Ray,

Your copper pipe is a circular cylinder and the volume is given by V = π r² 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

3 × 231 = π r² h

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

3 × 231 = π (3/8)² h

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