✂️ Seam Allowance Calculator
Adjust pattern pieces for any seam allowance
About this calculator
The Seam Allowance calculator adjusts pattern pieces when you change seam widths. Enter your pattern's original allowance and your preferred one; it shows how much to add or trim at each seam line and how the finished garment measurements shift. A common use is converting a 5/8-inch commercial pattern to a 1/4-inch allowance for quilting cotton.
Tip: mark the new seam line directly on the pattern tissue before cutting — trimming by eye at the machine produces wavy seams that never press flat.