I know that there is a formula for cutting squares to make half square triangles. It seems that you make it either 5/8" larger or 3/8...????... I can't recall. If anyone knows the formula, please let me know. If I'm not being clear, it's the sq. you cut and draw a diagional line through the center, corner to corner and sew 1/4" seam on either side to give you 2 half square triangles. Can anyone help me????

If you want a 4 1/2 square, start with two 4 7/8 squares, right side together, draw the line in the middle and sew 1/4 inch away from both sides of the drawn line. Open and you have two 4 1/2 squares.

I learned with 5 in. squares. Then after you mark, stitch and cut apart, you square them to 4 1/2 inches.

I do it like Inordyke said. Better to be a little to big and square up rather then worry about an exact 1/4 in. seam.

I use the 1" larger than the finished HST as my guide and trim. So if my finished HST is 4" I start with a 5" square. I draw the sewing lines 1/4" from the center and sew on them, cut down the center and press both sides open. Using this method you do have to trim each HST on all 4 sides but it makes for a nicer square. It also eliminates the need for the perfect scant 1/4" seam.

There is also a wonderful HST Tutorial by Rhonda and she uses a totally different method. It may work for you.
http://www.quiltingboard.com/t-37915-1.htm

I'm with Mad. 1 inch larger squares than the size of HST I want.

I totally cheat n use paper piece, like triangulations

I go bigger, then trim. If you want a 4.5" square and then do a 4 7/8 with 1/4" seams, right off the bat aren't you pulling it in MORE than 1/2 inch? Because of the angle?

Anyhoo, I'm just not that accurate. I'd rather sew, sew, sew and then trim, trim, trim. I've tried to do it the other way and ended up sew, sew, sewing some more...

exactly, skip the 5/8 or 7/8th..to hard to find on the ruler and the mat..just go 1 inch bigger..

Thanks everyone for the help...Now gotta' get 'er done...

Adding 7/8" to the FINISHED size of the block is the way I was taught.

I draw boxes on the back side of the lighter fabric. If my finished size is going to be 4", then I draw boxes that are 4 7/8". Then I draw a diagonal line through all the boxes and sew a quarter inch on either side of it, then you cut on all the lines. I have pictures at home if anyone wants to see them.

Yes! I would love to see your pictures....

pictures are always great!
they reason i use the 1 inch greater, is i did a whole quilt in the HST's and i was cutting strips first, and the 7/8th was to hard for me..to do the continual cutting of the strips..it was either,find the next measurement back from the last cut, or move the strip back to the one inch measure on the mat,and measure to the 7/8th again...i found both to be tiresome. i don't like moving my fabric once i start cutting.

the way you explained is the way i learned also..

If your entire quilt is going to be made out of the same HST's, then definitely round up to the next whole number. That will make the cutting much easier. But if your HST's need to fit next to other blocks that finish at the same size, then you need to do the 7/8" measurement.

OK, when I get home, I will post the pictures.

I also use this same grid method, adding 7/8 to the finished size. The other nice thing about this grid is you can sew it in a continuous line on either side of the diagonal if you lay your grid out right. I have found it described in several books. I think the book "Quilts Quilts Quilts!" might have instructions on how to do it this way (in the back section). I have found this method best for me. I have another book that talks about making them out of bias strips and fat quarters but have yet to try that way.

I'm with everybody else for cutting your square 1" larger than the finished size needed. When I first started quilting I did it by the rules (7/8") larger and no matter how careful I was they neer came out right so now I do it 1", trim them up and I'm ready to go. Lots of people use Thangles and I have tried them too but to me it just takes the same amount of time and I don't like pulling the paper off when I'm through.