Help with a square in a square block
 

I'm making a quilt that uses a square in a square for the border and I like it a lot. But the instructions for it are driving me crazy. I'm posting two pictures here so you can see one block that was finished per the instructions for the quilt. The second square in a square in the picture is one I tried to make via an easier way. As you can see, it isn't large enough. The center is exactly the right size. I started with a 3 inch block for the center and it finished at 2 1/2 as expected. But the finished block needs to finish at 5 3/4 inches and I don't know how to get my 3 inch center to finish at 5 3/4 with the white added around the edge. Does anyone here know the math for this?

This is where graph paper will serve you well.

Draw it out ... and then you have a PP pattern, waiting for you to copy.
.......with Perfect SIASIAS blocks to follow!

Look at your original block. Do you see the triangle that is formed by 3 white pieces? That is a much larger triangle than the one on your new block. Remember that a triangle has 2 equal sides and 1 longer side (the hypotenuse). You probably want the 2 equal sides to be straight-of-grain and the hypotenuse to be the bias.

Probably the easiest thing to do is to draw the block on graph paper. Draw the center square first, then extend lines until you can measure outside dimensions the size you need.

There are two different things you can do. The first, which is the easiest, is to cut 4 triangles per block of the needed size. Sew on two, then sew on the other two and cut off the excess white fabric. For this method, you would want to cut the triangles so that the 2 shorter sides of the triangle are about an inch bigger than your graph paper measurement. This allows for the seam allowance and ensures that your triangles will be big enough so that after you press (and starch, if you like), you will have plenty of leeway to square up your block to the exact size you need.

The other method is to cut two different types of pieces. If you look at your graph paper (or just look at your original block), you will see that once you outline the 2 large triangles on opposite sides, the remaining 2 pieces are a different shape. You can use your graph paper measurements to create a template for that shape, being careful to add 1/4" to each side of the template for the seam allowance. You could do the same for the large triangles, but it is easier to just cut those triangles with the 2 sides an inch longer than the graph paper, and trim down after the block is finished.

Edit: QuiltE described a 3rd method you can use. I always forget about paper piecing because I don't do it myself.

Prism99 ... The drawing I suggested, would accomplish the same as you mentioned. And would give QuilterDale the measurements she needs for her pieces to make the block via traditional piecing ... or as a PP block.

..... sometimes I do and sometimes I don't, but the joy of PPing is that I can pretty much do it when my brain is dead and end up with perfect blocks with points, corners, squares bang-on!

Bonnie Hunter's website Quiltville has a square in square tutorial as well as a cutting chart for various sizes. It can be found under "tips and techniques".

I think if you cut a 5 1/2" square twice diagonally and sew to the sides of your 3" block you should come close.
Try it with scraps first.

You could always put a one-inch white border around the one that is easier to make. That would still be fewer seams than the instructions in the original pattern, and it would give you a bit extra that you could trim down to make the perfect 5.75" square.

Is this what you are trying to do? (If this is not what you were trying to do - I did not understand your question - sorry)

The attached are for illustrating - the size is not what you want -

5.75 inches finished is an "unusual" size - is that what the pattern calls for? or is that what you ended up with? From the picture that you have on the left - it appears to me that your "finished" 5.75 inches is including the seam allowances?

After establishing the size you want, I would draft the pattern and make templates.

This is a fairly good tutorial on how to make them -
https://www.quiltingboard.com/tutorials-f10/how-make-plastic-templates-t57961.html

Or you could just make a pattern and paper piece it.

When you sew your smaller blocks together, you will not have a square in a square because you will lose all the corners with your seam allowance. Your white pieces need to be quite a bit bigger. Absolutely the easiest way is to draw on graph paper the finished block at the size you want. Number the pieces and cut them out. Redraw them with seam allowances added. Cut these out, sew them together and I'm sure you will be happy with your results.

I think you may be overthinking this block. It is a 9 patch block. The center blocks on the outside perimeter consists of a HST and a QST. The outside corners and center are simple squares.

Make each part as a separate unit making sure they match the size you need. I do not believe you have a 5.75 finished block there. It's not a size that is normally made. I think it should be a 6 inch finished, 6.5 unfinished block. This makes all your parts of the 9 patch, have to be 2.5" before they are sewn together. Cut your corner and center blocks, then overcut your 4 center side blocks and piece them as the QST first, then add the HST to them, then square them to 2.5 inches. Then put it together in rows. Can you give us a link to your pattern? and/or the name?

Measure your rows. Each of the 3 rows should be 2.5 inches by 6.5 inches before sewing together.