I have a 300' pipe that is .675" i.d. How many U.S. Gallons does this contain ? Thanks.
The volume of a prism (a cylinder is a prism) is it's cross-sectional area multiplied by it's length.
For a cylinder the cross sectional area is a circle: $\displaystyle V = \dfrac{\pi d^2h}{4}$ where d is the diameter and h the height/length of the pipe.
Make sure your units are consistent