Seam Allowance Calculator
Zero Waste & No Sign-up: Calculate exact cutting sizes by adding seam allowance to finished dimensions. Standard 1/4-inch quilting seam allowance applied automatically.
How to Use the Seam Allowance Calculator
Enter the finished width and height of a patch along with your seam allowance, and the tool returns the exact cutting size. Quilting's standard quarter-inch seam is the default, and because an allowance is consumed on every sewn edge, the calculator adds twice the seam to each dimension — half an inch in total at the standard setting.
The math works for any straight-sided patch: a 3-inch finished square cuts at 3-1/2 inches, and a 4 x 6 inch finished rectangle cuts at 4-1/2 x 6-1/2 inches. This is also the fastest way to resize someone else's pattern — strip the allowance off the published cut sizes, scale the finished dimensions, then add the allowance back.
Note that diagonal seams consume more fabric than the flat half-inch rule predicts. Half square triangles need 7/8 inch added to the finished size and quarter square triangles need 1-1/4 inches, so use the dedicated HST and QST calculators for triangle units rather than this general-purpose tool.
Frequently Asked Questions
What is the standard seam allowance in quilting?
The standard quilting seam allowance is 1/4 inch (6mm). This is added to all sides of each piece, so a finished 3-inch square requires a 3.5-inch cut square.
Why is an accurate seam allowance important?
Consistent seam allowances ensure blocks fit together correctly. Even a 1/16-inch variation per seam adds up across a large quilt top, throwing off the final dimensions.
How do I sew a scant 1/4 inch seam?
A scant 1/4 inch is just a thread's width less than a true 1/4 inch. It accounts for the fold of the seam when pressed, resulting in blocks that measure exactly right after pressing.