Seam Allowance Calculator
Calculates the cut fabric dimensions you need to achieve a specific finished size after sewing. Essential when drafting patterns or adjusting pieces to account for the fabric consumed by seams.
About this calculator
When you sew two pieces of fabric together, each seam consumes a strip of fabric on both sides of the seam line. To end up with a finished piece at the size you want, you must cut the fabric larger than the finished dimensions. The formula adds the seam allowance to both ends of each dimension: cut area = (finishedLength + seamAllowance × 2) × (finishedWidth + seamAllowance × 2). The ×2 accounts for both edges (top/bottom and left/right). Standard seam allowances in garment sewing are typically 5/8" (about 1.6 cm), while quilting commonly uses 1/4". The result is the total cut area in square inches; to find individual cut dimensions, apply the additions to each side separately: cut length = finishedLength + (seamAllowance × 2), and similarly for width.
How to use
You want a finished pillow cover of 18" × 18" with a 1/2" seam allowance. Step 1 — add seam allowance to each dimension: 18 + (0.5 × 2) = 19" cut length; 18 + (0.5 × 2) = 19" cut width. Step 2 — compute cut area using the formula: (18 + 1) × (18 + 1) = 19 × 19 = 361 sq in. So you would cut a 19" × 19" square of fabric. After stitching all four seams at 1/2", the finished pillow face will measure exactly 18" × 18".
Frequently asked questions
What is the standard seam allowance for garment sewing versus quilting?
Garment sewing patterns in the United States typically use a 5/8" (approximately 1.6 cm) seam allowance, which provides room to grade, clip, and press seams neatly. Quilting almost universally uses a 1/4" seam allowance to reduce bulk in the many seams of a quilt top. Home décor projects like cushions and curtains often use 1/2". Always check your pattern instructions, as seam allowances are not universal.
How do seam allowances affect the finished size of a sewn project?
Every seam you sew removes exactly one seam allowance width from each cut edge involved. Forgetting to add seam allowances when cutting means your finished piece will be smaller than intended — by twice the seam allowance per seamed dimension. For example, with a 5/8" allowance, a piece sewn on all four sides will finish 1.25" shorter and 1.25" narrower than cut. This is why calculating cut size before cutting fabric is critical.
Why do quilt patterns specify cutting dimensions instead of just finished block sizes?
Quilt patterns provide pre-calculated cut dimensions so you don't have to manually add seam allowances to every piece — a major time-saver when a quilt has hundreds of pieces. The cut dimensions already include the standard 1/4" quilting seam allowance on every edge. If you change the seam allowance from the pattern's assumption, you must recalculate all cutting dimensions to ensure the blocks finish at the correct size and the quilt top assembles accurately.