Trimming Half Square Triangles: Must-Have Tools for Accurate and Easy Results

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Learn how to trim Half Square Triangles with ease and accuracy using these essential tools: Omnigrid rotating mat, Bloc Loc ruler, Clearly Perfect Slotted Ruler, and Creative Grids ruler. Discover the best methods for trimming HSTs to ensure that your quilt blocks are perfectly square and beautiful.

Half Square Triangles (HSTs) are a common quilt block unit and a building block for many quilt designs. Because of this, it’s important that they are square and trimmed to the correct size.

In this post I’m sharing a few essential tools that will make trimming HSTs go smoothly and produce accurate blocks – whether you’re trimming a few for a quilt or 200+ for an entire HST quilt.

HSTs on rotary mat

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Omnigrid Rotating Mat

One tool that can make trimming HSTs more efficient is the Omnigrid rotating mat because it allows you to turn the mat and not the fabric when trimming HSTs.

The mat is available in two sizes: 8″ and 14″. I prefer the 14″ mat for the studio because it can handle HSTs up to 12.5″. But, I like the the 8″ mat for carrying to classes.

I’ve become totally besotted with this mat because you never have to move the block when trimming. And this ensures you’ll get accurately trimmed HSTs because nothing shifts during the trimming process. Accurate HSTs = blocks that go together effortlessly = my can’t live without out it tool for trimming HSTs.

To use this mat, place your HST on the mat and align the diagonal line on your ruler with the diagonal seam of the HST. Make sure the HST is centered on the mat. Use your rotary cutter to trim the edges of the HST, rotating the mat as needed to trim all four sides.

I find that this mat has the perfect amount of spin. It turns smoothly, but not too fast. And it securely grips to your table, so it’s not going anywhere and neither is your HST! This mat is also great for trimming flying geese, quarter square triangles, and simple block squaring.

trimming hst

Creative Grids Square Ruler

The Creative Grids Square Ruler is my go-to ruler for trimming a small number of Half Square Triangles (HSTs). The ruler has a non-slip surface that helps to keep the fabric in place, and it’s marked with diagonal lines that make it easy to align the ruler with the diagonal seam of the HST.

To use the Creative Grids Square Ruler for trimming HSTs, place your HST on a rotating cutting mat and align the diagonal seam with one of the diagonal lines on the ruler. Make sure the HST is centered under the ruler and that the edges of the HST extend beyond the edges of the ruler. Use your rotary cutter to trim the edges of the HST along the sides of the ruler. Rotate the mat and repeat the process for the remaining three sides of the HST.

If you have other Creative Grids rulers then it will be easy to read the markings and make the HST trimming process faster. I have a 6.5″ square ruler and a 4.5″ square ruler which allows me to trim multiple sizes of HSTs with the same ruler. I find that I can cut almost any size HST with these two rulers and switch between sizes as needed.

If you can only afford one ruler then I recommend the 6.5″ square ruler.

trimming hst with square ruler

Bloc Loc Ruler

The Bloc Loc ruler is a perfect tool for trimming HSTs because it has a groove that fits snugly over the diagonal seam, ensuring accurate trimming. This ruler is ideal when you are making a quilt that has a ton (think 200+) of HSTs that need to be trimmed.

This ruler takes a second to learn. There is a right and a wrong way to use it so I recommend a little practice before you dive into trimming HSTs for a quilt. The Bloc Loc ruler has a groove on the back and you line it up with the pressed seam. It locks the ruler along the seam line and allows you to slide it up and down until your HST corners are perfectly aligned.

There are a few details that are important to remember.

  • Your HST seams must be pressed to one side in order to use this ruler.
  • The Bloc Loc ruler must be placed correctly. I do this by making sure the label on the flat side of the HST.
  • This ruler is not super grippy (like the Creative Grids ruler) but the slot keeps everything in place and gives you the most accurate cuts – especially when you use the Omnigrid rotating mat.
using a bloc loc ruler

To use the Bloc Loc ruler, place your HST on a rotating cutting mat and line up the ruler with the groove over the diagonal seam.

Align the Bloc Loc ruler with the desired size and trim the edges of the HST. Turn the rotating mat and slide the ruler into place. The Bloc Loc ruler ensures that the HST is trimmed to the correct size and that the diagonal seam is not distorted.

This ruler is on the expensive side but a worthwhile investment if you’re making a quilt with a ton of HSTs. It will make the trimming process much easier. Add it to your wish list and let your family know that this ruler and the Omnigrid rotating mat would make a perfect stocking stuffer or a birthday/Mother’s Day gift!

When I have a large number of HSTs to trim, this ruler combined with the Omnigrid rotating mat are a perfect combo.

Clearly Perfect Slotted Ruler

This is a new HST trimming ruler for me but is quickly winning me over. What makes this ruler unique is that you trim your HST before pressing it open. That translates into only one alignment for two cuts that trim all four sides of a half square triangle.

The Clearly Perfect Slotted Ruler also has slots that allow you to perfectly trim away the dog ears by keeping the fabric in place and preventing any slipping or sliding.

To use the Clearly Perfect Slotted Ruler for trimming HSTs, place your HST on your rotating mat and align the seam allowance with the correct seam line on the ruler. Make sure the HST is centered under the ruler and that the edges of the HST extend beyond the edges of the ruler.

Trim the top edges – rotating the mat as needed. Then, insert the rotary cutter into the slots on the ruler and trim along the bottom edge. I recommend using a 28mm rotary cutter for trimming the dog ears in these slots.

One of the benefits of using the Clearly Perfect Slotted Ruler for trimming HSTs is that it helps prevent any distortion of the diagonal seam. The slots on the ruler ensure that the rotary cutter does not cut into the seam allowance, which can also cause distortion and affect the overall accuracy of the HST.

The Clearly Perfect Slotted Trimmers is a little to the pricy side, but it comes in a set with two rulers. One ruler is for trimming half-inch squares and the other is for trimming full-inch squares. In addition, the package includes details for squaring-up hourglass blocks; quarter square triangles, strip-piecing HSTs; and fussy cutting.

Personally, I like this ruler best when my HSTs are slightly over-sized. And, you’ll get the best results when your seam allowance is accurate. One thing to note: This ruler doesn’t have a good grip (like a Creative Grids ruler) but it is so great when you have tons of HSTs to trim.

Clearly Perfect Slotted Ruler

Which HST ruler is best?

The answer to this question depends on your skill level and your budget.

  • If you can only afford one ruler for trimming HSTs, get the Creative Grids 6.5″ square ruler. It doesn’t slip and you can trim a variety of sized HSTs.
  • For quilts with lots of HSTs I recommend the Bloc Loc Ruler for the most accurate cutting. I prefer the individual sized rulers, but a 6.5″ ruler will cover most of your HST cutting. It’s worth the investment when a quilt has a massive amount of HSTs.
  • The Clearly Perfect Slotted Trimmer trims before you press the HST open and this keeps the diagonal seam from warping. It’s also very efficient when making your HSTs using the strip-tube method. I recommend this ruler for someone who has a little more experience making HSTs.
stack of HSTs

Putting the Rulers to Use

If you’re looking for a quilt pattern that uses Half Square Triangles and want to try out your newly acquired trimming skills, check out the Tumbling Sea Glass quilt pattern. This quilt features a modern + fresh design that is perfect for all skill levels. The pattern includes detailed instructions, diagrams, and fabric requirements, making it easy to create your own beautiful Tumbling Sea Glass quilt.

Click the green button below to purchase the pattern and get started on your next quilting adventure!

printed quilt pattern with

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