Two ways to trim large Half Square Triangles without a square ruler

Today's tip came about when I found myself needing to trim a bunch of half square triangles that were larger than my 6 1/2" square ruler that I usually use to trim. I had to find an alternative and looked down at my cutting mat and hoped it could help me out!

Of course, if you happen to own larger square rulers then you can use those. But right now I don't and I don't plan to shop for rulers right now. But I am making a shopping list! ;)

Here are 2 different ways that I came up with to use the lines on a mat to help trim. I also used my 18" long ruler and my rotary cutter.

Hopefully one of them will work for you too.

First Way to Trim with Cutting Mat

Trim First Side

• Line up the diagonal seam with the 45° angle line on the cutting mat - the line might even read '45' on your mat.  See the black arrows in the photo.

• Line up two sides of the block with a lines on the mat. In this case, I lined up the top and left edges, but it might be easier to line up the bottom and right edges. 

• Use the 18" ruler to trim the side NOT on a line to the required size. See the pink arrow in the photo. Here I am trimming to the 1/2" using the line on my cutting mat and straight edge of the ruler.

Trim Second Side

• Turn the block clockwise to trim the next side. The first trimmed side is at the bottom.

• Line up two sides (top and left) with lines on the cutting mat. See the white arrow in the photo.

• Trim the right side using markings on mat and edge of ruler. See the pink arrow in the photo.

• NOTE the diagonal seam of the block is not lined up with any markings in this step.

Trim Third Side

• Turn the block clockwise to trim the next side. The second trimmed side is at the bottom and the first is now to the left lined up with the cutting mat.

• Line up the diagonal seam with the 45° line on the cutting mat. The top edge will also be on a line. See the black arrows in the photo.

• Trim the right side using markings on mat and edge of ruler. See the pink arrow in the photo.

Trim the Final Side

• Turn the block clockwise to trim the next side. The third trimmed side is at the bottom.

• Line up two sides (top and left) with lines on the cutting mat.

• Trim the right side using markings on mat and edge of ruler.

• NOTE the diagonal seam of the block is not lined up with any markings in this step.

Trimming is complete and the block should be the desired size.

But that was a LOT of turning! I decided to try the same steps, but with my rotating cutting mat so that I didn't have to lift and turn the block. It definitely made the trimming go faster. My rotating mat can only accomodate a block that untrimmed is 12" square, but maybe you have a bigger one!

Second Way to Trim - with a Rotating Cutting Mat

Trim First Side

• Line up the diagonal seam with the 45° angle line on the rotating cutting mat - the line might even read '45' on your mat.  See the black arrows in the photo.

• Line up two sides of the block with a lines on the mat. In this case, I lined up the top and left edges because I trim the right edge. You could do it the other way as well - line up the bottom and right edges to trim from the left.  

• Use the 18" ruler to trim the side NOT on a line to the required size. See the pink arrow in the photo. Here I am trimming to the 1/2" using the line on my rotating cutting mat and straight edge of the ruler.

Trim Second Side

• Rotate the mat! I went clockwise.

• Since I'm trimming the right edge, I now trim the new right edge and don't move the block. 

• For this edge, I trim to the marked line.

Trim the Third Side

• Rotate the mat! I went clockwise. 

• Since I'm trimming the right edge, I now trim the new right edge and don't move the block. 

• For this edge, I trim to the marked line.

Trim the Final Side

• Rotate the mat! I went clockwise again.

• Since I'm trimming the right edge, I now trim the new right edge and don't move the block. 

• For the final edge, I trim to the 1/2" mark on the rotating cutting mat.

The second way is much quicker, but you might not have a rotating cutting mat. 

Both are accurate ways to successfully trim large half square triangles.

I hope this tip helps you with your larger quilt blocks!

With Joy,

Sarah V.