Quilt backing overage
Backing has to be larger than the quilt top so it can be loaded, quilted, and trimmed. How much larger depends on the frame and the quilter.
Overage is per side
Four inches on each side adds eight inches to both the width and the height. Five inches on each side is offered as an alternate starting value because some longarm loading advice describes 4 to 5 inches as a comfortable range.
Neither number is locked. A domestic machine quilt can use less. Enter the overage your setup needs.
Seams consume two allowances
The seam allowance field is the allowance on one raw edge. Joining two panels consumes twice that amount. For a 1/2 inch allowance, each seam removes 1 inch from the combined width.
Coverage is then n × usable width − 2 × seam allowance × (n − 1). Both seam directions are compared. Directional fabric is not turned just because the other direction is shorter.