What mesh count means
Mesh count is the number of holes per inch of canvas, and each stitch covers one of them. On 13 mesh, 13 stitches make an inch; on 18 mesh the same design comes out smaller and finer. Divide the stitch count by the mesh to get the finished size.
Leave room around the design
Canvas is cut bigger than the design so it can be held in stretcher bars, blocked square and finished. A bare margin of 3 to 4 inches on every side is commonly recommended for needlepoint, and 2 to 3 inches for cross stitch. Allow more if a framer or finisher will make it into a pillow.
Cross stitch works the same way
Aida fabric is sold by count, the number of stitches per inch, so the same math applies. Switch the calculator to Aida and pick 11, 14, 16 or 18 count.