I want to end up with a 4 1/2" half square block, what size do I cut the two blocks to start with?

Finished size after it's pieced or cut?

If you want your block to be 4 1/2" when they are all put together then your blocks should be cut 5" to start with

I'm not sure what you are asking but I think I need cut size...what I'm saying is I want to end up with a 4 1/2 half square before i sew it on the row

According to this website, you should add 7/8" to the finished size. Finished size is after it's pieced, so I think yours would be 4 7/8".

http://www.trishstuart.com/pdf_files/math_hst_qst.pdf

This second website illustrates more, but it doesn't seem to be as straight forward.

http://www.trishstuart.com/pdf_files/HSTmath.pdf

I hope this helps!

I hate the 7/8 inch thing when making HST. I round up to nearest inch. After I sew the HST and press, I trim down to the exact size I want. I find that there is often distortion on the bias and this helps deal with it.