triangles
 

Hi - I have a 61/2" block - finished it will be 6" - what size square do I cut - so that when it's cut diagonally it will make a diamond shape around the 6" square - I know there is a formula for this - but I have forgotten it and did not right it down.

here is an online calculator for any size square in a square block. it works great.
http://www.quiltersparadiseesc.com/C...Calculator.php

the calculator gives your square for triangles as 5 1/8 . I would cut it at 5 1/4 for easier measurement and then that leaves wiggle room to square up your final block

Now that is really useful! Thank you

If you mean a sq-in-a-square block, I would probably cut one 7 1/5" square, (1" larger) and recut twice on the diagonal, then sew one to each side.

Gay's solution leaves you with bias edges all around. Two 5 1/4" squares, each cut one diagonal, makes outer edges on straight grain.