How to Make a Half Square Triangle Quilt Block

This site contains affiliate links from which we may receive a small commission at no extra cost to you. Thank you for your support!

Our third block in our Sampler Quilt is going to be another one of the simplest pieced quilt blocks – the half square triangle! As I said in our Sampler Quilt Introductory post, this series is a beginning quilting series, designed as the perfect project for first-time quilters. However, just because this is meant for a beginning quilter, it doesn’t mean quilters of all skill levels can’t join in!

My plan in all of this is to build a library of free quilt tutorials with easy blocks, and next year, start another series with more difficult ones, so that we can all gain new skills.

Let’s Talk Half Square Triangles!

The half square triangle block is another one of the basic blocks used in quilting. It is deceptively simple, just two right triangles sewn together, but a building block for many other quilt blocks, and a must for all quilters to learn how to sew – and to sew well!

New quilters are often scared of the idea of cutting and sewing triangles, but really, the construction is straightforward, and as with the other quilt blocks we have made so far, there are different methods to make half-square triangles (often referred to as HST or HSTs). Even after years of piecing, I have tried different methods, different fabric choices, and different ways to press and trim… and learned new skills!

The secret isn’t just knowing how to make one HST, it’s knowing which method saves you time and suits the project you are working on. It’s also learning how to avoid certain mistakes that leave you with warped, wonky blocks.

Using different fabrics and arranging them into different blocks makes this a great way to try to empty your scrap bin!

What Is a Half Square Triangle?

As I said above, a half square triangle is simply two right triangles sewn together. They can be made in any size you like, and you can certainly take fabric squares, cut them in half on the diagonal, then mix and match them with another triangle and sew together. If you have a whole bunch of scrappy triangles, for example left over from other blocks, it’s a fun way to use them up.

The problem with this method is that you are cutting them on the bias, and the fabric will be very stretchy. So most methods of making hsts avoid this.

Different Ways to Make Half Square Triangles

There are quite a few ways to make hsts, but I’ll be touching on the two main ways in this blog post.

The Two-at-a-Time HST Method

I find that this is the easiest way to make half-square triangles, and it’s the method I teach and the one we will be using today.

You’ll get two identical HST blocks from two squares of fabric, and the process is so simple you can chain-piece dozens in one sitting.

1. Cut two fabric squares 1 inch larger than your desired finished hst size
2. Place them right sides together and make sure all edges line up perfectly
3. Draw a diagonal line from one corner to the opposite corner on the wrong side of the lighter fabric
4. Sew a 1/4-inch seam on both sides of that diagonal line
5. Cut along the drawn diagonal line with your rotary cutter using a ruler as your guide
6. Press the seam toward the darker fabric or according to your quilt pattern’s pressing plan
7. Trim the dog ears and square up your block to the unfinished hst size your pattern requires

Four-at-a-Time HST Method

Another popular method is the four-at-a-time one. You start with two fabric squares cut 1-1/2 inches larger than your finished size. Layer them right sides together, and sew 1/4 inch all the way around the square. Then cut on the diagonal from corner to corner both ways. You’ll end up with four identical half-square triangle blocks.

I don’t like this method as much because you end up with bias edges, which can easily warp the half-square triangle block.

Common Mistakes to Avoid

Beware of the Bias!

As I said above, working with triangles always brings with it the challenge of the bias of the fabric. If there are bias edges on your half square triangles, I would suggest

• using spray starch to help make the bias more stable,
• use lots of pins
• sew slowly so as to avoid stretching the fabric more
• PRESS carefully; don’t iron or pull the fabric around.

Check Your Seam Allowance!

Most patterns in quilting have a quarter inch seam allowance. This is even more important while making half square triangles. Most patterns don’t allow for wider seam allowances because quilters don’t like wasting fabric.

A few ways to ensure your seam allowance is correct are:

• Use a quarter inch foot.
• Sew a practice block, measure the finished size, and adjust your needle position or presser foot until you’re hitting the exact unfinished measurement your pattern requires.
• When drawing a diagonal line on the back of your square, use a regular pen or pencil, not a marker. Markers have thicker lines that, when lined up with the side of the quarter-inch foot, will lead to a wider seam allowance.
• Thread matters! If you’re piecing with thick thread, it takes up more room in the seam allowance and can throw off your finished hst size. Stick with 50-weight cotton thread for piecing and save the decorative threads for quilting.

Trim if you need to!

I think most of us seasoned quilters would skip this step if we could, especially if we are making an entire quilt out of hsts. However, it is worth doing for accuracy in the finished block or quilt.

