Math question: Does a 4.25" square cut diagonally yield a 3" pinwheel?
 

My pattern says cut (68) 3" squares, then cut diagonally for pinwheel parts (times 2 fabrics)
I will end up with a total of 17 pinwheel blocks which are ultimately 4" square.

If I were to use Jennie Doan's easy pinwheel method of sewing all the way around two pieces, then cutting diagonal both ways, do I need to start with 4.25" squares?

I've drawn this out on a piece of paper, and measured, and I get the whole pythagorean theorem, but just need to confirm. And the second part of the question is: if it is the 4.25, do I need to be concerned about the decimals? (square root of 18 is 4.24264068712).

Should I bag it and just cut the flippin triangles?

I'd find out what size I need for the finished block (hst) and use either Thangles or Triangulations to make them in bulk. I enjoy math, but in the right time and place....

Final pinwheel blocks are to be 4" x 4"

Try it with scrap fabric. Then you'll know for sure.

I had bookmarked this site before and found it very helpful. I rounded my squares up and then trimmed to size.

edited to add the link. oops.

http://www.moonlightquilters.org/tipHSTfav.htm

Pinwheels are made from 4 - HSTs. For a finished 4" pinwheel then each corner of the HST should finish at 2".

When doing HSTs, I use method 2 in the following tutorial

http://www.thatgirlthatquilt.com/201...es-3-ways.html

The rule of thumb for assembling HSTs this way is "finished size plus 7/8". So a 3" square will be adequate using method 2 - giving you a little wiggle room for trimming the final pinwheel into a perfect square.


My only concern about Jenny Donan's method (method 1 in the first link) is that the resulting edges are not on the straight of the goods and can stretch, If you do want to do that method, the following site even has a chart for measurements

http://whipup.net/2011/03/17/guest-b...are-triangles/

I have to respectfully disagree entirely with the math in that whipup link above. The relationship is NOT proportional as the blogger states.

The math would be the same as for QST's. You're just sewing the first seam differently to get to the same result. For any finished size HST, add 1.25" and you have the size of the squares to start with. Your finished pinwheel will be twice the size of the finished HSTs, so for 4" finished pinwheels you need 2" finished HST's, which requires two 3.25" squares.

Awesome!! Thank you.

Thanks for this, I just printed it to hang on my bulletin board.

When I need several HSTs I use this method ( makes 8) except that I add 2" instead of 1 3/4 and trim down: