QUILTER'S MATH:Convert Blocks to the Size You Want

[Barbara B.]

I was asked for a "foolproof formula" to convert 6" blocks to 9". Here is my response more of you may find helpful. The blocks mentioned are from the book The New Quick and Easy Block Tool!

There is no simple formula to convert one block size to another. It's all geometry and math. The layout of the block is the first thing you need to know. Block 74 Ribbon Star is a 4-grid. Divide 9 by 4 and you see each unit is 2.25" finished. The A corner squares would be cut 2.75". The BC HST would be made from squares cut 3 1/8"--I would make that 3.25" and trim the HST to 2.75" when sewn. That block is easy to convert.

Look at Block 45 Home Grown. It is an 8 grid. 9/8=1.125". The A corner squares would be cut 2.75"--since the square = 2 units: 1.125 * 2= 2.25" + .5 seam allowance. The B triangles would be cut from squares 3.125": 1.125" * 2= 2.25" + .875" seam allowance. The C Quarter Square Triangles start with 3.5" squares, then cut on both diagonals. 2.25" + 1.25" seam allowance. The center F square is more challenging. It is 4.5" wide across the middle when finished. That means it has to be cut from a square that is 3.682"--bigger than 3 5/8, smaller than 3 3/4. No easy way to get that cut accurately. You would have to draw a square 4.5", add the 4 seam allowances and use that to create a template. Alex showed how to do this the other day.

Any block you can draw 9" can then be used to create templates so you don't have to "do the math". Alex says math makes her head hurt so she doesn't do it, she carefully draws the block on graph paper.

One of these days I will create a Tutorial on my blog called "Quilters Math"--with all the basic numbers every quilter should know.

For now, I hope this helps you.

[barbara]

Thank you. Definitely not the answer I was looking for. It is that final squaring up size of each unit I have the most problem with. I can over cut but never sure of what size I need to trim to. I understand what you explained and I am waiting for that page of SIMPLE math formulas your working on. (LOL) What a job ahead of you. Again thanks for respoding. Barbara Conklin

[Helen W.]

Thank you. Definitely not the answer I was looking for. It is that final squaring up size of each unit I have the most problem with. I can over cut but never sure of what size I need to trim to. I understand what you explained and I am waiting for that page of SIMPLE math formulas your working on. (LOL) What a job ahead of you. Again thanks for respoding. Barbara Conklin

Bardorcon, (Barbara Conklin), I don't have your issue in converting sizes since I use EQ8 software that does the math for me, but I did type in Resizing Quilt Blocks and there is
a lot of info out there. These are just couple screenshot to show you what I am talking about. If you are worrying about squaring up an oversized block, you just add 1/2 inch
to the finished size. That puts a quarter inch all around the block for the seam. Example 6 inch finished in the quilt would be squared up to 6 1/2 inches if you oversized it.
Not sure that was what you were asking. HelenW

[Kathryn S.]

Barbara C, if I am understanding what you are asking, you are wanting to know what size to trim your oversized block units to. Most block components are squares, for example in 4 patch or 9 patch blocks. In a 6 inch finished block, before it is sewn into the quilt, its size is 6-1/2 inches. If this 6 inch finished block is a 9 patch, then each of its components finishes at a 2 inch square. So, in order to get 2 inches finished size in the block, each of the squares making up the 9 patch would need to be trimmed to 2-1/2 inches square. Find your finished size in the block and add 1/2 inch to it to get trimmed size.

This is why a 6 inch finished block is versatile. You can divide 6 by 2 (as in a 4 patch), and get 3 inch finished components, which are 3-1/2 inches before you sew them into the block. Or divide by 3 (9 patch block) for a 2 inch component, or divide by 4 and get 1.5 inches.

Trying to make a 9 patch block fit a finished 8 inch size just leads to lots of crazy numbers- each component would be 8 divided by 3, or 2-2/3 inches finished. Not easy to do with our quilting rulers
Happy quilting, Kathy

Thank you. Definitely not the answer I was looking for. It is that final squaring up size of each unit I have the most problem with. I can over cut but never sure of what size I need to trim to. I understand what you explained and I am waiting for that page of SIMPLE math formulas your working on. (LOL) What a job ahead of you. Again thanks for respoding. Barbara Conklin