How to Trim Large Half Square Triangles

Half square triangles are a quilters staple. I love them because they can add so many options to a quilt design. I also kind of hate them because... trimming. The easiest way to trim half square triangles (HSTs) is with a ruler with a 45 degree angle. But what happens when your HSTs are too large for your rulers? We still gotta trim those guys. I'm going to show you how to use your cutting mat as a guide to trim extra large HSTs. 

Let's get started...

First off, you need a large cutting board with a 45 degree angle marked. This is important to make sure the points are kept when the block is trimmed. 

Large gridded cutting mat with a marked 45-degree line for aligning oversized half square triangle blocks.

Make sure your HSTs are pressed nice and flat. I prefer to press my seams open because I think it makes them flatter and easier to trim. But you do you, or what the pattern you are following says to do. 

Pink and orange half square triangle on a cutting mat, ready to be trimmed to 10 1/2 inches square.

I start by lining up the 45 degree angle on the mat with the seam on the block. In order to be able to see the line I shifted the block up and over one inch. So I'll be using the 11 1/2 in. and 1 in. lines on the mat as my guide (instead of the 10 1/2 in. and 0 in.). 

Half square triangle seam aligned with the mat's 45-degree line, overlapping the 1-inch and 11 1/2-inch guides.

Using the 11 1/2 in. line as a guide line up the ruler and trim off the edge of the block making sure it doesn't shift. 

Long quilting ruler positioned along the right edge of a large half square triangle, with a rotary cutter beside it.

Right edge of the pink and orange half square triangle trimmed straight, with a narrow fabric offcut beside the ruler.

Next, shift your ruler to the next side. In this case, I trimmed the bottom edge on the 1 inch line. Again, make sure the seam stays lined up with the 45 degree line on the mat. 

Ruler across the bottom edge of a half square triangle, with arrows showing seam alignment on the mat's 45-degree line.

Trim the next side on the 1 inch line. 

Long quilting ruler aligned vertically at the mat's 1-inch line to trim the left edge of the half square triangle.

Lastly, trim the final side at the 11 1/2 inch line. 

Long ruler aligned across the top of the half square triangle at the 11 1/2-inch mat line for the final trim.


Trimmed pink and orange half square triangle with sharp corner points and its seam following the 45-degree mat line.

Completed pink and orange half square triangle block trimmed to 10 1/2 by 10 1/2 inches on a cutting mat.

 

This method is for large HSTs only. It is nice not to have to flip the block if possible. I didn't have to because I can access all sides of my cutting table.

If trimming smaller HSTs it's always faster and more accurate to use a ruler with a 45 degree angle as your guide to trim with. 

The most important tip for these large ones? Go slow and make sure everything lines up before you trim! 

I use this method when trimming larger HSTs for my Easy Going Star Quilt Pattern. Some of the larger pattern sizes have blocks which I don't have square rulers large enough to trim. I am always pro saving money and using what you have. So this is a great way to use your long ruler instead of investing in a ruler you might not want to right now. 

Easy Going Star Quilt with colorful oversized stars and half square triangles displayed against weathered barn doors.

Above is a picture of the Easy Going Star Quilt. These large HSTs aren't hard to trim and there aren't very many so the quilt comes together quickly. You can buy a copy of the pattern here.

 I don't hate these extra large HSTs so much because there aren't so many of them. In fact I kind of love them. Give this trimming method a try next time you find yourself with super large half square triangles. 

Happy quilting!

Back to blog

Leave a comment

Please note, comments need to be approved before they are published.