A worked answer
An 85 by 90 inch full quilt has 350 inches of edge. Add 10 inches for joins, cut 2 1/2 inch strips from 42 inch fabric, and you are buying about 5/8 yard.
Perimeter is 350 inches plus 10 for joins. Nine 2 1/2 inch strips need 22 1/2 inches of bolt length, sold as 5/8 yard. The full quilt designer is 85 by 90 on the canvas. Queen binding is a different perimeter — do not copy a queen leftover onto a full without measuring.
Backing is how much backing for a full quilt. Twin binding is 5/8 yard as well on Quiltler's twin card — same strip count coincidence, different quilt. Confirm inches.
For an 85 by 90 inch quilt, cut 2 1/2 inch strips. The perimeter is 350 inches, plus 10 inches for joins. That is 9 strips from 42 inch fabric, which comes to 5/8 yard.
Not for 2 1/2 inch strips. Half a yard is 18 inches, which is seven strips. This quilt needs nine. Five-eighths of a yard covers it.
A queen is 90 by 95 and needs more strip. Use the queen binding page for that bed.