Need some mathmatical quilting help
 

I am doing a disappearing 4 patch. I want the final square to be 12 1/2 inches square. What size do the four patches have to be? I am cutting the four patches into 9 squares before I sew them back together.

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Here is a picture of the final block I am trying to make. I started with 61/2 inch four patches, but the final block is 1 1/2 inches too small all the way around.

Any help is appreciated :)

If you just added a quarter inch all the way around each of the 9 blocks (1/2 to each block) wouldn't that give you the size you need? I am a little mathematically challenged, but that seems logical to me.

I think the block you show is a disappearing hourglass, which is made up of HST's instead of 4-patches. MSQC has a tutorial video on it here: https://www.youtube.com/watch?time_c...&v=xXggafOP230

Since you want a specific finished size, I would use a different method to make the HST's than what Jenny demonstrates. Her method gives bias edges all around the block; I would use the 8-at-a-time method instead, so that I don't have to deal with that bias. If you start with two 15" squares and use this method: http://blocloc.com/wp-content/upload...rt-for-HST.pdf, you will end up with eight 7" HST's.

Then follow the MSQC video from 4:05 onward. Join 4 HST's into an hourglass block, cut it into 9 squares, and rearrange as you did before. This should give you a 12.5" finished block.

It's late, so I might have made some huge mistake somewhere, but I think this will work. Good luck!

That is a disappearing hour glass. I put a chart together to make them several sizes including 12.5" unfinished:
https://www.quiltingboard.com/tutori...t-t251803.html