I must admit that when I saw the request on the calculator creation/2658/, which sounded like "calculate area by perimeter". I was a little surprised because it looked somewhat surreal.
Although, after some search on the Internet I realized that the request just not complete, and often goes like this: "Calculate the area of a rectangle if its perimeter is equal to X and it is known that ..." there are different things that may be known and they are leading us to a solution. For example, the length of one of the sides or the aspect ratio. The calculator below calculates the area of the rectangle, depending on what else is known apart from the perimeter. I dedicate it to the students.