this information comes from a book called "Complete Guide to Quilting" from Better Homes and Gardens.
to calculate the size to cut a square for SIDE triangles:
multiply the finished block size by 1.414 and add 1 1/4" seam allowance. (for example 10" block X 1.414 = 14.14 + 1.25 = 15.39 rounded up to the measurement would be 15 1/2" )
then cut the 15 1/2" square corner to corner twice to make 4 side triangles.
to calculate the size to cut a square for CORNER triangles:
divide the finished block size by 1.414 and ad .875" for seam allowance. (for example 10" block divided by 1.414 = 7.07 + .875 =7.945 rounded up the measurement would be 8" )
cut the 8" square corner to corner to make 2 corner triangles. You will need 2 squares to equal 4 corner triangles.
I hope this helps. Grab a calculator and go for it. I have cut samples of paper before cutting my fabric just to make sure.....cuz my math skills ....ummmm....suck.