Seam allowance math
If both sides of a seam get allowance, add SA twice (seams=2). US apparel often uses 5/8" (0.625"); quilting uses 1/4"; some indie patterns use 3/8" or 1 cm.
Grading
graded = base + (toIndex − fromIndex) × gradePerSize. Index sizes consecutively (0=XS …). This is a linear helper, not a full multi-point grade rule set.
Worked example
Finished 20" width, 5/8" SA on both side seams → cut 21.25". Grading a 36" bust from size 0 to 2 at 1.5" per size → 39".
Tips
Check whether your pattern’s stated measurement is finished garment or body. Quilting: keep a consistent scant 1/4".
Common mistakes
Adding SA only once to a centered seam that needs both edges; confusing ease with SA.
Pattern drafting context
This helper supports simple arithmetic for makers adjusting PDF patterns or drafting rectangles (bags, facings, quilt-to-garment hacks). Full apparel grading uses multiple grade points (bust, waist, hip, length) with different increments—apply this linear grade only where a single measurement steps evenly between sizes.
Limits and how to use the results
Estimates run only in your browser. Prefer a published pattern’s tested yardage when it conflicts with this planner. Density factors and assumptions are documented on the methodology page; legal terms cover planning-use limits.
FAQ
What is a standard sewing seam allowance?
Many US garment patterns use 5/8 inch; always check your pattern sheet.
What seam allowance do quilters use?
A scant 1/4 inch is standard for piecing.