Originally Posted by qbquilts
Originally Posted by lee_stitches
Pythagorean Theorem? A squared + B squared = C squared is the formula for a right triangle. 8 squared + 8 squared = C squared. 64 + 64 = C squared. 128 = C squared. The square root of 128 = 11.3
You need to add in the seam allowance before doing the math. So the 8" finished block becomes 8.5". Plugged into the formula, you get 12.02. Just to demonstrate that you can use the formula OR the "Multiplying by 1.414" trick, multiplying gives you 12.019. Round UP to the nearest 1/8 for ease in cutting, so sashing strips would need to be cut at 12 1/8.
Sashing would be the length of the side of the block, just like straight set quilts. You sash the sides, not the diagonals, using the unfinished length.
Diagonals are needed for setting and corner triangles and to compute that measurement, you use the finished block size plus the finished sashing width....not adding the seam allowances. Here's a chart for you to use.
http://quiltville.com/onpointmath.shtml
How big do you want your quilt to be?