Half Square Triangle Charts and Formulas

Plan your next half-square triangle quilting project with the help of these HST measurement charts and cheat sheets. Whether you want to make half square triangles 2 at a time, 4 at a time, 8 at a time, or with the strip method, I’ll show you exactly what size fabric you need to cut.

Plus, I’ll give you my best tips and tricks to make piecing half-square triangles easier than ever!

Half square triangles are one of the most common quilting blocks, and for good reason! They’re simple to make, and you can arrange them in almost infinite ways.

A half-square triangle is a square quilt block unit made from two right-angle triangles. The triangles are sewn together along their long (diagonal) side. Each triangle forms one “half” of the square.

4 Half Square Triangle Methods

There are many ways to make half-square triangles. The method you choose will depend on how many squares you need, and what size fabric you have to start with.

Basic Method

You can make one half square triangle block from two fabric squares of the same starting size. For example, if you start with two 3″ squares, you can make one 3″ HST.

To make 1 half-square triangle at a time:

1. Stack two fabric squares right sides together.
2. Draw a diagonal line, from corner to corner, on the back of one square.
3. Sew on the line, and trim a 1/4″ seam allowance. (There will be quite a lot of fabric cut away.)

This basic method will give you one HST that is the same size as the two squares you started with. It’s quick and easy, but it only makes one HST at a time (plus a lot of scraps).

Better Methods

If you need more than one half-square triangle, there are a variety of other, better methods to choose from. In fact, it’s actually easier and faster to make half-square triangles with one of the following quick-piecing techniques.

Here are 4 easy methods to piece half-square triangles.

• 2 at a time
• 4 at a time
• 8 at a time
• strip piecing

Which HST method should you choose?

Each half-square triangle cutting method has it’s own pros and cons. The method you should choose depends on how many HSTs you need, how you like to sew, and what size fabric you have to start with. Plus, there are those pesky bias edges to take into account!

Here are some things to think about when choosing your cutting method:

Size: If you need larger size HSTs, you’ll likely choose the 1 or 2 at a time cutting method. Whereas, if you need small HSTs, it’s more efficient to use the 4 at a time or “magic 8” cutting hack.

Quantity: Cutting 4 or 8 HSTs at a time can definitely save you some time!

Fussy Cutting: You’ll have more control over the orientation of your prints with the “1 at a time” method.

Bias Edges: The “4 at a time” and the strip method are going to give you HSTs with bias edges. If you take care not to stretch the bias edges while pressing and sewing, they really shouldn’t give you any problems. But, they’re something to be aware of!

Accurate Cutting and Piecing

Before we get into the 4 methods and their corresponding charts, I want to make a small note about “accurate” cutting and sewing.

The measurements in these charts rely on precise cutting and sewing. If you are one of those lucky quilters that can piece with 100% accuracy, I must congratulate you! You may carry on with the chart measurements as written.

If like me, you are not blessed with the ability to sew perfect seam allowances, you may want to add 1/4″ or so to the cutting measurements written in the charts. I prefer to sew slightly oversized blocks, which I can then trim down to their exact unfinished size.

Adding a little bit to the starting fabric measurement will give you enough room to trim your half-square triangles to size. You can think of it as insurance against errors in the rotary cutting or sewing steps.

I know that trimming all of those triangles can feel tedious. But, having a stack of accurate HST blocks ready to go will make piecing your quilt much more fun! Your future self will thank you.

You may find that a rotating cutting mat and a quilting ruler help the trimming process go a lot faster. I like using square quilting rulers to trim HSTs. And, I’ve recently been using a triangle HST ruler that saves even more time.

How to Read the Charts

The finished size is the size of the pieced HST block, after you’ve sewn all 4 sides with a 1/4 seam allowance.

The unfinished size is the size of the pieced HST block, before you’ve sewn any of the outer edges. It’s also the measurement to which you would trim the pieced HST, when you’re working from a pattern.

The starting squares are the two large fabric pieces that will be sewn and cut into the HSTs.

Traditional Method: 2 at a Time

Here’s how to make HST blocks 2 at a time.

1. First, lay two fabric squares right sides together. Draw a diagonal line, from corner to corner, on the back of the lighter-colored square.
2. Next, sew 2 seam lines on either side of this diagonal line, each one 1/4″ away from the line. Cut the two pieces apart along the diagonal line.
3. Then, press the seam allowances toward the dark side (or whatever your preference.) Trim the “dog ears”, or those little corners that stick out beyond the edges of the square.

Using a ruler, trim the HSTs to their unfinished size. You may find that a rotating cutting mat and a ruler help the trimming process go a lot faster.

