How To Make A Quarter Square Triangle Block?

Learn how to make easy quarter square triangles that come out perfect every time! We’ve included the quarter square triangle formula, a cutting cheat sheet, and quarter square triangle trimming instructions.

What Is A Quarter Square Triangle?

A quarter square triangle, also known as a QST is a square quilt unit made of four identically sized 90-degree triangles. It is the building block of many traditional quilt blocks, like the Ohio Star. And because of its versatility, you’ll see it in many modern quilt designs, as well.

Variations Of The QST

There are different variations of the quarter square triangle block that all use the same building method.

The basic version is a square made up of four 90-degree triangles (of different fabrics):

If using only two different fabrics, you’ll get an hourglass block. The construction here is the same, but the combination of fabrics creates an hourglass shape – hence the hourglass block. It is also sometimes called the bowtie block.

A third option produces something that is not a QST exactly but uses the same method, so I think it deserves to be included here. It’s the so-called Split Quarter Square Triangle (or the 3 Patch Quarter Square Triangle). It is made up of two smaller and one large triangle.

All of these quarter square triangle blocks use the same construction method, so let’s take a look at making a quarter square triangle block.

How Do You Construct A 1/4 Square Triangle?

You might think the easiest way to sew quarter square triangles is to simply cut each individual 90-degree triangle and sew them together. Something like this:

Of course, you can do this, and I sometimes do when I’m just playing with scraps or testing something out. But sewing quarter square triangles this way is a pain for multiple reasons. The main problem is you have to sew the triangles on the bias edges, which is always a bit tricky as the edges can stretch etc. It also includes cutting triangles, which can be inaccurate. 

Luckily, there is a much more efficient way to do it and I’m about to show you how in this quarter square triangle tutorial.

Easy Quarter Square Triangles – 2 At-A-Time

These easy and quick quarter square triangles are made from two half-square triangles. 

If you’re new and haven’t done HSTs (or half square triangles), I recommend you look at how to make half square triangles. As you’ll see in our Half Square Triangle Tutorial there are different ways to make HSTs, depending also on how many you need. So it’s very handy to know the methods when you’re ready to speed things up.

But as I’ve said, for the purposes of this tutorial you will need two half square triangles. Once you’ve mastered the HST we can begin making quarter square triangles.

Making Hourglass Quarter Square Triangles

This tutorial will show you how to make two hourglass QSTs. We’ll start here because it’s somewhat easier to grasp the method when using only two fabrics.

1. Start with two half square triangles, each made of two different fabrics. Press the seams towards the same fabric on both HSTs (I recommend pressing towards the darker fabric).
   To learn how to make a half square triangle, follow the traditional method tutorial here.
   
   
2. Put one HST on top of the other, right sides together. Place it so the different colors are opposite each other (bright fabric on top of dark fabric, dark on top of bright). Because you pressed both HSTs towards the darker fabric, the seams will nest.
3. Draw a diagonal guideline on the backside of the top HST, perpendicular to the seam. IMPORTANT! It’s crucial the guideline is 90-degrees to the seam (and less important that it goes exactly from corner to corner).
4. Sew two seams, each a ¼’’ away from the guideline. 
5. Cut in half along the original guideline and press seams out. You’ve just made two identical quarter square triangles.
6. Trim to the desired (unfinished!) size.

Making Basic Quarter Square Triangles

To make the basic QST, meaning each triangle is a different color, the process is exactly the same. The only difference is you start with two HSTs, each made with two different fabrics – so 4 different fabrics in total:

Another thing to note here is that the two QSTs that this method yields are not identical. They are mirrored, as you can see in the illustration below. Keep this in mind when planning your layout.

The tutorial below shows how to make basic QSTs step-by-step:

1. Start with two different half square triangles, each made of two different fabrics – 4 different fabrics in total. Press the seams towards the same fabric on both HSTs (I recommend pressing towards the darker fabric).
   To learn how to make a half square triangle, follow the traditional method tutorial here.
   
2. Put one HST on top of the other, right sides together. Because you pressed both HSTs towards the darker fabric, the seams will nest.
3. Draw a diagonal guideline on the backside of the top HST, perpendicular to the seam. IMPORTANT! It’s crucial the guideline is 90-degrees to the seam (and less important that it goes exactly from corner to corner).
4. Sew two seams, each a ¼’’ away from the guideline. 
5. Cut in half along the original guideline and press seams out. You’ve just made two mirrored quarter square triangles.
6. Trim to the desired (unfinished!) size.

Making Split Quarter Square Triangles

To make split QSTs or 3 patch quarter square triangles you start with one square and one HST (instead of two HSTs as seen above).

