How to trim half square triangles is a basic quilting skill that, at least at some point in your quilting career, you will probably have to figure out. There are different ways to sew, press and trim half square triangles, but let me tell you, as someone who has made thousands of these wonderful blocks, I have recently landed on a favorite way to do it.
What makes this technique the best way to trim half square triangles? Easy – it's fast and accurate, saving me time and making me look good in the process. Double win!
Most quilt patterns will instruct you on how to sew half square triangles, but the trimming and pressing is more or less up to you and your prefered method. So how does one take some cut squares like this...
...and turn them into a beautiful quilt like this?
The quilt featured here is the Gather quilt pattern and can be purchased in the shop!
The answer is not a wag of the finger telling you to go slowly and always measure twice before you trim. Oh no, no, no. Please don't measure each of your blocks twice! The answer, is in fact, switching up the order in which we typically make these blocks.
In a past blog post on trimming HSTs (half square triangles) I sewed my blocks, pressed my blocks, and then trimmed my blocks. This method is still great and will also result in beautifully accurate quilt blocks. The one factor it doesn't take into account, however, is time. It's just not the fastest trimming method.
With this new method, as seen in the video below, my steps look like this:
- Sew the half square triangles.
- Trim the blocks.
- Press them open.
I know what you're thinking. That's the exact same amount of steps, how could it be faster? Well, it's faster because with a single HST still folded on top of itself, you trim all four sides without moving your ruler and by making only two cuts.
Previously, you needed to either rotate your mat and/or yourself and cut all four sides individually. Am I making sense? The video above will make it really clear.
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Supplies to Trim Half Square Triangles
Technically you only need a very basic ruler (square is the easiest), rotary cutter and cutting mat. However, in this tutorial I am using a really helpful HST-specific ruler that allows you to trim the dog ears off your half square triangle during the trimming process (so you don't have to go back and do it after pressing all of your blocks.)
When you buy a pack of Clearly Perfect Slotted Trimmers they come in sets of two. One question I had immediately after opening these rulers was, "Why is this frosted paper so difficult to remove?"
The answer is that you're not supposed to remove it. 😉 These rulers are designed with frosted paper on the back. Which actually turned out to be a nice feature because it helps the ruler not slide around as much as if it were just plain plastic on top of your HST.
EDIT! You ARE supposed to remove the paper, it's just kinda hard and takes some fingernail power, which I don't have! So remove that frosted paper, people! Sorry to steer you wrong initially. Blame it on my addled pregnancy brain!
In the video I also show you an alternate trimming method that does not use the Clearly Perfect Slotted Trimmer. Instead, I use a square ruler I had on hand and a piece of masking tape to give myself an obvious guideline.
The only major difference with not using the Clearly Perfect Slotted Trimmer is that I will have to trim the dog ears off after each block is pressed open. The supplies used in the video for this method includes:
- Omnigrip 7 1/2" square ruler (I own multiple sizes of square rulers, and picked the one closest to the unfinished block size I needed)
- Fiskars rotating cutting mat
- masking tape
Step 1: Line up the HST seam with the ruler.
This is actually something I messed up on a few practice blocks. Here's the truth – we don't always sew a perfect 1/4" seam (whaaaat?!?! I mean, obviously YOU do, but I don't.) So I initially made the mistake of lining up the edge of the ruler with the edge of my seam.
According to the size of your unfinished HST (in my case that would be 6"), line up the dotted seam line of the ruler with your sewn seam. This may mean that the edge of your seam and the edge of the ruler are off a little. That's OK! Ignore them. They don't matter.
See how my edges are ever so slightly off? What matters is that the seam and the dotten seam line on the ruler is lined up.
Step 2: Trim.
Hold down firmly with one hand, and with the other use your rotary cutter to trim all of the excess bits, including those dog ears.
Step 3: Press the HST
I like to use my tailor's clapper when pressing, but the important thing to notice here is that once you open up and press your half square triangle it's perfectly trimmed! See that?
For a full tutorial on how I press quilt seams, check out How to Press Seams in a Quilt (with Video Tutorial!)
