Straight-grain and bias binding
Straight-grain binding can be counted from the quilt perimeter and the cloth width. Continuous bias needs a construction sequence that an area formula does not finish.
Straight-grain strips
Required length is the perimeter plus an extra length you choose. The suggested extra is 12 inches. A published worked method also uses 10 inches, so the field stays editable.
For diagonal joins, usable length is n × width of fabric − (n − 1) × strip width. A 60 by 80 inch quilt with 12 extra inches, 2.5 inch strips, and 42 inch usable cloth needs eight strips and 5/8 yard after rounding.
The bias square
The area of the strip is length times cut width. A square with that area is the theoretical minimum. Fifty-five inches of a 2.5 inch strip points at about an 11.73 inch square, which rounds up to 12 inches.
Sewing the tube, trimming, and starting the spiral can use more cloth. The bias page labels that limit and does not present the square as a guaranteed yield.