Last updated on August 27th, 2020
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 quilts 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 half square triangles and I have tried them all. Many techniques involve drawing 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.
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 edge 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 seam and locked it in place, trim the block down to your perfect dimensions.
My formula for 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 half square triangle block, then I start with a 5 inch square of my chosen fabrics.
Half Square Triangle Size 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
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 posts.
3. Combine two triangles right sides together and sew along the diagonal edge. I let the feed dogs gently feed the fabric under the needle so that it does not distort the bias seam. By chain piecing, I can quickly sew very many HST blocks in a short amount of time.
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.
5. 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.
Voila! You now have a perfectly square half square triangle 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.
What I like about this method is that I end up with a half square triangle block 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 a finished unit in the quilt that is exactly the size that it is 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 HST sewn together for a second quilt top. I love being this efficient!
If you follow my half square triangle formula you too will be zipping out those perfect blocks in no time.
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