Okay, when I cut, I measure very carefully. Usually my pieces are a few threads too big. So for the sake of my upcoming problem/question, let's assume I cut the pieces right.

I sew with a 1/4" foot. I'm careful to line it up and not push the fabric too far. I pin (and sew over the pins (IBC silk pins)). I start and end with the frog or whatever the scrap piece of fabric you sew over is called)

And yet, almost every block I sew, every pattern, biased, unbiased, squares, rectangles, triangles, just about every single one comes out about 1/8" shorter then it's supposed to. And after I sew a seam, before I press it open, sometimes I'll measure and it is 1/4" so I know I'm not secretly doing 1/2" or something.

I did a 10" BOM where almost every block was 9 7/8". And I know one of the comments is going to be "don't worry, at least they're all the same size" and I agree, I just can't figure out why the blocks keep becoming smaller... it has to be something I'm constantly doing since they all shrink uniformly......... help!

the good news is that you have mastered the skill of consistent seam allowances.

the not so good news is that you are almost certainly stitching seams with an allowance of just-a-hair more than 1/4".

rotary rulers aren't necessarily the best tool for checking your seam allowance. because they are thick, they can distort the measure for a check like that. something about the way the light passes through the plastic ... yada yada yada.

find a thin, flat see-through ruler. when you line up the 1/4" line, your stitching should be lined up directly under the line on the ruler. if your stitching line is visible above the line - even if it's poker straight and right along the line, it's more than 1/4". like ... 7/32" ... or maybe 5/16". that teeny bit is just enough to throw off the finished size of your block.

move your needle 1 click to the right, but use the 1/4" foot as you normally do. don't change anything about that. i'll bet they come out the right size.

Sorry you're having so much trouble. Try this, it helped me. Take 3 pieces of fabric cut 6" long and 1 1/2" wide. Sew them together. I used one white piece in the middle and the outside pieces were dark, so I could really see them. Press as you normally would and then measure the center or white piece and see if it is 1" wide, should be. If not you may have to made your needle position until you get that 1" strip in the middle.
I had to do this several time until I got it just right, but it will work.
Good luck, Pam

I had this problem doing miniatures until someone told me about aurifil thread. It is finer but just as strong than the standard quilting cotton thread and has made a difference in that hair size of being off when blocks are made. Do you preshrink/pre wash your fabric? If not, you may be shrinking the fabric just enough when you press to make a difference. Since each fabric is different this could be an issue.

I used to have the same problem. I did two things to solve it:

I use a 50 weight thread, and if the quilt is a wallhanging, I use 60 weight. The little extra bulk from the thread adds up enough to make a difference.

MAke sure your seams are very flat. Sometimes you may think you pressed well, but when you look at the front of the block, you may notice that the seams and a little folded, but not totally flat. The measurements are given so exact that your pressing has to be perfect in order to achieve the right size.

In the long run, If all theblocks you are making are in the same quilt, it doesn't really matter what size they are, as they will match when you put them together.

Maria

This is the hardest thing to master in quilting. I work on using the SCANT quarter inch and it helps, but in a block with lots of seams, it can be troublesome! Pressing WELL is essential. Starch helps!

My blocks always come out to small until I reset my machine to even less that a scant 1/4 in. If I do a full 1/4 they are way to small even though I measure to make sure it is 1/4 in.
Now I set my machine for 1/4 in, then adjust my seam to 3.0. Now my blocks come out perfect. Each machine is different so you have to experiment. The strip test that Pam mentioned is a good way to measure.
I have used 1/4 inch feet but it still dosnt come out right.

My 1/4 foot isn't! I have to move my needle a click or so to make it work.

Your problem may also involve pressing. I have noticed that proper pressing can make a scant difference. I set the closed seam first and then press either to the darker side or I press open (depending on the pattern). If your individual pieces are cut correctly, and measure correctly, I would suggest to measure after each incremental sew and press.

What I did was cut a peice of fabric 1/4 inch wide. Then I put that on the guard of my 1/4 inch foot and lower my needle. The needle should just hit the edge of the fabric. if it is off the fabric then the needle needs moved until it is on the fabric. I keep this peice right by the machine so I can check the seam anytime I want. Maybe this will help.