Half Square Triangle Calculator

Our half square triangle calculator will calculate the size of the fabric squares needed to make half square triangles. 3 different half square formulas included!

Looking for a formula to make half square triangles? We’ve got something even better – an HST quilt calculator!

Our half square triangle calculator will tell you the exact size you need to cut your starting squares for HSTs. No matter which HST method you plan to use: 2-at-a-time, 4-at-a-time, or 8-at-a-time! The HST quilt calculator will do all the half square triangle math for you!

Calculating half square triangles has never been easier!

How To Use The Half Square Triangle Calculator?

To run your HST calculation, simply enter the desired finished size of the half square triangle you need.

We recommend using 0.25” increments for your finished HST size. (i.e. 2.00, 2.25, 2.50, 2.75 etc.)

NOTE: Finished size vs. unfinished size of half square triangle

The finished size of the half square triangle is the size you want your half square triangle (HST) to be AFTER it has been sewn into the finished project. This is the size of the half square triangle WITHOUT the seam allowances.

On the other hand, the unfinished size is the size of the HST BEFORE it’s sewn to the other pieces. Normally, the unfinished size is ½’’ bigger than the finished size. That is because the unfinished HST is sewn with a ¼’’ seam allowance on each side of the block – 2 times ¼’’ is ½’’.

Press CALCULATE to run the calculation.

Reading The Results Of The Half Square Triangle Calculator

The calculator will do the half square triangle math and provide three different calculations. 

They are the exact size of the starting square you need to make the desired size HST following the specified method:

  • 2-at-a-time half square triangles
  • 4-at-a-time half square triangles
  • 8-at-a-time half square triangles 

Half square triangle Dimensions

Additionally, the calculator provides the dimensions of your half square triangles:

Unfinished HST size: the size you need to trim your HSTs to in order to get the required finished HST size.

Finished HST size: the size of HSTs after they’ve been sewn in the finished project

All calculations are rounded UP to the nearest 0.25’’ increment. (i.e. If the calculation is 1.613, the calculator automatically rounds it up to 1.75.)


The provided calculations minimize fabric waste. This means the calculated starting squares are just big enough to make the required size HSTs.
Make sure you use a scant ¼’’ seam when sewing your half square triangles to allow for room for trimming.

Alternatively, if you want more room for trimming your half square triangles, increase the calculated starting square by a ¼’’.

Half Square Triangle Instructions

There are different construction methods for making HSTs. They differ in the number of resulting half square triangles and of course the fabric requirements.

We have step-by-step tutorials for each of the different construction methods. Click on the links below to get instructions for each of the methods:

HST Block Chart

If you want to have the construction methods AND the dimension charts all in one place, you are going to love this freebie!

It includes not only the dimensions of the starting squares required to make different-sized HSTs, but also handy construction diagrams. This way you’ll never wonder how different methods come together!

You can get this chart right in your inbox, if you just sign up for our newsletter using the form below.

Half Square Triangle Calculator FAQ

Explore Other Quilting Calculators

Disclaimer: Our calculators have been exclusively designed and copyrighted by Designed to Quilt. They are made available for unrestricted use, solely intended for personal, non-commercial purposes. Designed to Quilt does not provide any warranty or representation regarding the calculators’ functionality or precision. Designed to Quilt will not be held liable for any damages resulting from their utilization.


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