We get this question all the time, both for needlepoint and cross-stitch. You have a design that gives you a stitch count (or you've counted it for yourself) and now you need to know how much fabric to buy. You'll need to know the stitch count, your fabric count and what use you plan for the design. The reason you need the latter is that you need more margin for a piece that's going to be framed and matted vs. a piece that's going to be framed or something turned into a pillow.
We have a handy Fabric Calculations page that gives you all the details you need to know so you can do this calculation yourself. You're always welcome to ask us to figure it out for you and we'll ask you for the 3 things mentioned above. The page is a PDF file you can print and keep with your stitching for reference. And the math really is not hard :-).