Approximately 5.875 gallons of fluid fill 1 foot of a 12-inch pipe. To determine this, first find the volume of 1 foot of a 12-inch pipe, which is 1357.2 cubic inches, and then convert the volume from cubic inches into gallons.
The formula for the volume of a cylinder is pi multiplied by the radius squared multiplied by the height. In this case, the radius is 6 inches, as the size of the pipe is its diameter. The height is 12 inches, as this is simply the length of pipe used. To convert cubic inches into gallons, multiply the number of cubic inches by 0.004329.