Start from the backing rectangle, not the bolt
Add overhang to the finished top. That rectangle is what you cover. Then ask how many usable widths of fabric (often 42 inches) fit across it. One leftover strip along the edge is allowed; three skinny leftovers are a headache. Quiltler’s backing calculator counts panels and yardage. Size-specific pages such as throw, full, and queen freeze the common beds.
Where the seam should sit
If the top has a medallion or a strong centre seam, shift the backing join an inch or two so quilting does not stack two ridges. On a plain throw, dead centre is fine. Do not align a backing seam with a pieced seam on the front on purpose — that is where needles break and where a longarm hitch can snag.
Wide backing versus piecing
108 inch goods remove the join when one panel covers the quilt. They cost more per yard and often less in total on queens and kings. Width of fabric is the vocabulary; the backing card is the number. Diagonal piecing (John Flynn) is a special case on the diagonal backing calculator — it rarely beats three straight panels on a queen.
When the top is still in design, the studio counts backing with the rest of the pack after you subscribe. The free calculators never spend a saved-quilt slot. Design the quilt in the studio when you care about the top; piece the back from this math.