What am I doing wrong? :(
 

I'm working a on block of the month, where the finished block is supposed to be 15 1/2''. The first time I pieced three blocks, and they all came out to different sizes: 14'', 14.5'', 15''. So I investigated to see if I had a true 1/4 seam and I didn't, so I took out the blocks, and tried another block again, this time, trying to be careful with the 1/4'' seam... Well, my stupid block came out as 14.25-5''. I checked my 1/4'' seams and they were 1/4''.... I finally gave in and trimmed the block to 14'', because I couldn't get the right size.... does anybody have any ideas, why I keep getting different sizes?? Here is the block pattern:

https://i.ibb.co/ZdwQp90/IMG-5738.jpg

If its coming up that short I suspect cutting errors. Are you starching and pressing your fabric? How are you making your HST’s? When cutting fabric is the ruler on the fabric?

It looks like each unit should be 3.5 before sewing together, is each unit measuring up to the correct size before sewing the units together? Are you squaring up each unit before sewing them together?

I agree with tallchick, that the units need to be the correct size before they go into the block. If you sliver-trim them, ensuring that each unit is precisely the size it's supposed to be before putting them into the block, then the block will be accurate.

Are you using steam when pressing the blocks? It can cause shrinkage.

Check the individual square sizes. The outside ones should be 3 1/4 to account for seam allowance when sewn to the next block.

The inside squares should be measuring at 3 inches each finished.

Have you checked the pattern for corrections? There may have been corrections posted.

How are you making the HST? Is it 2 squares of fabric placed right sides together, sewn and cut? What size square are you starting with for the units?

I am using steam when pressing...
I'm making the HST with 2 squares of fabric, right sides together sewn and cut. Starting size is 4'' for the two triangle blocks and 4.5'' for the 4 triangle blocks.

I haven't been squaring up each unit before sewing them together, I didn't know I was supposed to do that.

The inside squares are not all measuring 3'', some are 3 1/8 or 3 1/4.... :(

And the outside squares are mostly running under 3'', like 2 7/8'' :(

I am starching and pressing the fabric, and the ruler is on the fabric when cutting.

Steam will often times shrink fabric it is why I never use it! Recheck your cutting and measure each piece of the unit before sewing together. Practice a few more times on scrap fabric until you get the desired results.

Also about the 1/4". It is good to sew 3 scraps together and measure the result instead of trying to measure the outside of the seam after sewing.

three scraps 3" wide. Sew together. Press as usual. Measure inside piece. It should be 2.5".