OK, I am making squares from HST's. I am having a hard time with my seams for a 3-7/8 square when cut from one corner to the other to line up. When I sew the two halves together to make a square, my square is too small.

I did measure the squares that I used to make HST's to be 3-7/8.
Now when I sew them together, they should measure 3-7/8?

You add 7/8" to the finished size and that is the size you cut your squares. Your HST should measure 3" finished.

You need to allow for the seam allowance. It seems to me, to get a 3 7/8 square made from HSTs, you'd need to cut your first square 4 3/8. I have some made from charm packs. I'll go measure.

Edit: I know I have read the 7/8, too, but my blocks made with 5" charms measure 4 1/2". ???

When I make HSTs, I just cut two squares about an inch or so bigger than I want my finished square, then I draw a line from corner to corner, sew a quarter inch from each side of the line, cut apart and I have 2 HST blocks! I then just trim it down to the right size. Takes a little longer with the extra trimming, but I don't have to think about the math of it. LOL

Rachel

if you want a 4 inch finished hst, sew 4 7/8 inch sqs diagonally to make two.
matching your seams of hst's is easier if you start with larger sqs and then trim. so sew 5 inch sqs diagonally and then cut and trim. but trim by laying your ruler on the hst's with the 45 degree line down the seam. that is a must. then trim, turn and trim again if necessary.
that way your seam lines will match up perfectly when sewing hst's together.

Oops double post

If you are taking 3 7/8 squares and drawing a line diagonally down the middle and sewing a 1/4" on either side of the drawn line then cutting along the drawn line to get two HST units, you should end up with 3" unfinished HST units and once in the quilt they would measure finished at 2 1/2"

If you are taking a 3 7/8 square and just sewing one seam diagonally down the middle I'm not sure what you get besides a lot of wasted fabric.

This is the method I use too. If I just add 7/8 to the finished size I sometimes come up just a tad short. So much more accurate when I cut down.

I have found this to be the easiest also. I starch and press the squares, and they HSTs are nice and even.

Cutting and sewing HST always were iffy for me. I never got them all the same. I have the Wondercut ruler, the June Taylor HST ruler, a speciality ruler that only measures 7/8" and the Tiger Tape, the Straight Stitch tape, and using the 1/2" divided ruler and using Thangles. Thangles are the fastest most accurate way to make a lot of HSTs. I now only make HSTs that are the size of my Go dies for them.