Seam Allowance Calculator
Calculate total fabric dimensions when adding seam allowances to pattern pieces.
Adjust finished measurements for any seam width in inches or centimeters.
Seam allowance is the strip of fabric between the cut edge and the stitch line. Getting it right determines whether your finished garment matches the intended size — too little and seams fray; too much and the fit is tight.
The core formula:
Finished Seam Width = Cut Piece Width − (2 × Seam Allowance)
Or rearranged to find the cut size from the finished size:
Cut Size = Finished Size + (2 × Seam Allowance)
What each variable means:
- Seam Allowance — the margin from cutting line to sewing line (both pieces have their own allowance)
- Finished Size — the measurement of the completed, sewn garment
- Cut Size — the measurement of the fabric before stitching
Worked example: You want a finished bodice width of 40 cm using 1.5 cm seam allowances on each side seam.
Cut size = 40 + (2 × 1.5) = 40 + 3 = 43 cm
If you accidentally use 1 cm instead of 1.5 cm seam allowances on all 4 side seams (2 per side × 2 sides), the bodice ends up 2 cm wider than intended.
Standard seam allowances by application:
- Home sewing patterns (US/Australia): 5/8 inch (1.6 cm)
- Dressmaking patterns (Europe/UK): 1.5 cm
- Industrial/commercial garments: 1 cm
- French seams (delicate fabrics): 6 mm first pass, trimmed to 3 mm, then 6 mm again
- Quilting: 1/4 inch (6 mm)
- Leather and faux leather: 1 cm (cannot be pressed open easily)
Always stay-stitch curved seams (necklines, armholes) before sewing to prevent stretching.