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3 Half-Square Triangle Formulas You Need to Know

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.

Half Square Triangles in progress | Half Square Triangle Math | HST | HST Trimming | Quilt Blocks

Photos and diagrams via Sheri Cifaldi-Morrill of Whole Circle Studio.

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:

Half Square Triangles | Quilt | HST | Traditional  |  HST Chart | HST Tutorial | HST Math | Making 2 Half Square TrianglesFormulas to make half square triangles two at a timeClick 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:

Half Square Triangles | Quilt | HST | Magic 8 | HST Chart | HST Tutorial | HST Math | Making 8 Half Square Triangles Half Square Triangles | Quilt | HST | Magic 8 | HST Chart | HST Tutorial | HST Math | Making 8 Half Square TrianglesClick 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:

Half Square Triangles | Quilt | HST | Strip Method | HST Chart

Formulas for half square triangles made with strips of fabric

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!
Quilt | Modern Quilt | Fish | Little Fishies | Half Square Triangle | HST | quilt pattern | water | pattern

Little Fishies quilt pattern

Quilt | Modern Quilt | Half Square Triangle | HST | quilt pattern | placemats | autumn | fall | pattern | Shifted Houndstooth | Houndstooth

Shifted Houndstooth placemat pattern

Quilt | Modern Quilt | Sunset | Sunrise | Ocean | Half Square Triangle | HST | quilt pattern | water | pattern | Sun Salutations

Sun Salutations quilt pattern

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 Craftsy Quilt Designer Fellowship. Learn more about Sheri and her work at wholecirclestudio.com and on Instagram or Facebook.

15 Comments

gmakitty

good job.. so many wonderful things to share…. these are great… putting in my “bucket basket” thanks for giving of your time and talent…Gmakitty

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karen eggert

making two at a time chart is wrong unless she uses different terms for raw and finished sizes

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K.L. Swift

please explain because I don’t see where it is incorrect.

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Sheri Cifaldi-Morrill

Hi Karen, there are a few typos in the charts that should be corrected and uploaded by tomorrow.

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Terry Carney

That’s a great summary and reference but can you also explain when to use which method?
Thank you

Reply
Sheri Cifaldi-Morrill

Hi Terry. It just depends on how many blocks you need. If you’re making a lot, the 8 up or strip method is more efficient than the 2 up method.

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K.L. Swift

If you need a lot of triangles, use the 8 at a time – quicker easier.

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Rhonda

Sheri, thank you so much for the math – I always appreciate anything shortcuts that help me put blocks together more quickly. Is the math on the strip set correct after the 3″ unfinished HST block size? I haven’t tested it, but it didn’t look right to me — but then again, math isn’t my strong suit (such a detriment when quilting, lol). If you have to edit this, can you also include the 4 at-a-time method? Thank you again!

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Sheri Cifaldi-Morrill

Hi Rhonda. You’re welcome. There are a few typos in the charts that should be corrected and uploaded by tomorrow.

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Rebecca Barlow

Great reference! I think though that the math for the ‘Making HSTs Two at a Time’ using a 5″ square is incorrect. The unfinished and finished sized should be switched;
Unfinished: 4-1/2″
Finished: 4″

Reply
Sheri Cifaldi-Morrill

Hi Rebecca, Thanks for your comment. After the post was made, it we noted that there were some typos in the charts. They should be corrected and uploaded by tomorrow.

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Kristina Hill, Seattle, WA

Making a cool scrappy quilt now with light and dark HSTS. Thanks for the tips!

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Sheri Cifaldi-Morrill

Hi Kristina—Excellent! While there is some wasted fabric making the HSTs larger and trimming, I find it makes my block sizes and alignments more accurate.

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Mary Malette

I’m about to share your charts with my class at the senior center. Have all the corrections been made. Thanks

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Toby Lischko

The problem with your last method is that they are quarter-square triangles with the long edge on the straight-of-grain. You don’t say how to measure and cut those. The right angle of the triangle needs to be on both straight-of-grains. This method is best used for templates with the corners trimmed.

Reply

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