Now go forth and make all of the HST quilts!
SQ HST Quilts!
And that's all there is on how to trim half square triangles! What do you think of this technique? Do you have a favorite trimming method? Share it with us in the comments!
22 thoughts on “How to Trim Half Square Triangles”
Thanks for the video. I looked at this tool: if you follow the directions for cutting, you end up with bias edges on the outside of the HST. I just don’t think that’s the best method: all those bias edges to piece. I do it with angles, of course; but a nice HST on the straight of grain is just better for me. I teach beginning quilters, and stretchy bias edges are not the best for them. I was so excited to see a method that was faster, but my choice is to take the extra time and have straight of grain HST. But thanks again. Any new tip is always appreciated. I’m always looking to find better and faster methods.
This trimming technique would still work with HSTs trimmed on grain. If you avoid this 4-at-a-time sewing method and stick to the 2-at-a-time or 8-at-a-time methods, you’ll only have the one bias edge instead of two. This post shows all three techniques – https://suzyquilts.com/half-square-triangles-tutorial/
Thanks for the tutorial. Wish I had read it before making so many HSTs for an Indian Summer quilt. Those rulers look awesome.
I have had the same thought! Where has this ruler been all my life??
With the Slotted rulers, what are the slots for and how are they used? Thanks
The slots are for trimming dog ears. So with this example I didn’t need to use the slots because the edge of my HST butted up to the edge of the ruler. However, if my HST was a size smaller, I would need to use the slots to trim.
Thanks for this Suzy!! I have been doing a version of this for awhile, but I kept thinking “Surely if this way works, everyone would be doing it. So I must be doing something wrong. And don’t call yourself Surely, your name is Peg.” 🙂 So I’m happy to know I have been kinda sorta doing it correctly!!! Yay! Pat that baby bump for me. My baby is fixing to graduate and go off to college! 😭
Of course I peeled the paper off!
Hahaha! Oh no! Does the ruler still have the markings? I think the markings are on the paper.
The markings are on the ruler not the paper. Feel free to peel the paper off, it is much easier to use that way.
Rest assured, the paper has been removed after the great paper saga! If you followed along with the Gather sew along, you got to see the many conversations about if the paper made it easier to use 🙂
I bought a Quilt In A Day ruler a couple of years ago. It does the same thing, except I still have to trim the dog ears, but I’ve never looked back. So much better than other methods.
When do we meet in zoom for the sew along? I haven’t gotten the link
We actually do not meet on zoom for the sew along, but I will have LIVE videos in Instagram and in the Suzy Quilts Patterns Facebook group Wednesday nights at 7:30 CST. Check out this post for details on Week 1 – https://suzyquilts.com/gather-quilt-sew-along-week-1-pick-your-fabric/
Random but I love the spray bottle and need a new one! Where did you get it?
Thanks! I got this at Grove Collaborative, but I bet you could find a similar one somewhere else. (Buying stuff from Grove is kind of annoying because you have to also become a member)
As a new quilter, I’ve tried both methods– trim before or after pressing. I still find that I have better luck with trimming after. I don’t always have a great seam line. Sometimes, my hst press wonky. Sometimes, I need to fussy cut a bit. I’ve had better luck with the bloc loc trimmers since I’m not messing with the hst again between trimming and sewing. YMMV.
Can you share which fabric is the dog backing on this quilt! Thank you xx
This is very helpful, thank you! One thing I’m not clear on is what do you mean by unfinished size of the HST?
“Unfinished” means the 1/4″ seam allowance is included in the size. For example an unfinished HST could measure 5 1/2″ square which would be the same as a finished HST that measures 5″ square.
Thank you so much for the video on trimming HSTs! It will save so much time and now one of my WIPs (with about 200) now has a chance of being finished.
Hah! I don’t like the term ‘dog ears’ either.
Before I knew they were called ‘dog ears’ I’d been calling them ‘sticky-outy fabric bits’ (which, while correct, sounded so wrong, lol) but have recently started using the term ‘seam flags’ instead.