Each joined edge uses fabric
Two panels sewn together each contribute a seam allowance. If you join two 40 inch wide panels with a ½ inch allowance on each edge, the joined width is 40 + 40 − ½ − ½ = 79 inches.
For several equal panels, the joined width is the total panel widths minus twice the allowance for every seam. Three panels have two seams.
Use the allowance your project calls for
Check the pattern or your quilter’s instructions. Also check whether a pattern’s cutting dimensions already include the allowance before adding more.
The seam allowance between backing panels is separate from the extra fabric around the outside of a quilt backing. Both affect the fabric you need.