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How to Sew a Half Square Triangle - Four at a Time

Written by Kimberly Coffin · This post may contain affiliate links · 83 Comments

65 shares

Learn how to sew a half square triangle unit fast and with no fabric waste - with the simple, 4-at-a-time HST method.

When you are first starting out in the quilting world, all the complex, multi-step quilt blocks can be quite intimidating - not to mention confusing. But did you know that some of the most intense quilt blocks are actually made up of really simple blocks?

4 green and white half square triangles

The half square triangle (HST) block is one of these “simple” blocks that is used extensively in creating all types of quilt blocks.

Today, we’re going to go over the steps in creating 4 HST blocks at one time. Plus, I'm sharing my math cheat sheet to make it super simple for you to make any size of HST you need!

Table of Contents

  • What Kind of Quilt Blocks Can I Make with HST?
  • Supplies You Need To Make HST Blocks
  • DOWNLOAD THE FREE MATH CHEAT SHEET HERE!
  • Half Square Triangles- “4 at a time” Method
  • Step 1: The Math
  • Step 2: Cut the Fabric
  • Step 3: Sew the Fabric
  • Step 4: Cut the 4 HSTs 
  • Step 5: Press the HST blocks
  • Step 6: Trim to size

What Kind of Quilt Blocks Can I Make with HST?

I know you’re probably wondering, is the HST block really “that” versatile? How many different blocks can it be used for? Here are just a few example blocks that use only HST in their construction:

Isn’t it amazing how completely different these 4 blocks look? Each one is completely made up of HST units! Now that we have your inspiration peaked, let’s look at what we will need to make some half square triangles for ourselves.

Supplies You Need To Make HST Blocks

  • Fabric- When you are practicing a new block, it’s always best to use some fabric that you aren’t in love with just in case you need to tweak your measurements or design. 
  • Sewing Machine
  • Thread- 100% cotton works great for quilts.
  • Cutting mat and rotary cutter-Yes, you can use scissors, but a mat and rotary cutter are going to help you get the precise cuts that you need for quilting. 
  • Ruler-Used for measuring as well as a straight edge to get perfectly straight cuts.
  • Marking Pen (optional)- I recommend using a pen that has washable or vanishing ink. 

DOWNLOAD THE FREE MATH CHEAT SHEET HERE!

downloadable math cheat sheet for 4 at a time half square triangles

Half Square Triangles- “4 at a time” Method

Now comes the fun part, actually making your first half square triangles! This first step is going to be a doozy (the math step), but don’t worry, I’ve broken it down for you so it will be much easier to understand.

various sizes of half square triangles laid out on a colorful purple backdrop

Step 1: The Math

Before we get into the equation, I want to make sure you know these terms:

  • Cut size- refers to the size that you cut your fabric to before any sewing. 
  • Goal size- refers to the size of the block after we have pieced it together and before we have sewn it into the quilt. Sometimes called the “unfinished size”.
  • Finished size- refers to the size of the block after it has been sewn into the quilt and there will be no more sewing on this block. 

Knowing these terms is very important when figuring out the math for your specific project. Now let’s look at the equation:

y= 0.7071x - 0.3535

I know…yikes! Don’t worry, I’ll break it down for you.

y= Goal size

x=Cut size

To find “y” we need to first choose “x”. For example, if you have a layer cake (a precut fabric bundle of 10” squares), you will take 10 and multiply it by 0.7071.

10 x 0.7071= 7.071

Now subtract 0.3535 from 7.071

7.071-0.3535= 6.7175

Now round that number down to the nearest quarter of an inch, ie 0.25, 0.5, 0.75, 1. You will need to trim the squares down to this new rounded number. 

6.7175 rounded down will be 6.5. This means I will be able to get 4- 6 ½” squares from a 10” square. 

Now, if you don't want to deal with all this math, I've put together a math cheat sheet for you. This sheet tells you exactly what size to cut your starting squares to create any size of HST! You can download the cheat sheet above.

Step 2: Cut the Fabric

You will need 2 equal squares to create 4 HST at a time. Choose 2 fun, coordinating fabrics.

I am going to cut 2- 10” squares for this example to go along with the math we did in Step 1. 

Step 3: Sew the Fabric

Optional:  If you want, you can mark a line all around the circumference of the square to make sure you sewing ¼” all the way around, but most sewing machines come with a ¼” foot that will help with this. 

Take your squares to your sewing machine and place them right sides together. Make sure they are lined up perfectly, if you need to, place a few pins in the fabric to hold it in place.

