The answer depends on the units used for measuring the circle: 12 WHAT? Inches, miles, millimetres? Since you have not bothered to provide that crucial bit of information, I cannot provide a more useful answer.
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-- 12 inches across the circle, along any route that passes through the center -- 6 inches along any radius -- 12 pi = 37.7 inches (rounded) around the circle's curvy line
Area of a circle = pi*radius2
21 feet = 21*12 inches = 252 inches - whether in a straight line or a circle or whatever.
The area of a circle with a diameter of 12 inches is: 113.1 square inches.
A circle with a radius of 6 inches has a circumference of 37.7 inches.