How to make Half Square Triangles: A Quilting Tutorial

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Learn how to make half square triangles to create blocks for use in a range of popular quilts with this tutorial.

Making half square triangles doesn’t have to be difficult and I’ve explained it below in simple and easy-to-follow directions.

On some estimates, up to 80% of quilt patterns include half square triangles, or HSTs as they are sometimes called, so the half square triangle block is an absolute must to master.

Objective

This tutorial will provide instruction to allow the user to piece half square triangles to make blocks commonly used in a wide range of popular quilt patterns.

Goals

  1. The learner will learn to accurately piece and square up half square triangle block
  2. The learner will learn to combine blocks to create different half square triangle quilt patterns

Specifications

This tutorial is designed to be used after our Quilt Piecing tutorial is mastered. The skills taught in this lesson will build on those learned in Piecing a Quilt.  

Quilting Terms to know

Diagonal

A line drawn from one corner to an opposite corner at a slant

Squaring up

Trimming an oversized block to make it square and the correct size

Arrangement

Moving the blocks around and creating the look you want

Quilting Materials and Equipment

  • ¼ yard each of a light and a dark colored 100% cotton fabric
  • 12.5 inch acrylic quilter’s square
  • 6.5 inch acrylic quilter’s square with 45 degree diagonal line
  • Rotary cutter
  • Cutting mat
  • Pressing surface
  • Steam iron

The first thing you need to do is cut 5 (five) 4 ½ inch blocks from your dark fabric and set them aside. (See Piecing a Quilt tutorial for how to do this)

IMPORTANT- Please read

As we learned in the Quilt Piecing tutorial, if we want to create 12 ½ “ blocks, we must cut our squares to 4 ½” to provide for the seam allowance.   To create a half square triangle we will be sewing on the diagonal of the block to create two new blocks.  If we begin with 4 ½” square blocks, our finished triangle blocks will be the wrong measurement. For that reason we will be using different sizes in our blocks.  Be sure to read all the instructions carefully.

Instructions for making half square triangles

1.Measuring and cutting your fabric

Begin by cutting 2 (two) 5 ½” fabric squares from your light fabric and 2 (two) 5 ½” squares from your dark fabric.  We must cut them this size to allow for seam allowances.

2. Stack the fabrics

Stack the fabrics right sides together with the dark fabric on the top.

Marking for the diagonal seams

Use the 12 ½” acrylic quilters square to draw a diagonal line across the wrong side of the dark square

(This will become the cut line after you stitch and press the fabrics.)

Draw a dashed line ¼” on either side of the diagonal lineas shown below.  These will be your stitching lines.

Stitching your pieces

Take the pieces to the sewing machine, being careful to keep the squares lined up.  Stitch on the first dashed line. Cut the thread and turn the piece around so you can stitch on the second dashed line.  

Pressing the seams

Carry your stitched units to your pressing surface.  Set the seams by pressing the entire square on the dark side, then on the light side.

To get the triangle units, you will need to press the dark squares back.  Flip the dark triangle from one side of your unit toward the top and press it in place.

Now, fold that piece back to its original position. It will not lay flat because of the pressed crease.  

Spin the piece around top to bottom and flip up the other blue piece and press it in place.

Flip that one back down. It will look like this.

Spin the piece around top to bottom and flip up the other blue piece and press it in place. Flip that one back down. It will look like this

Cutting the piece and pressing the half square triangle blocks open

Take your pressed unit back to your cutting surface.  Using the 12.5” acrylic ruler, cut on the solid diagonal line you drew earlier.You now have two units. 

Take these units to the pressing surface and press the seams again. DO NOT stretch or skew the shapes.

Measuring and squaring your half square triangle blocks

You now have two square units, but they are not the right size.  Remember we cut our blocks big to allow for the diagonal seam allowances.  We need to trim them and make sure they are square on the diagonal seam.

For this step, you will need your 6.5” acrylic quilter’s square with the 45 degree angle marked.

Place your first unit on the cutting surface.  Place your acrylic square on top, lining up the 45 degree line with your diagonal seam.

You can see that our pieced unit is too large, so we will need to trim it to size.

Use the methods you used in the Quilt Piecing tutorial to trim two sides of your pieced HST unit.

Turn the unit and place the 4 ½” mark lines along the edges you just cut. Your diagonal line should line up perfectly.  Cut the remaining two sides with your rotary cutter. 

You will have a 4 ½” pieced half square triangle unit.  Repeat with your other HST units.

You can arrange the half square triangle blocks with your solid color blocks to make many different quilt blocks, including these. 

80% of the quilt blocks in existence are a combination of squares and triangles.

Final thoughts on how to make half square triangles

This tutorial has given you all the techniques and steps you need to make half square triangles and create blocks that are the foundation of most types of quilt.

Half square triangles are relatively simple to make and they look great.

You can find half square triangles in a wide range of quilt patterns.