Pro Tip: Make sure you are not pulling or tugging the fabric while you are sewing. This will create puckers and over-stretch the fabric which is definitely not what you want. 

Sew around the entire square, making sure you are leaving a ¼” seam allowance. If you mess up, don’t be afraid to pull out your seam ripper, snip the stitches, and start again. You want this seam allowance to be precise. 

Step 4: Cut the 4 HSTs 

Take your sewn square back to your cutting mat and use your ruler as a guide to cut the square diagonally, creating 2 large triangles.

Now cut the opposite diagonal line to create the 4 half square triangles. Pretty neat, right? 

Step 5: Press the HST blocks

Lay each HST unit on your ironing board right sides facing up and press the seam towards the darker fabric. Pressing towards the darker fabric helps to avoid seam allowances showing through light-colored fabrics.

Pro Tip: Be careful when you are pressing that you aren’t running your iron back and forth on the fabric. You don’t want to accidentally stretch the HSTs out of shape. Just take it nice and easy and press with an up-and-down motion. Think press, not iron. 

Step 6: Trim to size

You definitely want to do Step 5 before Step 6. Pressing the blocks will ensure they lay nice and flat for trimming. Now it's time to trim the blocks to the desired size. For this example, I want 6 ½” blocks. 

See the excess fabric on the sides? It may not seem like a lot, but think about it: If you have that much excess fabric on each block of a quilt top, by the time you have completed the quilt the measurements will be WAY off. 

To square up these blocks perfectly, use the 45° line on your ruler and line it up with the seam of the block. Trim a small amount off the 2 edges emerging from the ruler.

Then, flip the ruler, lining the 45° line up with the seam in the other direction. Trim the remaining 2 edges, trimming the square down to exactly 6.5 inches.

Take a step back and admire your gorgeous half square triangle quadruplets!

Once you get past the math of this method, these blocks are so fun to make - and extremely versatile!

No matter what your skill level is, the HST unit will become your friend and you will thank yourself later for learning how to make this awesome foundational block.

a diagonally arranged row of half square triangles in various sizes and colors

 Want a method to make even more HST units at once? Try this: How to Make a Half Square Triangle 8-At-A-Time.

Want to remember this? Save it to your favorite board on Pinterest here.

« *NEW* Cricut Venture- Everything You Need to Know!
How to Sew a Half Square Triangle - Two at a Time »

Reader Interactions

Comments

  1. Tiffany says

    July 22, 2023 at 12:28 pm

    Love the look and patterns for the half square triangles

    Reply
  2. Michelle says

    July 22, 2023 at 12:38 pm

    I like how simple it is for a great look

    Reply
  3. Susan says

    July 22, 2023 at 12:51 pm

    I love how this is no waste!

    Reply
  4. Denise says

    July 22, 2023 at 12:52 pm

    Love this! Such a great idea.

    Reply
  5. Kristi says

    July 22, 2023 at 12:52 pm

    Great tutorial, thanks for sharing!

    Reply
  6. Bill Webb says

    July 22, 2023 at 12:56 pm

    I like the stress point of the formula, it stinks when your off on the final piece and the seam's don't line up. You put the work in and you want a great product coming out, not ''just okay''..

    Reply
  7. Sandra says

    July 22, 2023 at 1:09 pm

    Love HST projects and with the 4 at a time and 8 at a time method it makes it a breee. Thanks for this post and instruction.

    Reply
  8. Nancy says

    July 22, 2023 at 1:17 pm

    Lots of great time savers!!

    Reply
  9. Jan Hudson says

    July 22, 2023 at 1:21 pm

    A must know for quilters!

    Reply
  10. Chris says

    July 22, 2023 at 1:23 pm

    This would be so much faster than trying to cut with scissors!!

    Reply
  11. Vicky says

    July 22, 2023 at 1:30 pm

    I have been wanting to make a quilt this seems like a great way to start

    Reply
  12. Catherine B says

    July 22, 2023 at 2:13 pm

    This is a wonderful tutorial!

    Reply
  13. Helen J Roberts says

    July 22, 2023 at 2:20 pm

    Great cheat sheet and easy to follow tutorial. Thank you for sharing.

    Reply
  14. Jess says

    July 22, 2023 at 2:30 pm

    Learning to quilt has been a long time goal- so excited to try this!

    Reply
  15. Amber says

    July 22, 2023 at 2:31 pm

    These methods are definitely tried and truce! Thanks for the quilting tips. I need to print this now!

    Reply
  16. Tom Bellamy says

    July 22, 2023 at 2:37 pm

    Nice tutorial!

