The formulas for the perimeter P and the area A of the rectangle are used to write equations as follows:
P = 2 * L + 2 * W
A = L * W
Then these equations are solved for L and W which are the length and width of the rectangle.
How to use the calculator
Enter the perimeter P and area A as positive real numbers and press "enter". The outputs are the width, length and diagonal of the rectangle. There are conditions under which this problem has a solution (see formulation of problem below).