How to Sew Half-Square Triangles (HST) – 8 at a Time
I’m going to show you how to sew half-square triangles (HST), 8 at a time. This is great when you want a large number of identical HSTs.
Watch the video and follow along with the photo tutorial below.
The basic process is to start with 2 same-size squares, mark lines on one of the squares, layer, sew, cut, press, and trim.
In this tutorial, I’m starting with 10-inch squares, a standard size in the quilting industry. Another very common size is 5-inch squares. Stacks of these pre-cut squares are widely-available, found in most any quilt shop, so it’s no surprise that these are my two favorite sizes to work with.
- 10” squares make (8) 4” trimmed HSTs
- 5” squares make (8) 2” trimmed HSTs.
To calculate the size of the original 2 squares: take your desired HST size, add one, then multiply by two. So for 4″ HSTs: 4 + 1 = 5 x 2 = 10″ fabric squares.
1. Start with 2 same-size fabric squares. Quilting cotton is best.
2. Mark 4 centered lines on the wrong side of one of the squares: one vertical, one horizontal, and two diagonal. I like to use a regular pencil, but any fabric marker would work. You could even use creases instead, using an iron or a creasing tool.
3. Layer the 2 squares right sides together. You could pin them together, but I usually don’t. They always seem to stick together just fine.
4. Sew 4 seams, 1/4-inch from each side of the diagonal lines. Repeat, the diagonal lines only.
5. Cut on the marked lines. Use scissors or a rotary cutter, whichever you prefer.
6. Press the seams. I like to press the seams open. But if you like to press seams to one side, cool. Do what works best for you. I almost always press open so the quilt block lies nice and flat. Also, at this point I may not know the final arrangement of all these HSTs in the project I’m making. Depending on the HST layout, if I press seams to one side, some might double-up on each other making it bulky and bumpy. Not good. I don’t like my design decisions to be dictated by seam direction. Pressed-open seams give me design flexibility, which is good!
7. Trim the HSTs.
Why trim? Because it helps to ensure accuracy, which is important when trying to match all of those HST points when you sew them all together. Trimming is a last step that helps to attain precision despite any flaws made in the process, like imperfectly cut squares or a seam that is not an immaculate 1/4 inch. Also, you’ve just sewn on the bias, which is where fabric stretches most. So let’s correct any stretching or flaws with a simple trim, to align our seams the best we can!
You’ll need a cutting mat, a rotary cutter, and a gridded transparent ruler with a 45-degree line.
First, determine the desired cut size. In this example, we’re trimming the HST down to 4 inches square.
Line up the ruler’s 45-degree line with the diagonal seam, then make sure the desired cut size is within the appropriate lines on the ruler.
Here, I’m picturing the 4-inch square lines on the ruler, and I can see that I have excess fabric on all four sides.
Using the rotary cutter, trim off the right and top sides of the HST.
Lift the ruler and rotate the HST so the un-cut corner is now at the top-right. Just like we did before, line up the ruler’s 45-degree line with the diagonal seam, but this time make sure the freshly-cut corner (now at the bottom-left) is aligned nicely with the appropriate lines on the ruler. This quilting ruler makes it easy.
Again using the rotary cutter, trim off the right and top sides of the HST.
All trimmed and precise. Do this for each HST. See how the diagonal seam goes corner to corner? That’s important later on, for when you want to sew a bunch together and get the seams to match!