The formula for circumference is C= 2(pi)(r)^2. Because diameter is double the radius, and the diameter is 8 ft, the radius is 4 ft. So, plug that into the equation, and you get: C=2(pi)(4)^2. The answer is 32 pi ft. If the question asks for the exact answer, keep the pi as it is, but if it says to round, multiply 32 by pi and round.