Half-Square Triangles: The Work Horse of Quilt Blocks

Today I’d like to talk about a quilt block I consider to be the backbone of most quilt blocks – the half-square triangle.

For the sake of argument, I realize the square and rectangle are also pretty solid contenders for the title of Quilting Backbone, but they’re simply just….squares and rectangles.  Depending on the fabric they’re comprised of, they can be …well…boring.  At least half-square triangles have a little more pizazz and they’re actually pieced.  I have quilted so long, I tend to overlook these blocks – which can be blocks all by themselves, or a block unit (part of a block).  I’ve made them as long as I can remember, and it wasn’t until I made this quilt:

That I realized just how versatile they are.  And while they’re not complicated to make, due to the bias the block employs, they can be tricky.  In this blog, I want to share some of my favorite ways to make half-square triangles (HSTs), how I handle the bias,  the mathematical formula to turn any square into an HST, and my sure-fire trick to making sure all my HSTs turn out the right size.

With the beginning of my research about HSTs, I wanted to fine out how many quilt blocks use half-square triangles as block units.  I searched Google, Bing, and Duck, Duck, Go and you know what I found?

None of the search engines would touch that question.  They would show me blocks with HSTs in them, but none of the three would even proffer a number. Not to be daunted, I searched Electric Quilt 8, hoping it would give me some sort of answer (keep in mind I have Dear Jane and Barbara Brackman’s Block Base in my EQ8).  It responded with only 50 blocks in it’s data base with HSTs.  To me that number seemed deceptively low, but it’s a starting point.

The next question I needed answered was how many quilts can be made from only HSTs – that is the half-square triangle is considered the block, and not a block unit.  I was little daunted on this one, too.  Google came back with the answer 999+.  It seems after 999 quilts, Google threw up its hands and just said, “A lot…a lot of quilts can be made out of half-square triangles.  Please don’t make me count anymore.”

To start, let’s take a look at the formula used to determine how big to cut your fabric patches in order to make a HST.  It’s super-easy math.  You take the size of the finished square and add 7/8-inch.  So, if you’re looking at this quilt block,

and you decide you want to make it, but you’re not sure how to manage the half-square triangles, the first thing to keep in mind is even though the until is made out of triangles, you must think about it as a single block, and not two triangles in order to get your measurements correct. Let’s say the HST is 3-inches, finished (the term finished  means you’re measuring the HST after it’s been sewn in the quilt block).  You simply add 7/8-inch to the finished measurement:

3-inches + 7/8-inch = 3 7/8-inches.

You would cut two blocks of fabric (one of each color of the half-square triangle) each 3 7/8-inches square.  Keep this formula in mind because it will work with most of the construction methods I will share with you.

The first HST method is the standard one – you cut two squares of fabric, slice them once on the diagonal to form triangles.  Then take one color triangle and a triangle of the other color, place them right sides together, and sew them together along the long side of the triangle (hypotenuse). 

The HST formula works with this construction method.  One helpful hint I’d give you at this point is to blunt the ends of the triangle. This helps you line up the triangles correctly and there are no dog ears to trim off at the end.  If you’re a little worried about blunting your triangles, there is always this little tool.

It’s the Marti Michell Corner Trimmer.  You can use it to trim up your ends. 

In the spirit of transparency, this is my least favorite way to make triangles for a couple of reasons.  First, it’s easy for the ends of the triangles to get chewed up by the feed dogs on your sewing machine.  Second (and usually most important in my world), it’s a super, super slow way of making half-square triangles.

The second construction method is one of my favorites, probably because this is the method I use the most, and I’m comfortable with it.  The reason I like this technique is you don’t have to worry about the bias so much when you’re sewing.  With the first method, the bias is exposed along the hypotenuse, and you have to be careful not to stretch it as you sew.  The bias is not exposed until the last minute with this approach. The half-square triangle formula works with this method just fine, so cut two squares the finished size plus 7/8-inch.

Draw a line diagonally across one of the squares, from one corner to another.  As this line will be cut away, feel free to use whatever marking tool you have nearby.  The idea with this method is to sew ¼-inch away from either side of this marked line.  If you have a quarter-inch foot, such as this one:

Use it on your machine to sew ¼-inch seam on either side of the drawn line.  If you don’t have a quarter-inch foot, don’t despair.  Use a ruler to draw a line ¼-inch on either side of the line or there’s this handy-dandy little tool called the Quick Quarter:

With this marking tool, you can draw a dashed line on the diagonal from corner to corner, and without moving anything, draw solid sewing lines ¼-inch away from the dashed line. 

Put the fabric squares right sides together and sew along the lines on either side of the true diagonal line, then cut the square apart on the diagonal line.  This will give you two half-square triangles.  Helpful hint for this method:  Mark the lightest color of fabric with the cutting and sewing lines.  By sewing with this fabric on top, it makes pressing towards the darker fabric much easier.

