Seam Allowance Calculator
Convert between cut size and finished size — or finished size to cut size — for any seam allowance.
or enter custom:
Cut Size → Finished Size
Finished Size → Cut Size
Results are estimates. Always add extra for shrinkage, mistakes, and pattern matching.
Seam Allowance Quick Reference
| Seam Allowance | Total Removed (both sides) | Common Use |
|---|---|---|
| ¼" (0.25") | ½" | Quilting — standard for quilt blocks |
| ⅜" (0.375") | ¾" | Some commercial patterns, lightweight fabrics |
| ½" (0.5") | 1" | Home dec, some garment patterns |
| ⅝" (0.625") | 1¼" | Garment sewing — commercial pattern standard |
| 1" (1.0") | 2" | Heavy fabrics, outerwear, ease for fitting |
Common Decimal / Fraction Equivalents
| Fraction | Decimal | Fraction | Decimal |
|---|---|---|---|
| ⅛" | 0.125" | ⅝" | 0.625" |
| ¼" | 0.25" | ¾" | 0.75" |
| ⅜" | 0.375" | ⅞" | 0.875" |
| ½" | 0.5" | 1" | 1.0" |
How to Use the Seam Allowance Calculator
- First, select a preset seam allowance or type a custom value in inches (decimal, e.g. 0.625 for ⅝").
- Cut Size → Finished Size: Enter the measurement you cut your fabric. The calculator shows what the finished size will be after stitching seams on both sides.
- Finished Size → Cut Size: Enter the finished size you want. The calculator shows how large to cut the fabric piece.
- Results update live as you type.
How the Math Works
- Finished = Cut − (Seam Allowance × 2)
- Cut = Finished + (Seam Allowance × 2)
- The seam allowance is removed from both sides of a piece (left/right or top/bottom), so the total reduction is double the seam allowance.
Tips
- Quilting patterns almost always use ¼" seam allowances. Check your pattern instructions.
- Commercial garment patterns sold in stores use ⅝" seam allowances unless otherwise stated.
- When in doubt, make a test seam on scrap fabric to verify your machine stitches the correct seam width.