It takes 130.56 gallons of water to fill a 4-inch by 200-foot pipe. This is the result of calculating the volume of a cylinder or pipe in square feet and converting the square-foot volume to gallons.
The formula for volume of a cylinder is the cylinder’s radius at one end squared times the cylinder's height times the constant Pi, or 3.14. Using this formula, the volume of a 4-inch by 200-foot pipe is 30,159.36 cubic inches. Since 1 cubic foot contains 1,728 inches, the pipe’s volume is 17.45 cubic feet. Since 1 cubic foot of volume holds 7.48 liquid gallons, 130.56 gallons of water fill the pipe.