Find the cut size needed for a finished measurement plus seams.
Cut size = finished + two seam allowances. Quilting uses \u00bc\u2033; garments often \u215d\u2033.
The cut size of a piece is the finished size plus a seam allowance on every side. A piece that will be 4.5 inches when finished needs 5 inches if the seam allowance is a quarter inch on each side, since the two sides add half an inch. Enter the finished size and the seam allowance per side to get the size to cut.
Quilters generally use a quarter inch seam, while many garment patterns use five eighths of an inch, so check what your pattern assumes. Serged seams use a narrower allowance, and some designs use larger allowances to leave room for fitting.
Accuracy matters because errors add up across a seam. A tiny difference in the allowance on each of many blocks can leave the finished piece noticeably the wrong size. Use a seam guide or a quarter inch foot, and test on a scrap and measure before you sew the real piece.
Press seams flat after you sew them, since a bulky or uneven seam changes the finished size. Remember to add allowances for hems and facings as well, and mark them clearly on the pattern.