How to calculate fabric size for a cross stitch chart
A counted chart gives you a width and height in stitches. Divide each number by the effective stitch count per inch. A design that is 84 stitches wide on 14-count Aida will be 6 inches wide.
Then add your margin twice to each measurement. If your finisher requests 3 inches on each side, cut fabric 12 inches wide for that 6-inch design.
On evenweave stitched over two threads, halve the threads per inch to get the effective stitch count. For example, 28-count evenweave stitched over two gives 14 stitches per inch.