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.
About this Seam Allowance Calculator
Use this free Seam Allowance Calculator to get quick, accurate results directly in your browser. It is designed for everyday use with no signup, no downloads, and no account required. No signup is required, and the tool works on phones, tablets, and desktop browsers.