Measure the quilt top you have
Enter the actual raw, assembled quilt-top dimensions, with its outer seam allowances still present. Do not substitute the sum of the finished block sizes. Measure through the center; if the quilt varies, compare several measurements before choosing a border length. Remeasure after attaching the first pair.
The finished border width is the part that will show between the inner and outer seams. Cut width adds two seam allowances. A 4-inch finished border with ¼-inch seams needs 4½-inch-wide strips.
Square corners or diagonal corners
Butted borders meet at square corners. Choose whether sides or top/bottom go on first. The first pair matches the corresponding measured quilt dimension. The second pair includes the growth from the first borders.
For a measured 60 × 72 inch top and a 4-inch finished border, sides first means two 72-inch lengths and two 68-inch lengths. The new raw top is 68 × 80 inches. These dimensions account for the allowance removed from both pieces at every attachment seam; no second set of allowances is added to the measured top.
Mitered borders meet on a 45-degree diagonal. Each planned length is the matching measured dimension plus twice the cut strip width, plus the selected extra miter length. The editable extra defaults to 6 inches total per border, following American Patchwork & Quilting’s preparation method. It is separate from the extra fabric purchase allowance. Use your pattern’s corner technique and leave the attachment seam ends open for mitering.
A cutting plan you can follow
The tool cuts crosswise strips from your usable fabric width, after selvage removal. Each of the four borders gets its own full strips. A straight strip join loses two seam allowances; a diagonal strip join loses one cut strip width.
Offcuts are not reassigned to other borders. This conservative plan can use more fabric than a carefully optimized layout, but every listed border can be made from its assigned strips. The calculation adds the extra fabric length once, then rounds purchases upward independently to ⅛ yard or 0.1 meter.
One border round is supported. For another round, remeasure the assembled top and calculate that round separately, especially when using another fabric. Directional prints, shrinkage, squaring, matching motifs and cutting mistakes need your own allowance. The preview is a schematic, and the output top size is raw, before quilting and binding.