The area of a circle is 3.14*(radius*radius). To create the formula in Excel, I could suggest the exponential function however, since the goal is just to find the square, we can twist the formula ...

To find the radius from the area of a circle, use the inverse operations: Divide the area by π. Find the square root. To find the diameter, multiply the radius by 2.

If we take a square with this radius, then the area is (2r)(2r) = 4r² and the perimeter is 8r. The ratios A/r² and P/2r then both equal 4 and so we might call 4 "π" for the square.