Half Square Triangle (plus trimming tutorial!)

Ah, the half-square triangle. You can't get too far into quilting without encountering this little guy. It's a foundational unit, it can be used to create just about any shape and it pops up in traditional blocks such as the sawtooth star and the bear paw.

This little quilting unit is probably, definitely, one of the most used in quilting due to its versatility. Rotate it, turn it, make it bigger, make it smaller, pair it with a square, or a rectangle, or another half square triangle, and boom. You got yourself something new and exciting.

What Is a Half Square Triangle?

Before we jumped into the thick of half square triangle, or HST, magic, let's all get into the same page. A half square triangle is a square made up of two 90-degree triangles. Each triangle makes up half of a square.

Traditionally, the way to create a half square triangles is to sew two triangles together to create the unit. However, over the years, quilters have come up with a number of ways to piece half square triangle units to both save time and avoid the need to handle individual triangles with stretchy bias edges.

What are some of those methods, you ask? Today we'll cover two methods - Two-At-A-Time Method and the Magic-Eight Method, and then I'll cover some tips and tricks for keeping your points.

Before we dive into the methods, quick note.

Some formulas online add 7/8" (or 0.875) to get the size you'll need to cut your squares in Step 1 of both these methods. However, I add 1" which gives a little more wiggle room. You will need to trim down your HST at the end, however, your units will be much more accurate.

Download a PDF Cheat Sheet HERE!

Two At A Time HST Method

As the name suggests this method creates two half square triangle units from two pieces of fabric. To know which size to cut your squares you will need to know the size of your finished HST. This means, when the quilt top is all together, what size will your HST be? Once you have that number, add 1" and cut your fabric squares that size.

Finished HST	Cut Squares
2"	3"
2-½″	3-½″
3"	4"
3-½″	4-½″
x″	y″ = x + 1

Once you've cut your fabric squares, place them right sides together and draw a guideline from corner to corner on the diagonal (Fig 1).

Next, sew a ¼" seam on both sides from the guideline (Fig 2).

Cut along the guideline, press the seam and trim the HST unit to your unfinished size (Finished size + ½") (Fig 3). For tips on trimming, scroll to the bottom!

Magic Eight HST Method

Once again, as the name suggests this method creates eight half square triangle units from two pieces of fabric - and it's my preferred way to make HST.

Again, you will need to know the size of your finished HST. Once you have that number, add 1", then double this number and cut your fabric squares that size.

Finished HST	Cut Squares
2"	6"
2-½″	7"
3"	8"
3-½″	9"
x″	y″ = (x + 1) x 2

Once you've cut your fabric squares, place them right sides together and draw two guidelines diagonally from corner to corner (Fig 4).

Next, sew a ¼" seam on both sides from each guideline (Fig 5).

Now you will make four cuts total, the order doesn't have to be exactly like this, but here we go.

1. Cut the squares in half horizontally (Fig 6).
2. Cut the squares in half vertically (Fig 7).
3. Cut on the guidelines, in between the stitching lines (Fig 8).

When doing this, I try not to move the squares. I'm lucky to be able to walk around my cutting table, or if you have a rotating cutting mat, that comes in real handy. However, it's not a fast-and-hard rule. You can move them if necessary, but it's better if you don't.

Next, press seams and trim the HST units to your unfinished size (Finished size + ½").

Download a Half Square Triangle Cheat Sheet HERE!

Trimming Half Square Triangles

Here's the thing about half square triangles... you gotta trim them. Quilters love a good no waste method. However, when it comes to HST I've found that trimming down, that itty bitty extra waste, that extra time, is always worth it in the end. And I've realized all tutorials kind of assume quilters will know how to trim a HST correctly, but that's not the case!

So let's go over how to correctly trim an HST unit in order to keep those points (ps: there's a video at the bottom!)

After pressing your seams, lay your HST right side up on your cutting mat.

First, line up your ruler so that the 45-degree line matches up with the diagonal seam. Second, make sure you are leaving enough room on the left and bottom edges to trim/square up those edges.

In the picture above I'm trimming an HST unit to 4-1/2". You can see my 45-degree line runs right on top of the seam (red arrow) and my 4-1/2" lines are inside the rough edges of the square (blue arrows) with enough room to trim.

Next, you will square-up the unit. To do this rotate the HST so the newly straight edges are on the left and bottom. Once again line up the 45-degree line on your ruler with the diagonal seam (green arrow). And this time, make sure your desired size, in my case 4-1/2", line up with the left and bottom edges of your square (pink arrows).

And now you should have a perfectly square unit with a true 45-degree seam going from corner to corner to create the triangles!

I know the trimming is super tedious, however, taking your time with this step helps keep your points :)

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Want to give HST a try? Here are some Cotton and Joy patterns with HSTs!!