Making Precise Half Square Triangle Units for Quilting

This post is going to explore the variety of options for making half square triangle units to be used in quilt blocks. I will be sharing with you the techniques I use to get the most precise half square triangle units possible. This increases the accuracy of my quilt piecing in the whole quilt.

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What is a Half Square Triangle?

A half square triangle is a basic and widely used unit in quilting. It is quite simply a square block that is made from two triangles with a diagonal seam across the square. Half square triangles, or HSTs as they are sometimes referred to, can be the whole quilt block or multiple units can be combined with other units to make up a larger quilt block.

Examples of Half Square Triangles in Larger Quilt Blocks

Let’s first look at a few examples of how half square triangles can be used as part of a quilt block to make a variety of shapes.

Finished Size vs. Unfinished Size

Most patterns will indicate in their instructions the unfinished size of half square triangles needed. But what does that mean? When making a quilt, you typically will use a 1/4″ seam allowance to sew your pieces together. So the unfinished size of a block will be 1/2″ larger in both directions (width and length) than what the block will be in the finished quilt. This allows for a 1/4″ seam all the way around the block that will be used when you sew your blocks together.

Finished size refers to the size a block will be in your finished quilt, after the adjoining pieces are all stitched on.

For example, a 2″ UNFINISHED half square triangle will be a 1 1/2″ HST in the finished quilt block.

Methods for Making Half Square Triangles

I’m going to show you 5 methods for making half square triangles. There are only 2 that I regularly use, however, because I find them to be the most precise. The methods are as follows:

1. Half Square Triangle Foundation Papers (My top method for making lots of HST units)
2. 2-at-a-time HSTs (trim down method – my second favorite method, especially if I only need a few units)
3. 1-at-a-time HSTs
4. 4-at-a-time HSTs
5. 8-at-a-time HSTs

Half Square Triangle Foundation Papers

This method is my favorite for making lots of half square triangles all at once. Foundation papers are super easy to use. I purchase mine from Fat Quarter Shop.

1. Place your two fabrics right sides together and pin a foundation paper to the fabric sandwich.
2. Using a short stitch length, sew on all of the dotted lines on the paper.
3. Use a rotary cutter and ruler to cut on all of the solid lines on the foundation paper.
4. Press open as desired and remove the foundation paper by tearing along the perforated stitch lines.
5. Trim the little dog ears off with a rotary cutter or scissors. (the little tabs on the two corners you can see below)

Two-at-a-time HSTs

The two at a time method of making half square triangle units is my second favorite method and is the one I will normally use if I have just a few half square triangles to make with two fabrics. This method involves making the HSTs just a little bigger and trimming them down to the precise size that you need.

First you take two squares of fabric in the size you need according to the chart below or calculate using the following:

(Finished HST size + 7/8″)

The process for the 2-at-a-time half square triangles is as follows:

1. Draw a diagonal line corner to corner on the wrong side of one of the squares.
2. Place right sides together of the two fabric squares you will be using with the one with the drawn diagonal line on the top.
3. Stitch a line 1/4″ on each side of the diagonal line.
4. Using your rotary cutter and a ruler or a pair of scissors, cut on the drawn diagonal line to get two half square triangle units.
5. Press the unit open as desired.
6. Trim the unit to the proper size using the diagonal seam and your ruler as a guide to get a perfectly square unit.

One-at-a-time HSTs

This method is similar to the two at a time, but can be done a little faster. There is, however, more wasted fabric with this method. You will cut your fabric squares to the same size that you want your finished unit to be.

1. Place fabric pieces right sides together.
2. Sew a diagonal seam, corner to corner, using the method you desire (draw a line, use diagonal seam tape, etc)
3. Trim 1/4″ away from the seam and discard the trimmed piece (it can be stitched to create a secondary, smaller HST unit for another project)
4. Press open as desired.

Four-at-a-time HSTs

This method is one I don’t personally use. With this method of making 4 units at once, the sides of your half square triangles will be cut on the bias. This makes them very easy to stretch out of shape and can be hard to keep the blocks precise. The math for this is a little harder, but if you want to calculate it yourself, the math is:

(unfinished HST size x 1.6) and then round up to the nearest 1/8 inch

To make 4 HST units at a time:

1. Cut two larger squares of fabric. Use the chart provided, or calculate according to the math given above.
2. Sew a seam 1/4 away from all four sides of the squares completely enclosing them with the right sides together.
3. Cut across this unit diagonally in both directions.
4. Press as desired, being extra cautious to PRESS and not move the iron side to side as this will stretch the bias sides.
5. Trim to desired size, using the diagonal seam and your ruler as a guide.

Eight-at-a-time HSTs

This method can be very useful to make 8 HSTs at a time. I find that if I need this many, foundation paper is much faster and more precise for a very small added cost for the papers.

To calculate the size of squares you need to make 8 at a time half square triangles, I recommend the following calculation:

(Finished size desired + 1 inch) x 2

To make 8 Half Square Triangles at one time:

1. Cut two squares of fabric according to the chart of calculation above.
2. Draw a diagonal line in both directions on the back of one of the fabrics. Place the fabrics right sides together with your marked lines up.
3. Sew 1/4″ to the left and right of both of the diagonal lines. (4 seams total)
4. Cut the block in half both horizontally and vertically
5. Then, cut on the drawn diagonal lines to get 8 Half Square Triangle units
6. Press as desired
7. Trim to desired size, using the diagonal seam of the unit and your ruler as a guide.

Other Methods for Half Square Triangle Units

There are several other methods for making HSTs. I find the above methods to be the easiest that doesn’t require any special rulers. There are other products on the market that you might love. A couple of these products are the Bloc-loc rulers, and Square Up rulers. I personally am a minimalist when it comes to tools and rulers in my sewing room and try to save space and money with fewer tools.

Half Square Triangle Cutting Chart

I’ve made a handy chart for you to print out for the cut sizes for each of the methods of making half square triangles that I’ve discussed in this post. You can download it below.

If you have any questions of comments about half square triangles, feel free to leave a comment below! Happy Quilting!

Posted By: treeline quilting · In: Quilting, Quilting Tutorials