Squaring up each block ensures they’re all identical in size and that your diagonal seam is exactly where it should be. Also, not trimming the dog ears, those little fabric triangles sticking out past your seam, will create bulk at seams and keep your points from nesting cleanly.

So it may take a little extra time, but it is usually worth it.

With our block today, we won’t need to do much trimming if your quarter-inch seam is accurate, but you can always just trim the dog ears off with a pair of scissors.

Ok, enough chatter, let’s get started on making some half square triangles!

Half Square Triangle Tutorial

Fabric Requirements for a Single Block – Twelve and a Half Inch Block Unfinished

Fabric

• background fabric
• two scraps of feature fabrics

These are the fabrics I have chosen for this block:

Cutting Instructions

From the background fabric:

• Cut 2 (two) 7″ squares

From the scrap feature fabrics:

• Cut 1 (one) 7″ square of each fabric

for a total of 4 (four) squares:

Step 1

We are going to start by drawing a diagonal line from corner to corner on the back of the two background squares:

Step 2

Take one of the background squares and place it right sides together with one of the scrap fabrics and pin:

Step 3

Now take the square to your sewing machine and sew a quarter-inch seam next to the line you just drew:

Sorry, it is a bit blurry! Simply line up the left of your quarter inch foot with the drawn line and sew. Then turn the block around and do the same on the other side of the line so that you have a seam on either side of the drawn line:

Step 4

Now we are going to cut along the line we drew. You can use a ruler and rotary cutter as I did, or use a pair of scissors:

Step 5

Take the sewn units to the ironing board, and set the seams by pressing along the sewn seam. Then press towards the dark triangle:

This is what it will look like from the back:

Step 6

Now we need to trim the block to make sure it measures the correct size of 6 1/2″. If you have used a proper 1/4″ seam allowance, there will not be much to trim.

Most quilting rulers have a diagonal 45 degree line:

While you can get speciality rulers like these to trim half square triangles, since this is a beginner’s series and I don’t want you to have to spend more money than you need to, I want to show you how to use what you have.

We are going to place the 45-degree line along the diagonal seam like this, and line up the block close to the edges of the 6 1/2″ ruler:

You can see that there is a little bit that can still be trimmed.. but not a lot. If you want to skip the trimming step, you can; otherwise, you can simply run your rotary cutter along the right and top sides, flip the block around, line up the diagonal line and sides of the ruler again, and then trim.

It’s hard to show how to do this with photos – you can get a clearer explanation from my video tutorial here on YouTube.

Repeat steps 2 to 6 with the remaining background and scrappy square so that you have four half square triangle units.

Step 7

Lay the four half square triangle units out like this:

Step 8

We are now going to sew the block together in the same way as we did the four-patch. Take the two top pairs, place them right sides together as shown in the picture below, and sew:

Bring it back to the cutting board and set it back in place to make sure everything is in the right position:

Step 9

Now sew the bottom two pairs together and bring them back to the cutting board to place them in the correct position.

Step 10

Before we sew the two rows together, we need to take them to the ironing board. Again, we need to set the seams first, then we press the seams in opposite directions. I pressed my seam on the top row towards the dark triangle and on the bottom row towards the background fabric.

Bring them back to the cutting board – this is what it will look like from the back:

Place them back in the right positions for the next step.

Step 11

Time to sew the rows together.

Flip the top row down on top of the bottom row. As we did with the first block, we are going to pin them, starting with matching up the centers. With the seams pressed in opposite directions, they should nest together nicely:

You can tell if you have “nested” them properly if you go over the join with your thumb and finger, and it is nice and smooth – no bumps!

Pin!

I like to pin on an angle with one half of the pin in the left seam and the other half in the right seam, like this:

Pin at each end and then add a few more in the middle…

Sew the rows together using a quarter-inch seam allowance.

Step 12

Now we need to take it back to the ironing board and set that center seam that we just sewed.

Press the seam towards the side with the darkest amount of fabric.

This is how it will look from the back:

All Done

And here is the finished block on the design wall, all done!

The block should measure 12 1/2″.

Here it is next to our first two blocks:

Printable Tutorial Style Pattern

You can purchase a printer-friendly and ad-free version of this tutorial for a small price here:

Half Square Triangle Quilt Block ~ A Tutorial Style Pattern ~

Christmas Version

Here is a pic of the Christmas one I made on YouTube using Lori Holt’s Home Town Holiday Fabric from the Fat Quarter Shop:

and all three Christmas ones on the design wall:

YouTube Video

You can watch the video tutorial here if you like:

Let me know in the comments if you like making half square triangles and what your favorite method is!

Pin and Share!