Place the HST on top of the square and continue as you would with a normal QST. As with the basic QST, the two split QSTs you will get, will not be identical. They will be mirror images as shown below:

How to Square Up Quarter Square Triangles

If you’re making your QSTs oversized (which I definitely recommend), you’ll need to trim them to be the perfect size.

Sizing a quarter square triangle may seem a bit complicated, but it really isn’t if you’re using the right tools. You’ll need a square ruler that is larger than your desired finished QST size, preferably one with a diagonal line.

If your ruler doesn’t have a diagonal line, you can improvise using a piece of masking tape. Just take a piece of masking tape and tape it along the diagonal of the ruler (just make sure you tape it very accurately at a 45-degree angle). 

You’ll also (obviously) need a rotary cutter and a cutting mat.

So how to trim quarter square triangles?

This diagram will show you how to trim QSTs that are 3 ½’’  finished (so 4’’ unfinished). For other dimensions please check the QST cheat sheet that is included toward the end of this article.

1. Place your untrimmed QST on the cutting mat right side up. Place your ruler on top. Now, this is where the magic happens. When placing your ruler, there are 2 things to look for:
   
   • The center point of the block must be directly under the rulers’ diagonal and under ruler lines 2’’ (which is unfinished QST size divided by 2).
   • The diagonals of the block must bisect the corners on the ruler (at 4’’ marks).
     
2. Trim off the excess on the right-hand side and topside.
3. Rotate the block 180 degrees. Place the ruler on top and match the points as described in step 1
4. Again, trim off the excess on the right-hand side and topside.

That’s it! Not that hard, right? Remember, the positions of the center points are included in the quarter square triangle chart, so you’ll always know exactly how to place your ruler.

Quarter Square Triangle Trimming Problems

The number one problem that seems to arise when sizing quarter square triangles is the diagonals don’t line up with the corners. This is probably a problem of construction rather than trimming. It happens if the diagonal seams aren’t perfectly perpendicular. Make sure you check the construction tutorial above, especially Step 3 where I explain this in detail.

Dimensions Of A Quarter Square Triangle

A quarter square triangle is constructed using two half square triangles. This means that to figure out what dimension squares you need for a QST, you’ll really want to know the dimensions of squares to make the starting HSTs.

There are many different methods to make HSTs so it would be almost impossible to give you a quarter square cutting chart to cover all of the methods. That is why we’ll assume here that you’re making your HSTs using the traditional HST method (making 2 HSTs from two squares).

Of course, if you want to speed things up, you can use any of the methods to make your starting half square triangles, but you’ll need to follow the HST tutorial to get the cutting dimensions for your starting HSTs.

Oversized Quarter Square Triangle Formula

If you’re very precise when cutting and piecing you could use the exact formula to make your starting squares: finished QST size + 1 ¼’’. This however leaves very little room for error, so I strongly recommend you give yourself a little extra room for ‘mistakes’ and trim the QSTs to the perfect size later.

When cutting quarter square triangles or rather the starting squares for QSTs I recommend using the oversized QST formula:

Starting square = finished QST size + 1 ½’’ 

So to make 3’’ QSTs you’ll start with 4 ½’’ squares. Pretty simple right?

If you don’t like math, you can also use our quarter square triangle cutting chart below. Just look for the size of your desired finished QST and you’ll see how big you need your starting squares to be. 

Finished HST size	Cut starting squares*	Trim (unfinished) QSTs to	Place the center point at ruler lines:
1″	2.5″	1.5″	0.75″
1.5″	3″	2″	1″
2″	3.5″	2.5″	1.25″
2.5″	4″	3″	1.5″
3″	4.5″	3.5″	1.75″
3.5″	5″	4″	2″
4″	5.5″	4.5″	2.25″
4.5″	6″	5″	2.5″
5″	6.5″	5.5″	2.75″
5.5″	7″	6″	3″
6″	7.5″	6.5″	3.25″
6.5″	8″	7″	3.5″
7″	8.5″	7.5″	3.75″
7.5″	9″	8″	4″
8″	9.5″	8.5″	4.25″
8.5″	10″	9″	4.5″
9″	10.5″	9.5″	4.75″
9.5″	11″	10″	5″
10″	11.5″	10.5″	5.25″

*You’ll need 2 starting squares for hourglass QSTs, 3 starting squares for split QST and 4 starting squares for basic (4-color) QSTs

What’s really great about the quarter square triangle cheat sheet is that it will also tell you how big to trim your (unfinished) QSTs and where the center point is, so you can trim your QSTs perfectly using the method described in this article.

This is it. I think I’ve covered everything you need to know to start making your perfect quarter square triangles. If you have any other questions or maybe your own tips and tricks, let us know. Otherwise, happy sewing!