How to Make Half Square Triangles

The humble half square triangle, or HST as it is often affectionately known, is one of the most foundational yet most versatile units in all of quilting. 

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A common, versatile patchwork unit, half square triangles are the foundations of many quilt blocks and a great introduction to sewing with triangles. Click to learn several ways to make HSTs and discover your favorite!

From standing on its own to being used to create pinwheels, flying geese, diamonds, arrows, and more, the half-square triangle is essential to creative piecing in many quilts. Quilters covet "perfect points," where all the seams are sufficiently precise to create sharp points on all three tips of the triangle. Making and trimming accurately sized HSTs is critical, so let's look at several ways to do just that. 

A quick note on terms: 
-Unfinished is the size of the HST unit by itself BEFORE it is pieced into the quilt. This measurement includes seam allowances
-Finished is the size of the HST when it is pieced on all sides into the quilt top. This measurement does not include seam allowances. 

Video Tutorial

How to Make Half Square Triangles with HollyAnne Knight of String & Story

Two at a Time

I think most quilters learn to make the HSTs this way first because it is straight forward and, when done properly, results in lovely HSTs. Begin with two squares that are 1 inch larger than the desired FINISHED HST size. Lay the two squares right sides together and mark one diagonal. Then, stitch 1/4" away from the marked line on each side. Cut along the marked line and press the HSTs open. Trim to unfinished size. 

This method is wonderful... unless you need to make 47595 HSTs or if you're like me and don't have time to mark every single one, thank you very much. If either of those things is true, this method becomes inefficient and inaccurate. For a high volume of HSTs, I recommend considering some other methods. If you don't like marking, I recommend a tool like the Clearly Perfect Angles template which gives you a stitching guide right on the bed of your machine rather than having to mark each individual pair of squares. 

How to Make Half Square Triangles with HollyAnne Knight of String & Story

Four at a Time

This method is definitely quick, and I first encountered it on a tutorial for a quick finish quilt pattern. You'll take two squares, place them right sides together, and sew a quarter inch seam all the way around. Then, cut an X across the square to make four HSTs. This method creates bias edges on the triangles, so I recommend using lots of starch and being careful to PRESS and not iron when you open up the triangles. The math is a bit of a bear, so here's a quick chart for some common HST sizes. 

Four At a Time HST Chart with HollyAnne Knight on String & Story

For those who really do want the math: 

Cut Square Size = 2 x √((Finished HST Size + 3/4")^2 / 2) + 1/2"

How to Make Half Square Triangles with HollyAnne Knight of String & Story

Eight at a Time

I wish I had discovered (read: bothered to investigate and actually try) this method sooner! With only four seams, you make 8 beautiful HSTs lickety split. First, pair up your squares right sides together. Then, mark (or imagine if you are using the Clearly Perfect Angles Template) an X on the back of the squares. Sew 1/4" on each side of each line of the X (4 stitching lines total). Then, rotary cut the X as well as a + on the squares to yield 8 HSTs.

Fortunately, this math is pretty straight forward:
Cut Square Size = 2 x (Finished HST size + 1")

How to Make Half Square Triangles with HollyAnne Knight of String & Story

Essential Triangle Tool

If you love scrappy quilts and especially if you use Bonnie Hunter's Scrap User's System, then her amazing tool, The Essential Triangle Tool, is the trick for you. Simply lay a pair of strip down that are the same width as the desired unfinished HST size, line up the red line, and go to town cutting out pairs! The HST pairs are cut with one dog ear missing which is why we don't add that extra 3/8 of an inch to the width of the strip-- the width of a dog ear is 3/8 inch! This same tool can also be used to cut out quarter square triangles and to turn "bonus triangles" from stitch and flip shapes into HSTs.

How to Make Half Square Triangles with HollyAnne Knight of String & Story

Bonus tip: To keep the pairs of triangles together as you prepare to stitch them, use starch. Before cutting, mist each strip with starch then lay right sides together while still damp. THEN press the strip. This will "stick" them together, making it easier to cut and stitch the triangles accurately. In fact, I was totally amazed at the accuracy of this method (demonstrated above with the bloc-loc ruler). There was almost nothing to trim!!

How to Make Half Square Triangles with HollyAnne Knight of String & Story

Triangles on a Roll

Triangles on a Roll are also very accurate since they are essentially foundation paper pieced. I had some flying geese papers from my girl Kristin Esser (which she used in a recent flying geese post— go check out her post here!). They had enough of the right grid lines that I was able to use them to test the theory of the Triangle on a Roll Method. The theory is good-- the triangles are relatively easy to make and come out accurately. However, this method requires having a roll of paper for every size of HST you need to make which seems tedious to me. Regardless, Diane Knott has an excellent article on the method if you're curious. 

How to Make Half Square Triangles with HollyAnne Knight of String & Story

HSTs from Strips

This is another method for "pre sewing" your HST seams before you actually cut them out. Start once again with strips (the math is more complicated here than with Bonnie's ruler), sew a quarter inch all the way around the edge, then use your regular rotary ruler at a 45 degree angle to cut out HSTs. You can see an excellent tutorial with diagrams and the math on the Coral + Co Blog. The biggest drawbacks of this method are the wasted fabric at the beginning and end of the strip and the bias edges on the HSTs which are prone to stretch and distort (starch, please!), and the tendency to be less accurate. Consider adding more width to the strips than the general math suggests to make sure you have room to square up your triangles properly.

Quick, easy, and accurate half square triangles are a breeze to cut and piece with your AccuQuilt Go! Click through to see my tutorial that will you have excited to make gorgeous HSTs with no trimming!

Using Your AccuQuilt Go!

Now that I have an AccuQuilt, this is hands down my favorite HST method! It’s fast, accurate, and, with a bit of practice, low waste. (If no-waste is your top priority, go with the Essential Triangle tool above, but if you want an even easier method and don’t mind the bit of waste, THIS is the secret).


Closing Thoughts

The Half Square Triangle is a gorgeous unit and worth learning several ways to make. You might find that your favorite method depends on how much fabric you have handy (some methods require quite big squares!), how many HSTs you need, and what tools you have handy. I recommend trying all the ways to figure out your top two or three to "keep in your back pocket."

PS Don't forget to pin this post so you can find it later!

PPS put those gorgeous new HST skills to good use with one of my most poplar patterns, Dogwood Blossoms!