Even if you knew how many sides the polygon has, you still
could not calculate its perimeter with that much information.
Examples:
-- An equilateral triangle with area of 20 has perimeter of 20.3885 .
-- A square with area of 20 has perimeter of 17.889(rounded).
-- A rectangle with area of 20 can have any perimeter more than 17.889 .
4 by 5 . . . . area = 20, perimeter = 18
2 by 10 . . . area = 20, perimeter = 24
1 by 20 . . . area = 20, perimeter = 42
.
.
etc.
The conversion factor is 0.155: 20sq cm x 0.155 = 3.1 inches squared