The third half-square triangle technique yields four HSTs at a time, and it is a bit tricky, because so much bias is exposed at once.  The half-square triangle formula does not work with this method.  In order to determine how big to cut your fabric, take the finished size of the HST block or unit and add ½-inch to 1-inch.  Due to all the exposed bias, it works better to make the half-square triangles a bit larger and then cut them down.  How large you make your squares is really up to you, but I will tell you it’s a lot easier to cut down larger squares than deal with any fiddly bias issues which result from squares you wish you would have cut at least a bit larger ask me how I know.

Let’s say we want our half-square triangles to finish at 4-inches.  In order to make this happen, we know we need to cut our squares out at 5-inches (which, by the way, means this size is perfect for charm packs).  Cut the squares out and press them well with starch or a starch substitute.  This will help stabilize the bias. With the right sides of the squares together, sew around all four sides of the square with a ¼-inch seam allowance.  Once again, place the lighter fabric on top to make pressing towards the darker fabric easier.

Using a rotary cutter and ruler, cut the sewn-together squares twice on the diagonal.This should produce four slightly over-sized HSTs.  I’ll talk about a couple of different ways to trim these down a little later in the blog. 

There are a couple of other ways to protect the bias.  Make sure your rotary cutter has a sharp blade in it.  A dull blade can drag across the bias and stretch it.  And if you don’t plan on using the half-square triangles right away, put off cutting them until you are ready.  Then, once they are cut, handle them as little as possible.

The next method is known as “Magic Eight.”  This technique makes eight HSTs at a time and note the half-square formula does not work for this method.  Magic Eight has its own mathematical equation.  You’ll need to cut two fabric squares for this, one light and one dark.  The math for Magic Eight works like this:

  1.  Take the finished size of the half-square triangle needed.
  2. Add 7/8-inch to the finished size.
  3. Multiply that by two.

Let’s say we need 3-inch finished half-square triangles.  The math would look like this:

3-inches + 7/8-inch =  3 7/8-inches

3 7/8 x 2 = 7 ¾-inches

We will need to cut two fabric squares, each 7 ¾-inches.

On the lighter fabric, draw an X, from corner to corner. Now, just like we did with second HST method, we will draw two additional lines, one on each side of the X, ¼-inch away from the X.  Right sides together, place the lighter fabric square on top of the darker one, and sew along the lines marked ¼-inch away from the X. 

Once that is done, we need to cut the half-square triangles apart by the following steps:

  1.  Locate the middle of the square and cut it in half along the middle, vertically.
  2. Locate the middle of the square along the right and left side, and cut it again in the middle, horizontally.
  3. Now cut the four squares apart on the drawn diagonal line on each square.

Press the seam allowance towards the darker fabric.

The next method uses 2 ½-inch strips or a jelly roll.  Since we’re using a pre-determined size (2 ½-inches), we know the largest finished size HST we can produce is 2-inches.  However, remember you can trim down these half-square triangles to the size needed.

The first step is to sew the 2 ½-inch strips, right sides together, along the long sides of the fabric, using a ¼-inch seam allowance.

Next, line the ruler with unfinished size marking of the HST needed on the ¼-inch stitch line.  So, in this case, the 2 ½-inch marking of the ruler will be on the ¼-inch seam.  The ruler will be at a 45-degree angle.  Then cut out the fabric around the ruler. Rotate the ruler around to the other side and repeat the same steps. Remember to keep the lighter fabric on top, to make pressing towards the dark a little easier.

The final way to make half-square triangles is to paper piece them.  While I don’t mind traditionally piecing larger HSTs, I consider anything much smaller than 2 ½-inch finished half square triangles trickier than I want to deal with.  However, paper piecing will produce perfect smaller HSTs and the bonus is paper piecing will help protect the bias.  Half-square triangle papers come in lots of forms.  There are these by Moda, which are super fun to make:

They also come on rolls:

And in traditional paper.

The very best thing about HST papers is most sizes are available for free on the internet!  Simply Google the size you need, and all kinds of options come up:

Some of these make two half-square triangles at a time, and others make several at once.  These papers are directional, so make sure you sew in the directions the arrows point, then cut them apart according to the directions (each set of papers may have their own instructions, so be sure to read before sewing and cutting).  Helpful hint one:  Place the lighter fabric on directly beneath the pattern.  Keeping the lighter fabric on top makes pressing towards the darker fabric easier.   Helpful hint two:  I keep the papers on until I’m ready to sew the half-square triangles together.  This seems to protect the bias a bit better.

Finally, I’d like to leave you with a few tricks I employ to make sure my HSTs come out the correct size, with an unstretched bias.

  1.  You may get frustrated with your half-square triangles coming out different sizes, even though you’ve cut your squares out accurately.  Sometimes this happens.  I can tell you with all honesty, the more your make HSTs, the less this happens.  Until that day, here’s a couple of tricks I use to help.  First, let’s look at the sew lines on the square. 

