If the width is 18 then the diameter is 18 (if i've understood you right). The radius is therefore 9. The area of a circle (as the base of a cylinder is a circle) is calculated with the formula A = r * r * pi = 9*9*3.1415926535 = 254.47 length units.
If you want to calculate the volume too you use the formula V = B * H = Base * Height = 254.47 * 10 = 2544.7 Area units.
