below is the formula to calculate area of Farm assuming a quadrilateral (4 sides ). (Same as stated by agri_exec).
A = SqRt[(s-a)(s-b)(s-c)(s-d)], where a, b, c and d are the lengths of the four sides of the quadrilateral, and s = (a + b + c + d)/2.
Below is the formula you could use in a excel file .
Follow the steps.
*Add 4 sides in cells A2, B2, C2, D2 , place above formula in any of the cell s to see Area.
* place formula " =SUM(A2+B2+C2+D2)" to get perimeter of the farm (running feets to calculate fencing expenses).
Since its excel its easy to change parameters and understand Area.
If you know the exact area and one of the 3 sides you could calculate 4th side.
Same excel file can be modified to insert formula of perimeter x fencing cost to get aproximate expenses.