The volume of a pipe that is 1 foot long and 6 inches in diameter is 0.2 cubic foot. This correlates to approximately 1.5 gallons of standing water potentially inside the pipe.
The mathematical formula for the volume of a cylinder is volume (V) = pi x radius^2 x height. In this specific example, using feet as the units, that would become the equation V = pi x .25ft^2 x 1ft, or 0.2 cubic foot / ~1.5 gallons. While that is the total volume of the pipe, it does not necessarily mean that there is water filling the entire internal capacity.