Can you help me with my setting triangles??
 

I am planning a quilt that will be my first on-point setting. My quilt blocks will be 3 inches finished. I found a few sites that give formulas or charts for the size of triangles to cut.

For the corners of the quilt I found that I need to cut two 3 inch square in half diagonally. For each of the rows, I need to use 1/4 of a triangle from a 5 1/2 inch square. Do these measurements sound right??

Finally, my biggest question is if you recommend cutting the setting triangles the exact size (so the measurements I listed above) or do you like over-sizing them? If you use over-sized, how difficult is it to cut down to size afterwards (do you do that before quilting or after quilting??)?

Thanks for any help you can provide.

Why don't you cut a test square? Get a scrap, cut it to 5 1/2", then cut it diagonally into quarters. BASTE it (with a large stitch, so it's easy to un-sew) on the end of a row and see how it fits. If you think it's rather tight or small, you can go up half an inch or so.

I don't see a problem if you over-size them, as long as it doesn't throw off the assembly of your rows. And I would trim them after assembly, before quilting. A quilt that's been squared is much easier for a longarmer to load on a frame.

I think you should cut the pieces to be the proper size, because it will make it easier to assemble the quilt. But first measure your actual finished block size, not the theoretical block size, and use that figure to calculate the size needed for the setting triangles. If your finished block size is indeed 3", then the figures you've found are correct.

Hopefully someone will back me up, but, I think the square for the four corners should be at least 3 7/8". While the finished size is 3", the triangle is actually being sewn onto a 3 1/2" square. Also, though I am not positive, I think the setting square that will be cut in 4 should be more like 6". 5 1/2 might just make it, but you lose some with the triangle points when sewing them on. Mudd?

Their are special rulers that do the math for you. I think Fons + Porter have one and right now I can't think of the other manufacturer.

I cut my setting triangles over-sized and cut them down after adding them to the ends of a diagonal row. I do the same with the corner triangles. For me, there's nothing worse than being 1/4th inch off on cuts for setting triangles!

You multiply the size of your blocks by 1.414 for the side setting triangles, cut your squares that size then cut them diagonally twice (X). For the corners you divide by 1.414 cut your squares and cut them in half diagonally.

Your measurements look fine, if in fact all your 3" blocks are actually 3" finished

BUT .. I agree with Prism99​.

I too always cut oversize . When assembling the setting triangles to the block, I match the center of each so I have extra on each edge. Then each row will get squared up and straightened. Then after I assemble the whole quilt, I will square up the sides. I myself would rather have to trim off, than not have a straight edge.

I always cut my setting triangles bigger because sometime it is good to have a little breathing room between the quilt block and the border. Your idea won't work well because the edge of the quilt will be all bias. It will work for the corner triangles You need a much bigger square that you can cut diagonally into quarters.

Here is a popular website that might help you:
http://quiltville.com/onpointmath.shtml
I bought a ruler years ago and it was one of the best investments I made.

Thanks for this link to quiltville, ManiacQuilter2.