How to Make a Circle That Is the Same Area as a Square
(If I were doing this on a blackboard, I'd use superscripts and subscripts so that it would be much more readable)

AreaOfCircle = Pi X (radius)squared AKA A(c) = Pi X r X r
AreaOfSquare = (length)squared AKA A(s) = l X l

We know the area of the square; so taking the square root (Excel can do this using the SQRT function.) gives us the length. Or we could measure it.

We want AreaOfSquare=AreaOfCircle
l X l = Pi X r X r AKA l X l = 3.1416 X r X r
Taking the square root of both sides
l = 1.8 X r

Or, if we want to solve for r, knowing l,
r = l / (1.8)