How do I determine what size square to cut if I need the triangles to form a finished 3 1/2 in square?

Question

my pattern design calls for a finished 3 1/2 in square.  It consists of 1 right triangle of one color and a 2nd triangle of a different color.  What size square do I need to cut to form the triangles?  The pattern I am working with has me discarding the bottom half of the square.

Answer 1

Many of us blithly throw away the other half of the square (triangle).  But in frugal years gone by, that never would have happened.  Let's see, if I understand you correctly you want to cut triangles, that when sewn together will form a 4" square, because when that is pieced-in it will become 3 1/2" .

A rule of thumb is to cut the side or leg of the triangle 3/8" longer than the size you want when it's finished (or in this case it could be a square giving you two triangles).

Therefore, if you want to have a raw 4" triangle you would cut the new length 4 3/8". 

A.  You can cut the square diagonally, place 2 different colored triangles together, and then sew a 1/4" seam along the hypotenuse.

B. Or you can 1.) Place two 4 3/8" squares together, 2.) draw a line, diagonally from corner to corner on the top or lightest one, 3.) stitch 1/4" from that line on both sides of it, and 4.) cut on the drawn line to have two finished half-square triangles.

The 3/8" rule applies no matter what size you're making.

Best of Luck!

Judy

Answer 2

You're talking about half-square triangle units.  The squares need to be cut 7/8" larger than your finished squares need to be.  So you would need to cut 4 3/8" squares of each color in order to get a 3 1/2" finished HST unit ((square).. 

I'm not clear on what "discarding the bottom half of the square" means, sorry.  Do you mean you only need ONE HST unit?  Then you would indeed have an extra one by following this method.  The great thing about this method is that you never have to work with bias edges, so the quilt lies beautifully flat -if this idea appeals to you, then put the 2 squares right sides together and draw a pencil line diagonally between 2 corners.  I like to put in a pin or 2 at this point. Then sew 1/4" away from this diagonal line on both sides of it.  Then, putting your rotary cutting ruler along the center line, cut it.  When you open the 2 triangles, you'll have 2 perfect HST units

OR you can try starching your fabric (to keep the bias from stretching when you handle the triangles) and cutting your triangles and then sewing up the hypotenuses (is that a word?)..