How many strands for your fabric count
Strand counts, stitch size, and the count to buy when you already know how large the finished piece should be.
How this is worked out
The fabric is not what decides the strand count — the number of stitches per inch is. A stitch on 14 aida and a stitch on 28 count linen worked over two threads are the same size and want the same two strands, so both this page's answers are read off the effective count: the woven count divided by the threads a stitch is worked over.
Backstitch is always thinner than the cross it outlines. It is a drawn line rather than a filled block, and matching the cross's strand count turns an outline into a rope — one strand below the crosses, and never fewer than one.
Where a range is given, the range is real: coverage depends on the floss, on whether the cloth beneath is pale or dark, and on how tightly you stitch. The reliable answer is a corner of a scrap of the actual fabric, stitched with the actual thread, looked at in the light the piece will hang in.
For the reverse question the page lists the counts the chosen fabric is actually woven in rather than computing the ideal one, because the ideal one is usually a count nobody sells. The nearest is marked; the rest are there so you can see what the next step up or down costs you in size.