How to Add Seam Allowance

If a panel is joined on both sides, seam allowance usually has to be added on both edges, not once for the whole piece. That is why many width calculations use two side allowances.

This step is especially important when drafting rectangles, panels, waistbands, plackets or simplified pattern pieces from finished dimensions.

A hem often needs more depth than a standard joining seam because it folds back on itself. If both values are treated the same, the final cutting height can easily be too short.

Using separate seam and hem values creates a more realistic cutting plan and makes the result easier to adapt to different fabrics and garment finishes.