Making Half Square Triangles Using Your Rotary Cutter

Making half square triangles using your rotary cutter


Learn how to cut half square triangles using a rotary cutter by adding 7/8 inch to the finished size. You’ll also learn how to sew these triangles together to form a square.  You can download the handouts from the video below.

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Chart for Calculating Size of Square Needed to Make 2 Half Square Triangles (in inches) – FORMULA: Finished Size Plus 7/8”



This video is invaluable. Thank you so much for sharing.

Lee X


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Video Transcripts: CUTTING AND SEWING HALF SQUARE TRIANGLES - Making 1/2 Square Triangles Using Your Rotary Cutter (#2 of 8 videos)

In this video, I’ll show you how to use the rotary cutter, ruler and mat to cut these triangles and then I’ll sew a pair of these together.

Before you cut out your triangle, you first need to determine the finished size. That’s the size WITHOUT SEAM ALLOWANCES. Go to your quilt or your pattern and you measure one of the shorter sides. So let’s look. That’s 2” so my finished size of this triangle is 2”. Next you want to add 7/8” to the finished size. So 2 and 7/8 equals 2 7/8”. That’s the size of the square that I need to cut.

You want to make sure that your square is cut on the straight of grain or cross grain of the fabric. This ensures that at least 2 sides are going to be more stable than this 3rd side which that will be cut on the bias.

So let’s look at some fabric here. The straight of grain – it doesn’t mover very much when you try to stretch it. Here’s the cross grain – a little stretch but not much – and here’s the bias – stretches is a lot. The more the fabric stretches – right here- the harder it is to sew accurately. A triangle has three sides. If you can get two of those three sides on the straight of grain, that will make your life easier. Let me show you how this works on fabric.

Here is the selvage or the straight of grain and this is the cross grain. You can tell the selvage because sometimes it’s a different color or there’s a little white strip or there might be little dots or sometimes there’s little holes in the fabric there. That’s the part of the fabric you don’t want to use.

Since I have such a big piece of fabric here I like to get rid of some of it to make my job of cutting a little bit easier. So I know I am going to cut a little less than a 3 inch square. So let’s just get rid of some of this fabric. So here’s this smaller piece of fabric that I’ve cut out. I might be wasting a little bit of fabric but I think it helps with my accuracy in the long run and it is also much easier than dealing with that big piece of fabric.

So you notice, I’ve lined up the straight edges on the lines on the mat and I’ll take my ruler and 2 7/8 inches is 1, 2, almost up to 3; the line right before the number 3 on this 1/8” ruler. And coming down, it’s the line right before the 3. So I am going to line that up. I want to make sure that this line is even and I want to make sure that this line is even. And- the camera’s in the way here - but let’s see if I can work my way around that. I also want to see maybe if these lines on my ruler line up with the lines on
my mat. And I noticed that this little line here goes all the way down and I notice that this little line here goes not quite all the way across so I needed to move that a little. And I cut across. And here I have my 2 7/8” square.

The next thing I want to do is cut that diagonal in the fabric. You’ll notice that I put this square on this 45 degree line. That isn’t necessary but it really helps me see where I’ve got to line up. I want to come through this point and that point over there with the tip of that point. And I cut. And now I have two ½ square triangles that their finished size will be 2 inches.

Most of the time I am sewing two of these ½ square triangles together to form a square, so instead of cutting each one individually like this I like to take two scraps of fabric. Put right sides together. Cut my square out and then cut that square on the diagonal and I am ready to go straight over to the sewing machine with this.

Before start sewing, I want to make sure that all the edges line up. And since I will be sewing on the bias here and I don’t like to pull or stretch that bias, I am not going to use any pins. If you’re not comfortable with that then it’s OK to put a few pins in. I also like to use what I call a feeder or a leader which is a scrap of fabric folded in half. I’ll put it under my pressure foot. You’ll notice I have red thread so you can see. You’ll want to use a matching color of thread or a neutral color of thread. I’m using a regular stitch length.

This feeder gets things started. These tips of the triangles sometimes like to get stuck down here in the feed dogs and make a mess. So this leader or feeder piece helps pull those tips through. I have a ¼” foot on my sewing machine. This ¼” foot makes it easy to sew my ¼” seam. Let me put that through. Notice I am not trying to pull this. I am just gently guiding it. I don’t want to mess with that bias. And maybe let’s see – line up those tips. Coming through they start to come apart. Let me cut this off and bring it back here and sew right on to that. And there you have my triangle sewn together with ¼ inch seam. I’ll take this over to the ironing board and press those seams … um...I like to press them to one side. So my finished square will end up looking like this.

After you’ve sewn your two ½ square triangles together you’re going to get a square that looks like this. You’ll want to cut those tips off. You can either use scissors or your rotary cutter to do that. Or (this is optional) you can cut the tips beforehand. So let me show you what that looks like.

I have my two triangles here with right sides together. I am going to take my ruler. You’ll notice the rule says, “finished size plus ½ inch”. So the finished size is 2”. I am going to add ½” to that and make that 2 ½”. So 1, 2 – whew let’s get this – 2 ½. Take my ruler and I trimmed that off. Now let’s go up to this one. So I went 2 ½ across. Now let’s come 2 1/2 down. So, let’s see, I’ve got 1, 2, and ½ and I can see that. Let me trim across. There you have my two triangles that have been trimmed. What does that look like when you take it to the sewing machine? You just run your ¼ inch line there and when you open it up, you’ll see that all those tips have been taken off.

You can find more information about cutting and sewing half square triangles at under “Beginner Basics” – “Cutting” and “Sewing”.

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