565
A linear foot is the same as a foot. So, an 18-foot diameter circle would measure 18 linear feet in diameter!
24' 6"
There are 20 linear feet across the diameter; and 20π ≈ 62.83 linear feet around the circumference.
The answer depends on what aspect of the circle you would consider to be a linear measure. The diameter of the circle would be 7 ft.
Circumference = pi*d = 12.6 feet, approx.