Half Square Triangle Formula

Here is a simple formula for how to make half square triangles from squares.

If you are a very precise cutter and piecer, then cut your squares 7/8" larger than the FINISHED size of the half square triangle unit you need.

Exact formula is FINISHED size + 7/8"

Example 1: If you need your half square triangle to finish at 2", then you need to cut your squares at 2 7/8". Draw a line along the diagonal on the back of the lighter piece. Place your two 2 7/8" squares right sides together and sew 1/4" away on both sides of the drawn diagonal line. After sewing, cut along the drawn line, press toward the dark and you will have two half square triangle units that should measure 2 1/2" which means they will finish at 2".

However, if you are like me, it is nice to have some wiggle room in this process. So I don't do the above. I cut my squares 1" larger than the FINISHED size of the half square triangle unit I need.

Lynn's Formula is FINISHED size + 1"

Example 2: Again suppose we need 2" finished size half square triangle units. Then cut two squares (light and dark) at 3" square.

Draw a line along the diagonal on the back of the lighter piece.

Place your two 3" squares right sides together and sew 1/4" away on both sides of the drawn diagonal line.

After sewing, cut along the drawn line, press toward the dark and then you'll need to trim this down a bit.

Using a small square ruler, place the diagonal line on the ruler right along the sewn diagonal line making sure that the 2 1/2" lines of the ruler are on the half square triangle unit. There should be a bit of fabric on ALL sides that you can trim off. We will start by trimming off the excess on the right hand side and the top side, for right handed folks. (Lefties just need to turn the ruler and half square triangle 90 degrees to the left and trim the left side and top side)

Do this, then rotate the half square triangle unit 180 degrees and line up the diagonal line on the ruler right along the sewn diagonal line again. This time the 2 1/2" lines of the ruler should be right along the two edges you just cut off (i.e. squared up). There again should be a bit of fabric on two sides (right and top for righties) that you can trim off.

Now you have a perfect 2 1/2" half square triangle unit that will finish at 2".

P.S. This is a repost from my LQS blog. Since my LQS is closing, that blog will be disappearing. I was the main contributor of that blog and I wanted to keep the tutorial posts I did for it. So I am reposting them here.