I don’t sew exactly on the sew lines.  I sew one or two threads behind the line, towards the corners of the square.  This gives you a little more wiggle room.  Your thread will take up some space, and the fabric itself will occupy a little room in the seam when the half-square triangle is pressed.  Sewing a thread or two to the right of the sew line buys you a little extra wiggle-room space.

  •  Another technique I use is to make the HST larger and cut it down.  I know we do this automatically when we make four half-square triangles at a time, but for those we use the formula with, I add a full inch instead of the 7/8-inch.  This would apply to the first two methods, as well as the Magic Eight technique.

Once the larger-than-needed unfinished triangles are made, then you have to cut then down.  This can be done the traditional way with a square ruler:

You line the diagonal seam up with the diagonal line on a square ruler, and trim to the size needed.  Then rotate the HST and repeat on the next two sides.

My favorite half-square trimming ruler is Eleanor Burns Triangle Square Ruler.  It is extremely rare I have a ruler which can only serve one function, but this is the exception to my “it-must-be-a-multi-tasker” rule.  It’s super-easy and super-accurate and you don’t have to press your squares open to trim.  Seriously, if a lot of HSTs are in your future, you probably want this ruler.  It sells for $16.95 on Amazon, and slightly less on Eleanor’s website

  •  No matter which construction method used, the fact is you will deal with bias.  All of these HSTs have a hypotenuse (the base of the triangle).  And to get that long hypotenuse, the squares, at some point, have to be cut on the diagonal and this exposes the bias.  This bias must be carefully handled, so it won’t stretch out of shape and cause your half-square triangle to be wonky.  In order to keep the bias in shape, there are a few things you can do.
  • Starch your fabric with starch or a starch substitute.  Spray the fabric lightly and then with a hot iron, press the cloth until it’s dry.  Repeat this process several times until the fabric feels almost stiff.  This stabilizes the bias.
  • May sure your rotary cutter blade is sharp.  A dull blade can drag across the fabric and stretch the bias. 
  • Once they are constructed, press (use an up and down motion with the iron, not a back and forth one) toward the dark fabric.  Sliding the iron back and forth can stretch the bias.  Steam is a personal matter.  I don’t tend to use steam on any bias, as wet fabric will stretch more easily than dry.  If for some reason the HST is wrinkled to the point I feel like some steam is needed, I lightly mist the fabric with a spray bottle filled with water and then press it with a hot iron. 
  •  Handle the half square triangles as little as possible.  If you’re sewing them now and not planning on using them right away, you may want to hold off cutting them apart until you’re ready to sew.  Once they are sewn and cut apart, press them, and store them flat. 
  • If you are using any directional fabric such as stripes or plaids or material which has an obvious “up and down”, remember the orientation of these designs will change with HSTs.  You can see how the stripes in my green fabric rotated different ways in this little wreath quilt made from half-square triangles.  Care must be taken when using these in HSTs.  If I use these in a quilt and they absolutely must be oriented the same way, I use a template to cut the triangles out individually.  Yes, this does take a great deal of time, but when I do this, I’m guaranteed all my directional fabric will be correctly oriented. 

Between this blog and my blog on Charm Quilts, I hope you make a quilt from half-square triangles. Almost all quilters need to have a good grasp on how to make HSTs quickly and correctly  And really, what’s not to love about a good half-square triangle?  Mindless sewing and so many quilt options.

A couple of closing comments.  First, I’m not employed by any paper piecing publisher nor Eleanor Burn’s Quilt in a Day.  Any products I mention on the blog I use myself and pay for myself.  My bias, good or bad, is drawn from my experience with the product, not from any paid endorsement situation.

Second, thank you everyone who emailed, commented, or direct messaged me about the loss of my Sam.  Let me tell you, after 22 years of him being my constant companion, the loss is deeply felt.  I find myself looking for him without thinking and it’s the weirdest thing to completely ignore the cat food aisle in the grocery store.  Huge shout-out to Faithful Companions Pet Care in Greensboro.  They handled the situation of a sobbing cat owner who was completely incoherent with care and compassion throughout the entire process.  Sam was greatly loved and given the best life I could give him.  I miss him more than I can explain.

Will I get another cat?  Probably.  But not right now.

Again thank you all my quilting friends!

Until next week, Make Your Quilt Yours!

Love and Stitches,


3 replies on “Half-Square Triangles: The Work Horse of Quilt Blocks”

I’m so sorry about Sam! I didn’t get a chance to read last week’s blog until today. I’m sure he and your other 2 furbabies will be waiting for you. As for HSTs, I’ve spent a long time avoiding them but you’ve given me a couple of different ways to try them. Take care my friend.

Thanks Eileen. It’s been adjustment. After 22 years of Sam and then suddenly he’s not here — it’s taking some getting used to. Let me know which HST method works best for you. There’s more methods out there, too. These are just the ones I use.

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