How to Sew a Half Square Triangle Quilt Block (Free Pattern)

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ByCaroline Fairbanks-Critchfield Updated on

Learn to sew a big easy Half Square Triangle block! The HST (half square triangle) unit is found in many patchwork quilts. Sometimes a bunch of half square triangle units are sewn to make a single quilt block, and sometimes the half square triangle is the quilt block!

Every quilter has to know this easy HST technique!

This free quilt block pattern shares how to sew a big beautiful HST block that finishes at 12”.

These easy instructions will show you how to easily sew two HST blocks at a time.

There are 8 of these blocks in our Kitchen Sink Quiltalong Quilt, so the cutting instructions are for 8 blocks. Of course, you can easily adapt the technique and sew from 2 to 200 blocks!

Or you could sew 16 big HST blocks and make any one of these gorgeous throw quilts! My free Modern Half Square Triangle Quilt 4 Ways Pattern includes four half square triangle quilt layouts.

Plus I have even more patterns for half square triangle quilts.

Download the HST block pattern as a free printable PDF file.

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Instructions for sewing 12” (finished size) Half Square Triangle Quilt Blocks

These instructions make 8 HST blocks. Finished block size: 12’’ x 12’’ (they are 12 1/2’’ x 12 1/2’’ before you sew them in the quilt).

You will need:

  • 2 different colors or prints of fabric
  • a rotary cutter acrylic ruler, and mat
  • sewing machine
  • thread (I suggest thread matching the light gray fabric)
  • ironing board and iron

Cutting:

  • Light Gray (or other light fabric):
    • cut 4 squares 13” x 13”
  • Deep Coral (or other contrasting fabric):
    • cut 4 squares 13” x 13”

Variations:

If you are not making our quiltalong quilt, here are some ideas:

  • Two squares of fabric will yield two HST blocks
  • If you would like different size HST units, start with squares of fabric 1” larger than your final size. Trim and square the HST units to be 1/2” larger than your final size, then sew them in your quilt block or quilt!

Assembly:

Use a scant 1/4’’ seam allowance and a medium stitch length such as 2-2.5.

1. Place one light gray square and one deep coral square right sides together. Use a pencil or fabric pen to draw a diagonal line across the light gray square. This is a cutting line.

Pin the squares together.

2. Sew a scant 1/4’’ away from the cutting line on on both sides of the drawn line.

3. Cut the piece in half diagonally along the line. 

4. Open and press the two blocks. Trim each block to 12 1/2’’. 

Repeat to make 8 blocks. Trim and square to 12 1/2” for accuracy.

This is Block #3 in our Kitchen Sink Quiltalong

Get ready to sew all your favorite quilt blocks this year! This is the year that we are going to sew everything but the kitchen sink in our Kitchen Sink 2022 Quiltalong.

This bold graphic quilt is perfect for learning new quilt making skills that build on one another, or for putting skills you already have into a big beautiful quilt.

Tips: As you work block by block on your kitchen sink quilt, save your scraps because you will be able to use them later on.

These awesome scrapbooking bins are perfect for keeping your finished blocks safe until it’s time to sew them together into a quilt top.

Here are the 10 free quilt block patterns in the Kitchen Sink Quiltalong:

Don’t miss all my other free quilt patterns, too!

xoxo,

Disclosure: some of my posts contain affiliate links. If you purchase something through one of those links I may receive a small commission, so thank you for supporting SewCanShe when you shop! All of the opinions are my own and I only suggest products that I actually use. 🙂

Caroline Fairbanks-Critchfield is an author and publisher who is passionate about the world of sewing and quilting. You can find her work in books, sewing patterns, magazines, and articles all over the web. She founded and publishes her ideas on the popular sewing website SewCanShe.com.