Allow for both edges of a square
The seam allowance is the fabric between the cut edge and the line of stitching. For a simple square, add it to both sides of the finished dimension. A 3-inch finished square with ¼-inch seams needs a 3½-inch cut square. The same allowance is included along the top and bottom.
A join uses fabric from two pieces
When two pieces are joined, each contributes a seam allowance. Two backing panels that are 40 inches wide make a 79-inch joined span with ½-inch seams: the join uses ½ inch from each panel. Long strip sets follow the same principle, with each join reducing the assembled width.
Check whether the input is finished or cut
The block calculators start with finished sizes and add the seam allowance. The square cutting calculator starts with cut sizes, so there is nothing more to add. Backing starts with your measured top and adds overhang; it doesn't enlarge the quilt top with another hidden seam allowance.
Let a sewn sample settle the setting
Use the allowance called for by your pattern and check a small sewn unit before batch cutting. Pressing and the way the fabric folds over the seam can affect the measured result. If you adjust your sewing or the calculator setting, check the sample again before applying the change to every piece.