Seam Allowance Fabric Calculator — Extra Fabric for Pattern Pieces

Each pattern piece is enlarged by the seam allowance on every seamed edge and by the hem allowance on hemmed edges.

Cut dimensions per piece

Each rectangular piece has two width-running edges (top and bottom) and two height-running edges (left and right). The seamed edges you enter are spread across these pairs, and every seamed edge adds one seam allowance while every remaining edge adds the hem allowance.

Cut width = Finished width + allowance on each left/right edge

Cut height = Finished height + allowance on each top/bottom edge

Cutting layout

The calculator works out how many cut pieces fit across the fabric width, taking layout efficiency into account, then multiplies the rows needed by the cut height. The finished-size (no-allowance) layout is built with the very same packing rules, so the difference between the two is the seam allowance cost on its own.

Waste percentage

Waste = (Total fabric area − Total piece area) / Total fabric area × 100. It counts the layout-efficiency margin and any empty space in a partial final row, so it runs higher than the 15–25% norm for projects with only a handful of pieces.