    Reply
  17. Kelly says

    July 22, 2023 at 2:38 pm

    Good job. I love it

    Reply
  18. Darshna says

    July 22, 2023 at 3:05 pm

    Thank you for sharing easy to follow tutorial!

    Reply
  19. Sam says

    July 22, 2023 at 3:12 pm

    Great tutorial on a classic quilt style

    Reply
  20. Lacey Jurke says

    July 22, 2023 at 3:24 pm

    Going to try my first quilt this summer!

    Reply
  21. Laura says

    July 22, 2023 at 3:37 pm

    I would LOVE to improve my quilting!

    Reply
  22. Phyllis Huebbe says

    July 22, 2023 at 4:13 pm

    Great tutorial!

    Reply
  23. Cathy L Brown says

    July 22, 2023 at 4:36 pm

    I love and need directions when doing this.

    Reply
  24. Michelle Miller says

    July 22, 2023 at 4:53 pm

    Thanks for the tips!

    Reply
  25. Ericka La Tour says

    July 22, 2023 at 5:03 pm

    Love this!

    Reply
  26. Fran Evans says

    July 22, 2023 at 5:12 pm

    Love the explanation of it! Sounds very simple.have to try it thank you for sharing

    Reply
  27. Claire Cohen says

    July 22, 2023 at 6:02 pm

    Love the samples of different quilt blocks and the cheat sheet!

    Reply
  28. Wendy Whitbeck says

    July 22, 2023 at 6:25 pm

    This take so much of the work out of it! Love it!!

    Reply
  29. Dan says

    July 22, 2023 at 8:03 pm

    Great explanation!

    Reply
  30. Amy says

    July 22, 2023 at 8:36 pm

    I am a beginner and I absolutely love your tips and tricks

    Reply
  31. Kathleen B says

    July 22, 2023 at 8:44 pm

    Thanks for the guidance

    Reply
  32. Pamela Rodriguez says

    July 22, 2023 at 8:50 pm

    I’m just getting back into sewing, so I appreciate all of your tutorials!

    Reply
  33. TracyZ says

    July 22, 2023 at 8:54 pm

    Thank you!

    Reply
  34. Daphne says

    July 22, 2023 at 9:13 pm

    Thank you for sharing!

    Reply
  35. Maggie says

    July 22, 2023 at 10:14 pm

    very clear!

    Reply
  36. Michael Robles says

    July 23, 2023 at 12:16 am

    Loved this. Great idea

    Reply
  37. Kmarie says

    July 23, 2023 at 1:16 am

    Thank you for the easy to follow instructions!

    Reply
  38. Jennylyn Gross says

    July 23, 2023 at 1:23 am

    thanks for the info

    Reply
  39. Elaine Lozano says

    July 23, 2023 at 2:25 am

    Great tips!

    Reply
  40. Maddje says

    July 23, 2023 at 3:38 am

    That’s awesome!!

    Reply
  41. Diane R. says

    July 23, 2023 at 4:36 am

    I liked this tutorial so much, I think I downloaded it more than once. Thank you for posting.

    Reply
  42. Taryn says

    July 23, 2023 at 5:53 am

    Thanks for your amazing tips and instructions.

    Reply
  43. Dianna D. says

    July 23, 2023 at 6:36 am

    Thanks for the cheat sheet!

    Reply
  44. Jeff J. says

    July 23, 2023 at 7:18 am

    Nice tutorial for triangle squares!

    Reply
  45. Carolee Cucinella says

    July 23, 2023 at 10:00 am

    Thanks for the tutorial!

    Reply
  46. Catherine B says

    July 23, 2023 at 10:27 am

    Love the chevron design!

    Reply
  47. Cathy says

    July 23, 2023 at 1:45 pm

    I wish I could quilt ... this looks cool!

    Reply
  48. Mary says

    July 23, 2023 at 2:42 pm

    Feeling motivated to try this!

    Reply
  49. Phyllis Huebbe says

    July 23, 2023 at 3:11 pm

    I don't quilt but this is good info!

    Reply
  50. Barbara says

    July 23, 2023 at 8:28 pm

    I don't sew but this looks interesting

    Reply
  51. Kathleen B says

    July 23, 2023 at 11:02 pm

    looks so simple

    Reply
  52. Maureen Clark says

    July 24, 2023 at 12:11 am

    thanks for sharing!

    Reply
  53. Elisabeth Lenzi says

    July 24, 2023 at 2:35 am

    idk if i would be able to quilt

    Reply
  54. Dena says

    July 24, 2023 at 4:50 am

    Wow, you are soo talented! wish I could think of those things!

