Hi Gordon,

The formula for the volume of a circular cylinder is $\pi \; r^2 h$ where $\pi$ is approximately 3.1416, $r$ is the radius of the cylinder and $h$ is its height. I am going to use inches as the units since I know that one US gallon is 231 cubic inches.

I assume that the 8 inch pipe has an 8 inches inside diameter and hence its inside radius is 4 inches. There are 12 inches in a foot an hence the volume of the pipe is

\[\pi \; \times 4^2 \times 50 \times 12 = 30159.289 \mbox{ cubic inches}\]

which is

\[\frac{30159.289}{231} = 130.56 \mbox{ gallons.}\]

If you would rather not do the calculation yourself you can use our volume calculator.

Penny