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Are you a quilter in the market for a half-square triangle calculator? If so, then we have you covered. Quilting can be a great way to pass the time and even make a little money on the side. However, preparing your half squares can prove time-consuming, and the finished product may not meet your liking if you do not know how to calculate the measurements for your half squares.

Continue reading to learn:

  • The formula for calculating half-square triangles;
  • The tools you will need to make half-square triangles;
  • The easiest way to make half-square triangles;
  • How to cut half-square triangles like a pro; and
  • How to sew half-square triangles.

We also have a half-square triangle size chart that may interest you.

What is a half square triangle?

A half-square triangle is a unit used to create half square triangle quilts. These are right-angle triangles that you sew together to make a square.

Picture of the half square triangle.

Tools you need to make half square triangles

As with any job, making half-square triangles like a pro requires some specific tools. These are:

  • The rotary cutter;
  • The square quilting ruler; and
  • The 1/2 inch quilters ruler.

You will learn how to make half-square triangles for quilting with these tools.

Another tool that may interest you is our fabric calculator. This is perfect for helping you to figure out the amount of fabric you will need for your sewing project.

What size do you cut half square triangles

The size you choose to cut a half square triangle depends on the desired result. If you wish to have a 5-inch square finish, you should put that measurement into the field marked 'finished length and width' of our half-square triangle calculator. Our half-square triangle calculator will calculate the raw measurement for you. On the other hand, if you wish to do the calculation yourself, add 7/8ᵗʰ inch to the intended size of your finished product to allow for the seams.

This means that for a finished 5-inch square, the raw fabric must be measured and cut 5 7/8ᵗʰ inches for the length and width. It would be best to use a square quilting ruler to mark and cut the square at a 45-degree angle along the diagonal line to create perfect half squares.

Do you have other projects that require you to calculate the properties of a square? Our square calculator may interest you.

How to make half square triangles like a pro

Getting the measurements right and cutting the fabric correctly can influence the result of your half square triangle. As such, we must use the tools mentioned above.

Use your square quilting ruler and your rotary cutter to measure and cut the fabric into equal size squares 7/8ᵗʰ of an inch larger than the intended size of the finished piece. Next, using the 45-degree line of the square quilting ruler, mark the diagonal line from one point to the other. Finally, taking the 1/2 inch quilting ruler, measure and draw a line 1/4ᵗʰ of an inch from the diagonal on either side. These are to be used to sew our center seams.

How to sew a half square triangle for quilting

Sewing a half-square triangle is not at all problematic.

  1. Place the fabrics together, right sides facing each other.
  2. Sew along the lines that you drew parallel to the diagonal.

Take your square quilting ruler along with the rotary cutter and cut the square along the diagonal to get two equally sized triangles. Next, go over to your ironing board, open the square flat, and taking your iron, press the square with the seam facing the darker side.

Finally, use the rotary cutter to cut away the two dog ears at either end of the diagonal.

Half square triangle size chart

If you wonder what size you cut half-square triangles and do not wish to measure, our half square triangle size chart below is another great tool you can use.

Raw squares (inches)

Finished squares (inches)

2 7/8


3 3/8

2 1/2

3 7/8


4 3/8

3 1/2

4 7/8


5 3/8

4 1/2

5 7/8


6 3/8

5 1/2

6 7/8


7 3/8

6 1/2

7 7/8


8 3/8

7 1/2

8 7/8


9 3/8

8 1/2

9 7/8


10 3/8

9 1/2

10 7/8


11 3/8

10 1/2

11 7/8


12 3/8

11 1/2

12 7/8


The easiest way to make half square triangles

The easiest way to make half-square triangles is to use pre-marked patterns. This removes the need to measure. However, it may restrict you in terms of the size you can create.

These patterns sit on top of the fabric while we sew along the marked lines parallel to the diagonal.

First, take your pattern and attach it securely to the fabric using sewing pins. Next, sew along the lines marked on either side of the diagonal. Once you have completed sewing, the center seams are cut along the diagonals to separate the fabric into half-square triangles. Lastly, open the fabric flat with the pattern still attached and press the material with your iron. Tear the pattern along the stitched line to remove it. Finish up by cutting away the dog ears on either side of the diagonal.

Are you looking for more quilting tools? Our quilt binding calculator and this hexagon quilt calculator are sure to make your life easier.


Is a triangle always half a square?

No, a triangle is not always half a square.

A triangle that is half of a square has two equal sides and one of its internal angles is a right angle.

How to cut half square triangles for quilting?

To cut half-square triangles, we need to:

  1. Measure and cut the fabric 7/8ᵗʰ of an inch greater than the desired finished measurement.
  2. Use your square quilting ruler to mark along the diagonal of the square.
  3. Use your 1/2 inch quilters rule to mark a line 1/4ᵗʰ of an inch in from the diagonal
  4. Stitch along the parallel lines you marked.
  5. Finally, cut along the diagonal line.

What are the best fabrics for quilting?

The best fabric for quilting are:

  • Cotton;
  • Linen;
  • Voile; and
  • Silk.

Beginner quilters should start with cotton or linen since the others are much more difficult to handle.

When choosing cotton or linen for quilting, we should always aim for high-grade fabrics. For cotton, the lowest thread count should be 60 × 60. For linen fabric, medium weight is best.

Adena Benn
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