Seam Allowance Calculator

Seam allowance is the extra fabric beyond the finished stitch line that gets sewn into the seam. Standard garment seams use ⅝ inch (1.5 cm) in US patterns or 1 cm in many European patterns. Quilting often uses ¼ inch.

To find cut size, add twice the seam allowance to each dimension — once per side. A 10 × 12 inch finished rectangle with ½ inch allowance needs an 11 × 13 inch fabric cut.

Hems, facings, and zippers may use different allowances on specific edges. Pattern instructions usually note exceptions — this calculator applies one uniform allowance to all four sides for simple blocks and panels.

Worked example with defaults: finished 10 × 12 inches plus 0.5 inch allowance per side → cut 11 × 13 inches. Each dimension grows by 2 × 0.5 = 1 inch total (0.5 inch left + 0.5 inch right, or top + bottom).

Metric example: finished 20 × 30 cm with 1 cm allowance → cut 22 × 32 cm. Toggle the unit switcher to enter centimeters; the same formula applies regardless of unit.

Quilters cutting ¼ inch seams for piecing should enter 0.25 (or 0.635 cm). Garment sewists using commercial US patterns often start at 0.625 inch (⅝ inch) unless the pattern specifies otherwise.

French seams and flat-felled seams consume more fabric than a single pass — this calculator assumes one row of stitching per edge. Add extra width manually if your technique folds the allowance twice.

Binding and facing strips cut on the straight grain use the same cut-size math — only the seam allowance value changes if your pattern specifies a narrower join. Write the finished and cut dimensions on your pattern pieces so you do not double-add allowance at cutting time.

Limits: this tool handles rectangular pieces with equal allowance on all sides. Curved pattern pieces, mitred corners, and bias binding need pattern-specific math not covered here.