The area of a circle is:
pi times radius squared
So, the area of a circle with a diameter of 1ft 6 in would be
(diameter = radius times 2)
3.14 (pi) times 1ft 6 in = area
3.14 times (1 ft 6 in = 18in) = area
3.14 times ((18/2)² ) = area
Area = 254.34