How much is the tablecloth with a round table of 2.2 meters?
If the wall thickness of the vessel is thicker, it cannot be neglected. Because the bottom area is calculated by multiplying the volume. You can't mix volume and volume.
If you ignore the thickness of the container wall, then at 18 meters, 4.2 meters height, the diameter of the cylinder volume is: 2.1 * 2.1 * 18 * =79.38 PI (square meters)