Use this simple math to figure out how to cut setting triangles and corner squares for diagonal set quilts. They are cut differently in order to get straight of grain on the outside edge of the quilt. I saw this recently on YouTube from The National Quilters Circle, and retrieved the graphics off the internet from Spruce Crafts.
These are cut from squares, on the diagonal. Simple math lets you determine the size of the square.
Use your finished block size and divide this by 1.414. Add 7/8” and round up to nearest eighth. Or, you could round up to the nearest ¼, and trim after sewing.
Example: 12” finished block
Size of square = 12” / 1.414 = 8.49”
8.49” + .875” = 9.365”
9.365” rounded up is 9.375” or 9-3/8”
Option to round up to 9-1/2” and trim after sewing
These are cut from squares and cut twice on the diagonal. Use simple math to determine the size of the square.
Your finished block size is multiplied by 1.414. Add 1-1/4” and round up to the nearest eighth. Or, you could round up the nearest ¼” and trim after sewing.
12” finished block
Size of square = 12” x 1.414 = 16”
16” + 1.25” = 17.25 or 17-1/4”
Option to round up to 17-1/2” and trim after sewing.
I hope this helps the next time you make a diagonal set quilt.