I think this'll do it:


Pick a point on the polygon and begin moving in a counter-clockwise fashion around the shape. (If you accidentally move clockwise, you’ll get the same number but as a negative value.) Assuming the first point you choose is (x1, y1) and the last point, before you close the shape, is (xn, yn), you multiply the first x value with the second y value and subtract the second x value times the first y value. You continue this process all the way around the shape. Obviously, the last point will be the first point (x1, y1), since you’re dealing with a closed shape. Finally, take half of the entire value.
½[(x1 y2 - x2 y1)  + (x2 y3 - x3 y2) + . . . (xn-1 yn - xn yn-1)]