How to Make Half Square Triangles – The Easiest, Fail-proof Method

Learn how to make half square triangles the easy way. I have found this method to be the most straight forward and foolproof technique for a perfect quilt block.

learn the easyway how to make half square traingles

Last Updated on 04/21/22 by Rose Palmer

I am fascinated by the many quilting geometries you can achieve with a half square triangle block. Over the years I have made well over a dozen half square triangle quilt patterns and in the process I have perfected the easiest and most fail-proof method for how to make half square triangles.

A half square triangle block is one of the most versatile blocks in the quilting design lexicon. Whether you are combining two, four, sixteen or more half square triangle blocks together, the geometric possibilities are endless. Add to that different color combinations, and you have an infinite number of design possibilities.

In my Quilt Gallery you can see many of the half square triangle quilts I have made so far. A half square triangle block (or HST for short) looks deceptively easy to make. The principle is easy enough – combine two equally sized right triangles together to form a square. Then sew a whole bunch of these squares together to form a design.

Yet, the first time I tried this, my HST blocks were slightly wonky and not all the same size. Then, when I went to sew the HST blocks together, the points and intersections did not match. I was frustrated and a little discouraged (and that quilt is still not finished as a result).

There are many methods out there for making easy half square triangles and I have tried them all. Many techniques involve drawing diagonal lines on the back side of the fabric and using that as a sewing guide. For me this was an unnecessary extra step that I did not want to do. I want to just cut and sew.

Eventually, I identified the method that works best for me. It is quick, easy and allows me to make a perfect HST block every time. Now when I sew my half square triangle blocks together, the points and intersections always match perfectly.

Detail from "Ribbon Dance" - my quilt that was displayed at the 2017 Paducah Quilt Show
Detail from “Ribbon Dance” – my quilt that was displayed at the 2017 Paducah Quilt Show

In fact, one of my quilts with a modern half square triangle design won a second place ribbon at the 2016 Pennsylvania National Quilt Extravaganza and was accepted as a finalist in the 2017 Spring Paducah Quilt Show. (My inaugural blog post was about my Pilgrimage to Paducah to see my quilt hanging in the show).

How to make half square triangles the easy way

The challenge with making half square triangles is that you are sewing along the bias edges of the fabric. The bias is the diagonal of the fabric as opposed to the horizontal or vertical straight of grain. Fabric bias is stretchy and can easily be distorted out of shape, unlike the straight of grain.

As a result, when sewing or pressing the HST block it is easy to end up with a distorted square rather than the perfect one that you need. The key is to cut triangles slightly larger than what you need and then after you’ve sewn that pesky diagonal line and locked the seam in place, you trim the block down to your perfect dimensions.

My formula for making half square triangles is to start with a square that is 1 inch larger than the finished sized of the half square triangle block in the finished quilts. So, if the pattern calls for a 4 inch finished size square triangle block, then I start with a 5 inch square of my chosen fabrics.

Half Square Triangle Chart

The process for cutting and sewing the perfect half square triangle is very straight forward.

1. Cut a square in the desired size. I often cut out as many as four at one time. (Make sure the blade on your rotary cutter is sharp).

2. Cut the square diagonally from one corner to the opposite corner. Again, I cut through as many as four layers which creates 8 half square triangles

cut the HST units on the diagonal
Cut the HST units on the diagonal

If you do this with two contrasting fabrics and make 16 HSQ of each color, this leads to all sorts of design possibilities as I explore in my block-of-the-month free quilt pattern posts.

3. Combine two triangles right sides together and sew along the diagonal edge with a standard 1/4 inch seam allowance. I let the feed dogs gently feed the fabric under the needle so that it does not distort the bias edge seam. By chain piecing, I can quickly sew very many HST blocks in a short amount of time.

Sew the HST units along the cut diagonal edge

I use a quarter inch foot on my sewing machine which has a little lip on the right edge of the foot. I line up the edge of my fabric to that lip as I sew. This always gives me a consistent quarter inch seam.

4. Press open the diagonal seam. Pressing the seam open makes it less bulky and easier to sew the intersecting diagonals when you sew together the HST blocks.

Iron open the seams of the HST unit
Iron open the seams of the HST unit

5. At this point, the unfinished size of the HST is just a little bit larger than what you need. Using a square ruler with a diagonal line, trim the block to the desired size (I use a ruler by Creative Grids such as this one). You’ll only be taking off 1/8 inch or less on each side so there isn’t much wasted fabric.

The beauty of this method is that if your seam allowances are not all quite the same, you still end up with a perfectly sized HST unit.

Square up each HST to desired size
Square up each HST to desired size

Voila! You now have a perfectly square half square triangle quilt block that is ready to be sewn with other HST blocks that are all exactly the same size. Now your points and intersections should match up nicely.

A 4 patch HST unit with perfectly matched points
A 4 patch HST unit with perfectly matched points

What I like about this method is that I end up with half square triangle quilt blocks where all the edges are on the straight of grain of the fabric. This means that once the block is trimmed up, I can sew it together with other blocks without any resulting distortions. This helps to produce finished units in the quilt that are exactly the size that they are supposed to be with points and intersections that match up perfectly.

When I am working on a quilt project, I will also cut a whole bunch of triangles at the same time. Then I use Bonnie Hunter’s “leaders and enders” technique to sew them together as I sew up the blocks for my primary project. Before I know it, by the time I have finished sewing the main quilt, I also have enough HSTs sewn together for a second quilt top. I love being this efficient!

Hope you find this easy half square triangles tutorial helpful. If you follow my half square triangle formula you too will be zipping out those perfect blocks in no time.


Other quilting related posts you may like:

Spice up your quilt back with these ideas: 18 Ideas for a Unique Pieced Quilt Back

I design and make quilt patterns inspired by my travelsAround the World in 25 Travel Quilts

A quilting class with Kaffe FassetMaking a Kaffe Fasset Quilt – Taking a Class With the Master of Glorious color

A list of the best quilting classes and retreats: My Wish List of Unique Quilting Classes, Quilting Workshops, Quilting Retreats and Quilting Holidays to Inspire Your Next Creative Adventure

A comprehensive list of where to donate quilts: Be a Quilt Volunteer – Where to Donate Quilts to Charity


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A step by step tutorial to learn an easy and fail proof method for how to make half square triangles for quiltingin any size you need An easy tutorial to make perfect half square triangles every timeLearn how to make perfect half square triangles quilt blocks with this easy step by step tutorial