Ok, I am going out on a limb and making a tshirt quilt for my son. I have sketched it out with 12.5" tshirt squares and then using 3" square in a square blocks as sashing. I just need to know (have verified) how big to cut my half-square pieces to get 3.5" unfinished diamond blocks. I think 2.5" squares sewn together and then cut in half and paired up with another set of two will give me the right size, but need someone to verify it for me. This is what I want the finished piece to look like, but sized to 3.5" square.

Are you wanting a single component to equal 3.5 inches? The way I make my 1/2 square triangles is I go up 1/2 inch bigger than what I want my finished square to be then trim.

3 1/2 inch unfinished 1/2 square triangle--start with 2 pieces of 4 inch square fabric, right sides together, mark corner to corner, stitch 1/4 inch on each side of mark, cut on mark, press open, trim to correct size.

4 1/2 unfinished--2 pieces of 5 inch square fabric
5 1/2 unfinished--2 pieces of 6 inch square fabric

If you want what the picture is to be 3 1/2 inches each component has to be 2 inches unfinished each.

When I say unfinished I mean before it is sewn together with any other parts.

Does this help?

You want the HST to measure 2" unfinished.

When you sew 4 of them together , you will get a 3.5" unfinished Square-in-a-Square, just as pictured in your op.

That said, would you consider starting with a 2.25"-ish square and attaching oversize triangles on the four sides (cut two squares in half on the diagonal)?

You get the same exact image. No matching up of design points with all those the seams in the center, and then you can trim it down to a perfect 3.5" block to be used as cornerstones. It's a cleaner look.

Just a thought.

This is what I was going to suggest. Less sewing too!

Yes Sharon, that helps. I just kept trying to get it right by making samples!