Sometimes I use a quilt pattern that outlines exactly how many yards of each kind of fabric are needed, how many pieces of what size to cut, and how to put them all together. Other times, like when I'm starting with a specific block that I want to use like Kansas Troubles, and with a limited amount of fabric, I have to figure that out myself.
So for this block:
The whole block is probably going to be 8, 9, 10, or 12 inches on a side. Divide the block into quarters, and then put a 4x4 grid over each quarter. With all of those fours, a block that's 9 or 10 inches on a side will be difficult to work with; so we'll try the 8 or 12 inches.
For a 12" block, there are four sub-blocks that are 6" on a side; that 4x4 grid has little squares that are 1.5" wide. For an 8" block, those little squares would be one inch on a side. An inch is pretty small, and those aren't just little squares, they're little triangles being sewn together to make squares. The standard seam is a quarter of an inch wide, so I'm thinking these would be too small to work with. So, the block will be 12" square.
For each of the quarters of the block, which are now 6" square, there's one big triangle in the background fabric that takes up half of the square. The rule of thumb when cutting up fabric is to add 7/8" to the square so that when you cut it in half and stitch it to another triangle using that 1/4" seam, you'll get a square of the desired size. So, this means for every 12" block, I need 2 squares that are 6 7/8" on a side. For the medium-sized triangles, I need 2 squares that are 3 7/8" on a side. And for the little triangles, I need 12 squares of the dark fabric and 8 squares of the background fabric that are 2 3/8" on a side, plus four squares of background fabric that are 1 1/2" wide.
I've written this post out twice so far, once where I forgot I needed little triangles from the background fabric, too, and once where I forgot I needed the 1 1/2" squares from the background fabric. So this actually does serve a purpose besides boring you with a lot of math. :) In fact, once I realized I only had enough background fabric to make, like, three blocks once I took into account all of the little pieces I need, I had to go buy some more. This seemed appropriate for an SPN quilt. ;)
Now, I have enough for 16 blocks, 4x4, which would make the quilt 48" on a side. I also want a 6" border, which would make it 60" on a side, which is definitely big enough for a lap quilt. (I have a reason for making it square which you'll see at the end...)
So. From each of my two background fabrics--the gas pumps with cars and the ropes--I'll be cutting 16 squares that are 6 7/8", 64 squares that are 2 3/8", both of which then need to be cut into triangles, and 32 squares that are 1 1/2". From the four plaids for the medium triangles, I'll need 8 squares each that are 3 7/8". And from the miscellaneous dark prints with the coffee, skulls, etc., I need a total of 192 squares that are 2 3/8", which will be cut into triangles.
Fortunately, cutting fabric is something I can do while watching TV! More updates as events warrant. :)