A half square triangle refers to the two triangles that are cut from one square. No quilt templates are used in this quick cutting method. These two triangles include seam allowance. Quilters use this technique to rotary cut several triangles at once. The ‘formula’ that works every time is listed below.
Half Square Triangle (HST) Formula
- Measure the FINISHED SIDE of the triangle (one of the right angle sides).
- Add 7/8 inch to this measurement.
- Cut a square FINISHED SIDE plus 7/8 inches.
- Then cut on the diagonal.
This method does not work on all triangles. The triangle must have a right angle and two equal sides in order to be considered a half square triangle.
View the video below for more info about half square triangles or visit the Beginner Basics section where you will find a series of videos showing different methods for making half square triangles. The second video points out the different triangles that quilters encounter and some quick cutting/sewing rules for each.
NOTE: to enlarge the video, click the PLAY button, then click the full-screen icon at the bottom right of the video.
Half square triangles, are squares that have been cut on a diagonal, and the result is two equal triangles. Each one of these triangles has a 90 degree angle in the corner and they have 2 equal legs, or sides.
Half square triangles are very popular in quilt blocks. There's a fast way to make these by using your rotary cutter. The rule is that you need to add 7/8 of in inch to the finished size. So what does that mean? Let's look at this 6 inch pin wheel block. That means when I sew this block into my quilt it's going to be 6 inches big. This pattern does not have a seam allowance on it.
Quilters use a ¼ inch seam in order to sew these blocks together. So this is a 3 inch square right here, if I were to cut that in half, this side would be 3 inches, and this side would be 3 inches. And that would not help me put this block together. So instead, I need to add seam allowance. So if I, here's my 3 inch block, if I add a ¼ of an inch, on each side, I've got a ¼ of an inch here, a ¼ of an inch there, I've cut this block 3 ½ inches and I have two ½ square triangles, but that's still not going to help me because I need to sew a seam on the diagonal. That means that I'm going to not only have to add a ¼ of an inch on this side and a ¼ of inch on that side, but I'm also going to have to add a seam allowance in the center here. A ¼ of an inch for this triangle and a ¼ of an inch for this triangle. And because it's on a diagonal you would think it would be a ¼ and a ¼, makes it a ½ but because it's turned, it's not quite ½ and inch; it turns out to be 7/8 of an inch. And so I've cut this square at 3 and 7/8 inches.
The finished size was 3 I add 7/8 of an inch to that so I cut this 3 and 7/8 inches and that means, I'm going to have enough seam allowance to sew this square, this triangle, these two triangles into my finished quilt block.