How many cubic feet in an 18 ft circle?

Answer 1

There are none.

CUBIC feet are a measure of volume and volume is an attribute of a three dimensional object.

A circle is a two dimensional shape; it can be considered to have third dimension of zero length so its volume is area (in sq ft) × 0 ft = 0 cu ft

Answer 2

There can be no answer.

An 18 foot circle is a shape in 2 dimensional space whereas cubic feet are a measure in 3-dimensional space. According to the most basic rules of dimensional analyses, there can be no conversion between measures whose dimensions do not match.