How to Sew a Basic Half Square Triangle Block

Posted by Elaine Huff on

The Half-Square Triangle quilt block (also known as HST) is another basic building block for a ton of blocks.

half square triangle block

The math behind how to figure out how big to cut the squares is pretty easy. If your finished patch is to be 4” square, cut the squares at least 4⅞” square. I prefer to cut the squares at 5”, so that I have more wiggle room when trimming the resulting patch to 4.5” square. So a 2” finished patch would start with two 2⅞” or 3” squares, etc.

Fabric Requirements for Two 4" Finished Half Square Triangle Blocks:

  • Light/Green: 1 - 5" square
  • Medium/Pink: 1 - 5" square

half square triangle fabric requirements

Sewing Directions:

Step 1:

Match the two fabric squares right sides together.  Mark a diagonal line from corner to corner on the back of one of the squares and sew 1/4" (or a little less) on each side of the line.

half square triangle 2

Step 2:

Cut along the diagonal line.  You will have 2 half-square triangle patches.

half square triangle 2

Step 3:

Open up the patch and press the seam allowance toward the darker fabric.

half square triangle 3

Squaring Up:

Step 4:

Place the diagonal line on a square up ruler along the seam line of the patch with a little bit of fabric showing around all 4 sides. Trim the right and top sides with a rotary cutter as shown below.

half square triangle 4

Step 5:

Lift the ruler up, turn the patch 180 degrees, and line up the diagonal line of the square up ruler with the seam line. This time match up the left and bottom sides of the ruler with the left and bottom sides of the patch and trim the right and top sides of the patch with a rotary cutter as shown in the picture below.

half square triangle 5



Follow the attached links for four select tutorials YOU can make now that you understand the Half Square Triangle block!

P.S. To print a PDF of this tutorial, check out the free app at A reader and I tried it out and it worked great.

P.P.S. To figure out yardage for a quilt, check out this post:

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