3 Half-Square Triangle Formulas You Need to Know

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With variations in fabric, color and arrangement, half-square triangle quilt design possibilities are endless! And when you know how to make your HSTs just the right size, you can create new designs with just a little bit of math.

Many quilters want to know how to figure how large their initial fabric piece needs to be in order to achieve the size of the half-square triangle they want to make.

Here are three formulas I use to calculate fabric requirements for a specific size half-square triangle.

1. Making HSTs two at a time (the “traditional” method)

First, determine the size you want your unfinished HST block to be (the size before you sew your blocks together, taking into account your typical ¼" seam allowance). Add 1" to that number. This is the measurement you want to cut the squares to make your two half-square triangles.

Unfinished HST block + 1" = square sizes to make HSTs using the “traditional” two-up method

Here’s a handy chart and an example:

Click here for a tutorial on how to make two HSTs at a time.

2. Making HSTs eight at a time (also called the “Magic 8” method)

Again, determine the size you want your unfinished HST block to be (the size before you sew your blocks together, taking into account the ¼" seam allowance). Add 1". Then, multiply that number by 2. This is the measurement you want to cut the squares to make your half-square triangles.

(Unfinished HST block + 1") x 2 = square sizes to make HSTs using the “Magic 8” method

Here’s a handy chart and an example:

Click here for a tutorial on how to make eight HSTs at a time.  You’ll notice that I use more fabric to make my HSTs than this tutorial recommends. Again, I find it easier to have more fabric during the squaring up process, giving more precise results.

3. Making HSTs with strips of fabric

Determine the size you want your unfinished HST block to be (the size before you sew your blocks together, taking in account the ¼" seam allowance). This is the minimum measurement you want to cut the widths of the strips to make your half-square triangles.

Unfinished HST block =  strip width to make HSTs using the strip method*

*For unfinished HSTs that are 2½" or smaller, add an extra ¼" to the strip width as indicated in the chart below.

Here’s a handy chart and an example:

Click here for a tutorial on how to make HSTs using strips.

Don’t forget to square up your blocks before using them!

Click here for a few tips for achieving better, more precise half-square triangles.

Designing with half-square triangles

Once you master these formulas, you can design all kinds of quilted projects, from traditional designs to more modern projects. Here are just a few of my designs that use half-square triangles. You can see how versatile they are!

What will you design and make with the super versatile half-square triangle block?

ABOUT THE AUTHOR

Sheri Cifaldi-Morrill is a designer, pattern writer and award-winning quilter. With the help of books and the Internet, she taught herself how to sew and discovered her love of designing and making quilts. As a trained and practicing graphic designer, her quilts start with a concept and research shapes the design. Her work is inspired by her everyday life and experiences. Sheri strives to make beautiful objects that inspire others to make and learn by providing clear instruction and encouragement. In 2016, she was awarded the first annual Bluprint Quilt Designer Fellowship. Learn more about Sheri and her work at wholecirclestudio.com and on Instagram or Facebook .

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3 Half-Square Triangle Formulas You Need to Know