Half Square Triangles (8 at a Time) Math

Last week I showed you how to make 8 half square triangles all at once, but we still need to talk about how to figure out what size square to start with to get the right sized half square triangle (HST)!

First, it’s important to understand the difference between finished and unfinished block sizes. Finished is referring to the measurement of the block after it’s sewn into the quilt. Unfinished is after the components of the block have been sewn together but before it is sewn into the quilt.

For this calculation we start with the measurement of the finished block size and multiply by 2, then add 1¾.

Let’s use an example. Say I have a quilt with a finished HST block size of 4″.

4 x 2 = 8

8 + 1¾  = 9¾  

So we’ll need a starting block size of at least 9 ¾”. I say at least because that will give you exactly the right size HST without trimming. I always add at least ¼” and trim down the HST blocks to size afterwards because I’m not a particularly precise sewer.

Don’t want to do the math? Lucky for you I’ve put together this handy printable with several common sizes already mathed out for you! I also included the formula if you have a size that’s not listed. Click on the download button below to get it!

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Jan Hill
Jan Hill
May 19, 2020 4:23 am

Thank you so much for this. My pattern says to cut all the triangles individually first!! Nightmare. I knew there was an easier way but couldn’t work out the lines 🌹

May 23, 2020 6:08 pm

If I wanted to make 8 at a time and unfinished block to be 7.5 inches how large would I need to start out with please? I don’t trust anything I do anymore