Measurements for a 9" hourglass block

I am trying to make 9" hourglass blocks and I am trying to figure out how big I need to cut my fabric squares in order to make them come out to 9" after they are put together. Actually I guess they would need to be 9 1/2" with the seam allowance. They need to be 9" squares once they are sewn together. I have 9" 9 Patch blocks that I'm trying to match the hourglass blocks up to so that I can sew them all together into a quilt. I am horrible at figuring out how to get the right measurements when squares are cut diagonally to make triangles. Thank you.