Half-Square Triangle Unit

Other than the “square”, the Half-Square-Triangle unit (HST) is the most utilized unit when creating block designs for quilts. So, it’s important for beginners to learn how to make them.

A Half-Square-Triangle unit gets its name because a triangle makes up “half” of the square unit. Each half of the square is a triangle.

A square is made by joining  two equal-area right triangles.

Years ago, before rotary cutters were invented, people used to make HST units by cutting the same size triangles from two different fabrics and sewing them together to make a square.  It’s still a legitimate way to make HST units, but, there are also more opportunities for stretching that exposed bias edge.


There are a few ways to make a half-square-triangle unit., but first,  some measurements must be done. I also want to mention some quilting terms that you will be hearing often. They are “finished” and “unfinished” or “raw”.

The “unfinished” or “raw” measurement is what a unit or block measures BEFORE it has been sewn into a quilt.

A “finished” size is the measurement of a unit or block AFTER it has been sewn into a quilt.

The square on the bottom is “finished”. It has been sewn into the quilt and measures 10″ square. The square that I pinned above that is my first attempt at making the cowboy hat and boots block. I didn’t like the results, so it did not get used in this quilt. It measures 10 1/2″ x 10 1/2″from raw (unsewn) edge to raw edge.

First, know what your finished square unit should measure after it’s been sewn into a block.

I have some standard measuring rules for determining what size fabric pieces to cut.

Now, for a simple solid square whose finished size is 4″, you need to have a square of fabric that is 4 1/2″ by 4 1/2″.  This allows for a 1/4″ seam allowance on each side. But, a HST unit has an extra seam sewn diagonally before the unit is ready to be sewn into a quilt block. So an extra 3/8″ must be added to the basic  measurement to allow extra fabric for the diagonal seam. 4″ + 1/2″ + 3/8″ = 4  7/8″

For precise measuring, cutting, and sewing, the formula is:       Finished size + 7/8″.

This way to measure and cut is precise, so your cutting, and 1/4″ seam also needs to be precise. Precise.

To me, it’s much easier to cut the pieces a little larger than needed and then trim the excess later.

Using the example above, instead of cutting squares 4  7/8″, I would cut them at least 5″ to 5 1/4″.  This just makes accuracy easier to attain because the cutting is the last thing I do to the unit before joining it in the block.

Okay, so now, let’s put that information into practice.

I cut two 5″ squares. One a light fabric and the other a medium. One the wrong side of the lighter fabric, I marked a line.

I like to use a sandpaper board for marking lines on fabric.

You could simply mark a line from corner to corner and sew 1/4″ away from the line on each side.

Oh, you noticed the three lines I have drawn on tape on the bed of my sewing machine. I use that for piecing accuracy. (Hey, every little trick I can use to sew more accurately, I’ll do it!)  See Less Marking, More Sewing for details.

You can also use a tool like this Quick Quarter ruler.  Simply line-up the center slot, corner to corner, and mark a line on each side of it. Then sew on  the lines.




Try each method and see which one works best for you.

No matter which way you sew them, they will most likely end up looking something like this.

But, don’t worry, pressing with an iron will relax those stitches and let the unit lay flat. I do this before cutting them apart.

Cut the unit in half from corner to corner, between the sewn lines, either with scissors or a rotary cutter.


Place your HST unit on the pressing surface with the dark fabric up. Open up the unit and press flat.


If you started with larger sized squares, it’s time to trim.

Here, I lined up the 45 degree line on my ruler with the diagonal seam of the unit.

I also, made sure that the edges and the 4 1/2″ mark were well within the HST unit.

Then I trimmed the side and the top.


I rotated the unit 1/2 turn, and trimmed the remaining two sides.

Now, it’s exactly 4 1/2″ square and ready to be sewn into a block.


If you have any questions about anything in this tutorial, email me at diane@lookoutmountainquilter.com and I will answer promptly.