x7lillies

I just made a sailboat quilt top with pinwheels (lots of HSTs). I decided to do the math and cut my squares the correct size, since my machine is pretty good about straight lines and has a helpful 1/4" seam line guide (I use a clear foot). I prefer not to mark the fabric, so I cut the squares in half first, then sew a 1/4" seam on the two triangles right sides together.

To make your HST squares come out to the right size as your whole squares, you need to add 1/(2*sqrt(2))" to the sides, or 0.354" (slightly less than 3/8"). You have to add less than the 2 x 1/4" (1/2") seam allowance because in the square dimensions your seam is actually applying on a diagonal, and the horizontal distance eaten up by the diagonal is less. So for me, I wanted 3" squares. That meant cutting 3.35" squares (I cut at 3 3/8"). Technically, they end up being 3.021" square, but that's within my sewing margin of error I think. I could have squared them up to 3" exactly, but the difference was so small it didn't really matter to me (it's only a crib-sized quilt), as I just squared up the terminating edges of the quilt once everything was pieced and that worked fine with my pattern. If you have a large quilt, or all your blocks are HST, then you're better off squaring them up to exactly the size you want or it'll be really noticeable once you start to match seams at opposite ends. For instance, joining two blocks together puts you off by 0.042", but 10 blocks puts you off 0.21" (nearly a quarter of an inch), 50 puts you off by 1.05", and 100 puts you off by 2.1"!