If you are measuring the length of something with a constant cross-sectional area, you multiply the length times the area.
Example
The area of a 1 ft inside diameter pipe is about .7854 square feet. 10 linear feet of such pipe will have a volume of (10 ft)*(.7854 ft2) = 7.854 ft3.
Example
The area of a 1 ft inside diameter pipe is about .7854 square feet. 10 linear feet of such pipe will have a volume of (10 ft)*(.7854 ft2) = 7.854 ft3.