“2 at a Time” HST chart

Finished HST	Unfinished HST	Starting Squares
x	x + 1/2	x + 7/8
1	1 1/2	1 7/8
1 1/2	2	2 3/8
2	2 1/2	2 7/8
2 1/2	3	3 3/8
3	3 1/2	3 7/8
3 1/2	4	4 3/8
4	4 1/2	4 7/8
4 1/2	5	5 3/8
5	5 1/2	5 7/8
5 1/2	6	6 3/8
6	6 1/2	6 7/8

Quick Method: 4 at a Time

Here’s how to make HST blocks 4 at a time:

1. First, lay two fabric squares right sides together. With a seam allowance of 1/4″, sew all the way around the perimeter of the stacked squares.
2. Then, draw two diagonal lines from corner to corner. The lines will cross at the center point, forming an X. Cut along these diagonal lines to make 4 half-square triangle units.
3. Press the seam allowances toward the dark side (or whatever your preference.) Using a ruler, trim the HSTs to their unfinished size.

Dealing with Bias Edges

There is one potential downside to the “4 at a time” method: the edges of the resulting HSTs will be on the bias. Bias edges can be problematic because they stretch. If you aren’t careful, you can end up with wonky HSTs.

However, if you are mindful of this issue and take care not to stretch the bias edges while pressing and sewing, you shouldn’t have a problem.

“4 at a Time” HST Size Chart

The math for these “4 at a time” HST is a little more complex, and the fractions aren’t as tidy. So, in this chart, I’ve rounded up to the neartest 1/8 of an inch.

Since you’ll be trimming anyway, I recommend that you add another 1/4″ to the starting square measurement. This will give you a little extra wiggle room when it comes time to square up your blocks.

Finished HST	Unfinished HST	Starting Squares
1	1 1/2	2 3/8
1 1/2	2	3 1/8
2	2 1/2	4
2 1/2	3	4 3/4
3	3 1/2	5 1/2
3 1/2	4	6 1/4
4	4 1/2	7
4 1/2	5	7 7/8
5	5 1/2	8 5/8
5 1/2	6	9 3/8
6	6 1/2	10 1/4

Even Quicker Method: 8 at a Time

Here’s how to make HST blocks 8 at a time.

1. First, lay two fabric squares right sides together. Draw two diagonal lines, from corner to corner, on the back of the lighter colored square. The lines will intersect at the center point, forming an X.
2. Next, sew 2 seam lines on either side of each diagonal lines (for a total of 4 seams). Each seam line should be 1/4″ away from the drawn lines.
3. Cut along each of the diagonal lines, and then again along the vertical middle and horizontal middle lines.
4. Press the seam allowances toward the dark side (or whatever your preference.) Using a ruler, trim the HSTs to their unfinished size.

You will have made 8 HSTs. Look at you go!

“8 at a Time” HST Size Chart

Finished HST	Unfinished HST	Starting Squares
x	x + 1/2	(x + 7/8) x 2
1	1 1/2	3 3/4
1 1/2	2	4 3/4
2	2 1/2	5 3/4
2 1/2	3	6 3/4
3	3 1/2	7 3/4
3 1/2	4	8 3/4
4	4 1/2	9 3/4
4 1/2	5	10 3/4
5	5 1/2	11 3/4
5 1/2	6	12 3/4
6	6 1/2	13 3/4

Strip Piecing Method

This method is really useful if you’re working with jelly rolls or other precut strips of fabric. This technique is perfect for using scrap fabric from your stash.

I think you’ll be surprised by what size HSTs you can make from precut strings. For example, did you know you can make 2″ finished HSTs with a standard 2 1/2″ jelly-roll strip?

Here’s how to make HST from a jelly roll or other strip of fabric.

1. First, place two fabric strips right sides together. Then, sew along both of the long edges with a 1/4 seam allowance.
2. Next, use a quilting ruler to cut the triangles. Align the seam with the measurements for your desired unfinished measurement. (In the photo below, I am cutting 3 1/2” unfinished HSTs, so I placed the ruler such that the seam line touches both 3 1/2” markings.)
3. Cut the first triangle. Flip the ruler over, and line it up with the seam on the other side. Cut out the second triangle. Repeat along the rest of the strip.

Bias edges: Just like with the “4 at a time method”, the edges of HSTs made with the strip method will be on the bias.

Strip Method HST Size Chart

Finished HST	Unfinished HST	Starting Strip
1	1 1/2	2
1 1/2	2	2 1/4
2	2.5	2 1/2
2 1/2	3	3
3	3 1/2	3 1/2
3 1/2	4	3 1/2
4	4 1/2	4
4 1/2	5	4 1/2
5	5 1/2	4 3/4
5 1/2	6	5
6	6 1/2	5 1/2

Tips for Trimming HSTs

If you’re making a lot of half-square triangles, you might want to invest in a few tools to make them easier, faster, and more accurate.

• Rotating Cutting Mat: You may find that a rotating cutting mat saves you time, and helps make those rotary cuts more accurate.
• Square rulers: A pack of square quilting rulers in a variety of sizes can help you trim HSTs much more quickly.
• Triangle ruler: I’ve recently been using a triangle HST ruler that saves even more time. You can use this type of ruler on the folded HST block before you press it open. That way, you only trim two sides, instead of all four.
• Starch: Last but not least, a little starch can make sewing accurate HSTs even easier.

Explore more Quilting Ideas

If you like quilting as much as I do, I like you’ll like these other projects.

• 50 Free & Easy Quilt Patterns for Beginners
• Free Quilted Christmas Stocking Pattern

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