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Seam Allowance Calculator

Add seam allowance to pattern pieces. Enter finished dimensions, set your seam allowance — get the cut size instantly. Supports inches, cm, and multiple pieces.

Pattern Pieces

✂️ Cut Sizes

Common Seam Allowances

Quilting 1/4" (0.64 cm)
Knits / Serging 3/8" (1 cm)
Home dec / Crafts 1/2" (1.27 cm)
US Garments 5/8" (1.6 cm)
Metric Standard 1 cm (3/8")
Metric Garments 1.5 cm (5/8")

About This Tool

Cutting fabric pieces and need to add seam allowance? This calculator does the math so you don't have to — just enter your finished dimensions and seam allowance, and get the exact cut size. Works for rectangles, squares, and any pattern piece with straight edges. Choose which sides need allowance (all four, two opposite, or single edges), and see the final cut dimensions instantly. Perfect for quilting, garment sewing, home dec projects, or any time you're working from finished measurements. The tool handles both inches and centimeters, shows the total allowance added, and lets you calculate multiple pieces at once. No more mental math or marking errors.

How to Use

1. Enter the finished dimensions of your pattern piece (width × height) 2. Set your seam allowance (common: 1/4" for quilting, 5/8" for garments, 1.5 cm metric) 3. Choose which sides need allowance: all sides, two sides (width or height), or custom 4. View the cut dimensions — the size to actually cut your fabric 5. Add more pieces if you have multiple pattern pieces 6. Click "Copy" to get a list of all cut sizes

Formula

Cut width = Finished width + (Seam allowance × Number of width edges) Cut height = Finished height + (Seam allowance × Number of height edges) Example: 10" × 8" finished, 1/2" seam allowance on all sides Cut width = 10 + (0.5 × 2) = 11" Cut height = 8 + (0.5 × 2) = 9" Cut size = 11" × 9" Common seam allowances: Quilting: 1/4" (0.64 cm) Garments (US): 5/8" (1.6 cm) Garments (metric): 1.5 cm Home dec: 1/2" (1.27 cm)

Frequently Asked Questions

What is seam allowance?
Seam allowance is the extra fabric between the cut edge and the stitch line. It's the margin that gets sewn into the seam. Standard seam allowances vary: 1/4 inch for quilting, 5/8 inch for garment patterns (US), 1.5 cm for metric patterns. Without seam allowance, your finished piece would be smaller than intended.
How do I add seam allowance to a pattern?
Add the seam allowance to each edge that will be sewn. For a rectangle with seams on all sides, add seam allowance twice to both width and height (once for each edge). A 10×8 inch piece with 1/2 inch seam allowance on all sides becomes 11×9 inches to cut.
What is the standard seam allowance for quilting?
Quilting uses 1/4 inch (6.4 mm) seam allowance. This narrow allowance reduces bulk where multiple seams meet. Quilt patterns typically include seam allowance in the cutting instructions, but if you're working from finished block sizes, add 1/2 inch total to each dimension.
What is the standard seam allowance for garments?
US garment patterns typically use 5/8 inch (1.6 cm) seam allowance. Metric patterns commonly use 1.5 cm. Commercial patterns include seam allowance; indie or self-drafted patterns may give finished measurements. Always check the pattern instructions.
Do I add seam allowance to all sides?
Add seam allowance only to edges that will be sewn. A neckline facing needs allowance on the outer edge but not the fold line. A hem needs allowance on the bottom but the width stays the same. This calculator lets you choose which sides need allowance.
How do I convert between inch and cm seam allowance?
Common conversions: 1/4" = 0.64 cm, 3/8" = 1 cm, 1/2" = 1.27 cm, 5/8" = 1.6 cm, 1" = 2.54 cm. For metric sewing, 1 cm and 1.5 cm are standard. This calculator works in either unit and shows both.
What seam allowance for knit fabrics?
Knits typically use 1/4 to 3/8 inch (0.6-1 cm) seam allowance because they're sewn with stretch stitches that don't need as much margin. Serged/overlocked seams use about 3/8 inch. Some knit patterns use 5/8 inch for easier handling.

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