i want to finish with --6 1/2 x 6 1/2 block---what size do i need to start with using the method 2 squares RST sewing all arround the sides then cutting across both diagonals

http://www.quiltingboard.com/t-57124-1.htm#1377835

see if this helps.

Add 7/8" but most just add a full inch and then square them up :D:D:D

That's what I would do too. Start an inch larger and square them up to your finished size.
Janie

we have such nice helpers here

i have 6 1/2 inch pinwheels thank you all

I'm using 5" squares with the sew around method to cut down to 5 1/2" pinwheels.Try cutting 6" sqs & see how that works.

The formula for the sew around the edges according to Ceil is:
size of block you start with minus .5, times 1.4, minus .5 for the unfinished size.

So if you start with 6" blocks it would be:
6 - .5 = 5.5
5.5 x 1.4 = 7.7
7.7 - .5 = 7.2 trim to 7"
Take off seam allowance and it finishes to 6.5".
HTH