I was silly and was getting my ranges messed up, I wasn't shifting the formula by one row. By correcting this, it meant that the formula in post 4 of this thread is now working for me:
http://www.mrexcel.com/forum/excel-questions/495100-irregular-polygon-area.html
However, with the formula...
@NARAYANK991
Having tried your formula:
=0.5*SUMPRODUCT((H5:H200)*(I6:I201)-(H6:H201)*(I5:I200))
I get an #N/A error.
It doesn't seem to like the fact that the arrays in the formula are of different lengths, but surely this is intentional. Any thoughts?
Thank you,
John