Making the disappearing 9 patch...
 

I can't find anywhere what size to cut the squares to end up with a 10" or 12".. Most of what I read says "cut any size you want."
Any help?

Let me see how my math is this morning:
5 inch squares will leave 4.5" after seam allowance (4.5 x 3 = 13.5" finished square) I figured 3 across and 3 down. Now cut that again into 4 and sew back together - these squares will be 6 inch (not counting seam allowance - you will end up with a 12 inch square.


Hopefully I did my math correctly. I love using larger beginning squares for this.

I have this chart which comes in handy:

Thanks for the chart!!!!

(assuming 1/4 inch seams and pieces are cut accurately)

A basic formula

Finished size plus 2 inches
Divide that number by 3

Example:

In the attached example, I would end up with a 4 inch finished unit.
I cut each of my squares at 2 inches (2 x 3 = 6)
I "lost" 2 inches to seam allowances


I would cut the squares 4 inches to get a 10 inch finished unit
I would cut squares 4-2/3 inches to get a 12 inch finished unit

Great reasoning and charts everyone ... most helpful. I also thank you!

You will end up with a 13 inch "finished" square if you start with 5 inch squares
What happens:
The original corner squares will still "finish" at 4.5
The center side cut squares lose 0.5 inch and now "finishes" at 4.0
4.5 + 4.0 + 4.5 = 13.0

Part 2 of my original post:

What one would "see" after cutting the 9 patch apart and sewing it back together.

Thanks, everyone! I knew I would get an answer quickly… Yeah….. The chart will come become very handy.
Thanks again…….

GREAT post Onebyone with posting that chart. That is going to be one of the best references I will use.