I can never remember how to do the math... When you want to make a block out of two half-square triangles (and who doesn't!), how big do you cut out the original squares before you sew them and cut them into triangles? If you want an 8" finished block, do you add 5/8" to the original blocks? 7/8"?
I googled it just now, and came across a great website that had the answer, so I am posting it here so I will never forget. I suggest that you visit Patchwork-and-Quilting.com to find the answer to this and many other questions about quilting.
By the way, the answer to the question about half-square triangles is 1". Add 1" to the original squares and then cut them down slightly to get the exact size you need. You can read about it here.