The perimeter of a shape is the measure of the length of a shape around its outermost extremities. The area of a shape is the amount of two-dimensional space that it covers. The ratio of the perimeter to the area of a shape is simply the perimeter divided by the area. This is easily calculated.
Find the radius of your circle. This is the distance from the center to the outer edge.
Calculate the perimeter or circumference of the circle. This can be done using the equation "C=2pi*r" where "r" is the radius.
Calculate the area of the circle. This can be done using the equation "Area=pi*r^2" where "r" is the radius of the circle.
Divide the perimeter by the area to obtain the ration of perimeter to area.
Find the length and width of the rectangle. This will be the measurement of one of the long sides and one of the short sides.
Calculate the perimeter using the formula "2(b+w)=P" where "b" is the base and "w" is the width.
Calculate the area using the formula "A=b*w," where "b" is the base and "w" is the width.
Divide the perimeter by the area to obtain the ration of the perimeter to the area.
TL;DR (Too Long; Didn't Read)
There are an infinite variety of shapes. If you don't know the formulas for a shape, get creative and try breaking it down into smaller rectangles.