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Favorite tools for trimming HST’s

Every quilt pattern is different, but I often find myself trimming LOTS of half-square triangle units (HST’s). As I work on my latest pattern, I’m doing just that! This seems like the perfect time to share my favorite tools for trimming HST’s.

My favorite tools for trimming HST's include the Omnigrid rotating mat with scalloped edges and the Bloc Loc Half-square Triangle Ruler.

But first, what are HST’s?

The half-square triangle unit is common in quilting. It’s a square that is made up of two triangles. They can be made many ways, but my favorite is to start with two over-sized squares.

  • I draw a line (from corner to corner) on the wrong side of the lighter square.
  • Place squares right sides together and stitch 1/4″ seam on both sides of the diagonal line.
  • Cut apart on the line and press towards one side.
  • Trim on all four sides to the required size.

If I’m making tons of HST’s, I usually make them using the 8-at-a-time method.

  • To get the size needed for each square, I multiply the cut size listed in the pattern by two. For example, if the pattern says to cut a 4″ square, I will cut a 8″ square instead.
  • Draw a diagonal line on the wrong side of one of the squares in both directions, forming an “X”. Then draw a vertical and horizontal line in the center.
  • Stitch 1/4″ on both sides of the diagonal lines and cut along the drawn lines.
  • Then press and trim.
How to make HST units 8-at-a-time @ The Crafty Quilter.

My favorite tools for trimming HST’s

There are two tools I use for trimming HST’s that work together for a fast and efficient process. They are the Omnigrid rotating cutting mat with scalloped edges and the Half-Square Triangle Bloc Loc Ruler. Together, they create a dynamic duo that work together beautifully. Let’s talk details.

My favorite tools for trimming HST's include the Omnigrid rotating mat with scalloped edges and the Bloc Loc Half-square Triangle Ruler.

Omnigrid rotating cutting mat with scalloped edges

I have other rotating mats, but the scalloped edges on this specific mat make it really easy to rotate. It’s a game changer. When using this mat, I don’t have to lift the HST to trim the other sides; I just rotate the mat.

The Omnigrid Rotating Mat with Scalloped Edges is perfect for trimming HST's (half-square triangle units).

The Omnigrid rotating cutting mat with scalloped edges is great for trimming anything that requires all four sides to be trimmed. It’s available in 8″ and 14″ (diameter) sizes. I’m using the 8″ size and it’s perfect for units smaller than 6 1/2″. It’s also a good size for taking to classes or retreats. The larger, 14″ size, would be great for trimming square quilt blocks up to 12 1/2″.

The Omnigrid Rotating Mat with Scalloped Edges is perfect for trimming HST's (half-square triangle units).

Of course this cutting mat is useful for trimming any kind of unit, not just HST’s. Squaring up blocks, trimming flying geese units and QST’s are just a few of the ways it can be used.

Bloc Loc Half-square Triangle Ruler

I love the Half-square Triangle Bloc Loc rulers; I’ve been using the 5 1/2″ square version. These rulers have an etched groove on the back that slides along the seamline. This makes it really fast and easy to lock the ruler into place. It also assures that the seam ends in each corner. Another advantage: the Bloc Loc ruler lays flat on the HST which prevents any rocking movement while you’re trimming.

The Omnigrid Rotating Mat with Scalloped Edges is perfect for trimming HST's (half-square triangle units).

It’s worth mentioning that using this ruler requires you to press your seams to one side, not open. If you’re an open-seam kind of gal/guy, this is not the ruler for you.

I’ve heard complaints that Bloc Loc rulers are expensive, and they are (compared to other rulers). However, this is the ruler I reach for time and again. It’s worth every penny to me. My 5 1/2″ ruler can trim squares from 5 1/2″ down to 1″ in 1/8″ increments. Of course, there are many sizes available; I suggest you choose one that will fit your needs.

It’s important to place the ruler so that the seam allowance is on the correct side for trimming. Notice if the corner of the ruler lands on the seam or if it’s offset. An easy clue for placement is to make sure the Bloc Loc label is on the flatter side of the unit (usually the lighter fabric, but not always).

Correct placement of the Bloc Loc Ruler for trimming HST's (half-square triangle units).
Favorite tools for trimming HST's includes this Bloc Loc ruler. Make sure the Bloc Loc label is placed on the lower seam allowance side of the HST unit.

See it in action

Here’s a short video that I shared on Instagram. This is helpful for those that need a good visual of the process in action.

I hope you found these tips helpful. I’m always looking for ways to make quilting more efficient (and fast) while still having fun. If you have a favorite tool/process for trimming HST units, please share in the comments. There’s always something to learn from each other!

For more tool tips, check out my Five Favorite Quilting Tools and My Favorite Tools for Machine Applique.

Trimming half-square triangle units has never been easier. Check out my favorite tools for trimming HST's @ The Crafty Quilter.

Thanks for reading and happy quilting!

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  1. Great video. I haven’t bought these yet, but it is nice to see how you can use a bigger size for smaller blocks, like I do with my square rulers. As for the rotating mat, I just use a smaller square or rectangular mat and rotate it; it is not as fancy nor as expensive as the one you used, but it works.

  2. I have never figured out why tutorials like these have you draw a diagonal line. If you KNOW you sew an accurate 1/4″ seam and if, as here, you trim the HST anyway, just pair up the squares, cut diagonally, and sew the two seams.

  3. I found the New Leaf Slotted Rulers to work well for trimming half square triangles. Since I have trouble trimming after pressing, I find these rulers work well, as you don’t press them open before trimming. This allows for either open or closed seams. I do use mine with my Omnigrid rotating mat.

    There multiple sizes, but I only have the 1 ½” to 6 ½ sizes. the package inclues two trimmers, one for ½” and one for inch sizes. All I have needed so far!

  4. I love this tutorial…so helpful. I use another tool, but I always thought you needed each size of the BlocLoc; I am glad to hear you don’t! So glad you linked up to tips & tutorials this month.

  5. Julie,
    I just went to a quilt retreat over the weekend and forgot to bring my Bloc-Loc ruler. I had to trim 36 two inch half-square triangles. I certainly missed my ruler!

  6. I agree, Bloc-Loc rulers are the best. Love my rotating mat too. I don’t draw a line though, my machine has a laser light that I can line up with the needle. Game changer for me.

  7. I couldn’t agree more about the Bloc-Loc ruler. Trimming can be tedious, but it’s so rewarding to see a perfect HST.

  8. Hi Julie, Bloc-Loc rulers are a game changer for me. I got a set a number of years ago and use them constantly! I don’t usually buy a lot of specialty rulers but those are wonderful. As you said- worth every penny!

  9. Additionally, instead of drawing corner to corner lines I love diagonal seam tape from Cluck Cluck Sew (and others) taped on my sewing machine.

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