polygon - Converting Longitude and Latitude Coordinates to Square Miles?

I have a set of Long and Lat co-ordinates making up a five point polygon which I have converted in excel using the following formula:

=((X1*Y2)-(X2*Y1)+(X2*Y3)-(X3*Y2)+(X3*Y4)-(X4*Y3)+(X4*Y5)-(X5*Y4)+(X5*Y1)-(X1*Y5))*0.5

*Replacing X,Y with cell references of course.

This gives me a negative number but how do I convert this number to square miles or kilometres?