US gallon so you have
I assume the 3/4 inch measurement is the inside diameter of the pipe so the radius is 3/4 ÷ 2 = 3/8 inches. Thus
Solve for h which will be in inches. After you have found h you can use our volume calculator to check that the volume is in fact 3 gallons. Harley  


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