First, number the vertices in order, going either clockwise or counter-clockwise, starting at any vertex.
The area is then given by the formula
Where x1 is the x coordinate of vertex 1 and yn is the y coordinate of the nth vertex etc.
Notice that the in the last term, the expression wraps around back to the first vertex again.
Try it here
Adjust the quadrilateral ABCD by dragging any vertex. The area is calculated using this method as you drag. A detailed explanation
follows the diagram.
The above diagram shows how to do this manually.
Make a table with the x,y coordinates of each vertex. Start at any vertex and go around the polygon in either direction.
Add the starting vertex again at the end. You should get a table that looks like the leftmost gray box in the figure above.
Combine the first two rows by:
Multiplying the first row x by the second row y. (red)
Multiplying the first row y by the second row x (blue)
Subtract the second product form the first.
Repeat this for rows 2 and 3, then rows 3 and 4 and so on.
Add these results, make it positive if required, and divide by two.
See Polygon area calculator for a pre-programmed calculator that does the arithmetic for you.
Just enter the coordinates.