    Reply
  55. Dawn Curtis says

    July 24, 2023 at 10:46 am

    Great tips!

    Reply
  56. EDELGARD says

    July 24, 2023 at 1:34 pm

    Great tutorial, thanks for sharing!

    Reply
  57. Catherine B says

    July 24, 2023 at 2:04 pm

    Love the versatility of what you can do with the half square triangle.

    Reply
  58. Denise says

    July 24, 2023 at 4:17 pm

    Thank you for the great ideas

    Reply
  59. Valerie says

    July 24, 2023 at 6:43 pm

    I am definatley going to try this!

    Reply
  60. Phyllis Huebbe says

    July 24, 2023 at 9:49 pm

    Great tutorial if you make quilts!

    Reply
  61. Kathleen B says

    July 24, 2023 at 10:56 pm

    Great idea

    Reply
  62. Elaine Lozano says

    July 25, 2023 at 1:12 am

    Thanks for sharing this tutorial!

    Reply
  63. Gloria says

    July 25, 2023 at 2:27 am

    Thank you for this and the reminder...for all newbies...don't stretch the fabric and you will have beautiful perfect HSTs. Thanks for all you share!!

    Reply
  64. Phyllis Huebbe says

    July 25, 2023 at 4:04 pm

    For someone who quilts this is a great tutorial!

    Reply
  65. Lorena z says

    July 25, 2023 at 6:05 pm

    Thanks for the tutorial!

    Reply
  66. Liz says

    July 25, 2023 at 8:20 pm

    Love this

    Reply
  67. Krista says

    July 25, 2023 at 9:11 pm

    So many patterns can be madee with the 1/2 squares. Thanks for sharing!

    Reply
  68. Pye says

    July 25, 2023 at 9:19 pm

    It's nice to know that even complex projects can be broken down into much simpler steps 🙂

    Reply
  69. Susan says

    July 26, 2023 at 12:38 am

    Thanks for the cheat sheet. I like all the different ways the1/2 squares can go together.

    Reply
  70. dee says

    July 26, 2023 at 3:56 am

    You're so talented. I need to learn more from you!

    Reply
  71. Kathleen B says

    July 26, 2023 at 3:59 am

    Love it

    Reply
  72. Catherine B says

    July 26, 2023 at 12:44 pm

    Love the bright contrasting colors you used in your instructions.

    Reply
  73. Phyllis Huebbe says

    July 26, 2023 at 2:09 pm

    The math is what usually keeps me from trying to quilt.

    Reply
  74. Kathleen B says

    July 26, 2023 at 7:42 pm

    So efficient

    Reply
  75. Dan says

    July 26, 2023 at 11:43 pm

    Thank you

    Reply
  76. Brooke says

    July 27, 2023 at 12:57 am

    Thanks for doing the math! This awesome!!

    Reply
  77. Dee says

    July 27, 2023 at 5:41 am

    My goodness, you are so talented! Great idea!

    Reply
  78. Joy Gifford says

    July 27, 2023 at 3:28 pm

    This was so much simpler and easier, thank you.

    Reply
  79. Phyllis Huebbe says

    July 27, 2023 at 4:23 pm

    I wonder if my grandmother knew this technique

    Reply
  80. Kathleen B says

    July 27, 2023 at 11:41 pm

    Love the patterns you can make

    Reply
  81. Kathleen B says

    July 28, 2023 at 7:49 pm

    I had a quilt like this growing up

    Reply
  82. Sharon Aurora says

    July 28, 2023 at 11:26 pm

    Great tutorial!

    Reply
  83. Bob Lozano says

    July 29, 2023 at 1:36 am

    Thanks--this looks great!

    Reply

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Ready, Set, Sew! An online sewing course for beginners by top US sewing blogger, Sweet Red Poppy.

Hello, I’m Kimberly, the creator of Sweet Red Poppy!

I’m a wife, mother to 3, seamstress, and lover of all things crafty! Sweet Red Poppy is my creative outlet where I find and create inspiration. It’s a place where I can share my never-ending sewing projects and my love of learning!

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Ready, Set, Sew! An online sewing course for beginners by top US sewing blogger, Sweet Red Poppy.

Hello, I’m Kimberly, the creator of Sweet Red Poppy!

I’m a wife, mother to 3, seamstress, and lover of all things crafty! Sweet Red Poppy is my creative outlet where I find and create inspiration. It’s a place where I can share my never-ending sewing projects and my love of learning!

Read more about me →

The Latest

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  • Aeon Mira5 S Sweet Red Poppy
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