Free Half Square Triangle Calculator

What Is a Half Square Triangle and Why Use an HST Calculator?

If you're into quilting, you've likely encountered half square triangles (HSTs). These versatile units are created by sewing two right-angle triangles together to form a perfect square, and they appear in countless quilt patterns. Whether you are planning a subtle geometric design or a bold graphic layout, having reliable references—such as an HST calculator, a half square triangle size chart, or a quilting triangle calculator—makes the preparation stage smooth and accurate. This tool handles all the math, so you can focus on the creative aspects of your project.

Tools of the Trade

Achieving professional-looking half square triangles starts with the right equipment. You'll need:

• A rotary cutter for clean cuts.
• A square quilting ruler to measure and mark accurately.
• A 1/2‑inch quilting ruler for marking seam allowances.

These simple tools, combined with an HST quilt calculator that tells you exactly what size to cut, give every quilter—from novice to expert—the confidence to produce consistent blocks.

Calculating the Correct Size

The secret to a perfect HST block lies in the seam allowance. For a finished block of a certain size, you must cut the raw squares $\dfrac{7}{8}$ inch larger. In other words:

$$\text{Raw square side length} = \text{Finished block side length} + \dfrac{7}{8}\,\text{inch}$$

For example, if you want a 5‑inch finished square, your raw fabric should be cut at $5\dfrac{7}{8}$ inches. The half square triangle cutting chart built into this calculator gives you those numbers instantly, so you don’t have to reach for a pen and paper.

Step-by-Step: Cutting and Marking

1. Using your square quilting ruler and rotary cutter, cut the fabric into squares that are $\dfrac{7}{8}$ inch larger than the desired finished size.
2. Align the 45‑degree line of your square quilting ruler from one corner to the opposite corner and draw the diagonal.
3. With the 1/2‑inch quilting ruler, draw two parallel lines, each $\dfrac{1}{4}$ inch away from the diagonal. These are your stitch lines.

Sewing the HST Blocks

• Place two fabric squares with right sides together.
• Sew along the two drawn lines (not the diagonal itself).
• Cut along the diagonal line between the stitch lines to separate the two triangles.
• Open each unit and press the seam toward the darker fabric. Trim the “dog ears” at the corners with your rotary cutter.

Half Square Triangle Size Chart

The following reference table shows common raw-to-finished conversions. Use it when you prefer to work without a calculator, or simply cross‑check the numbers provided by the tool.

Raw square (inches)	Finished square (inches)
2 7/8	2
3 3/8	2 1/2
3 7/8	3
4 3/8	3 1/2
4 7/8	4
5 3/8	4 1/2
5 7/8	5
6 3/8	5 1/2
6 7/8	6
7 3/8	6 1/2
7 7/8	7
8 3/8	7 1/2
8 7/8	8
9 3/8	8 1/2
9 7/8	9
10 3/8	9 1/2
10 7/8	10
11 3/8	10 1/2
11 7/8	11
12 3/8	11 1/2
12 7/8	12

This half square triangle size chart covers sizes up to 12‑inch finished blocks. For uncommon dimensions, simply apply the formula or let the HST quilt calculator determine the cut size.

The Easiest Method: Pre‑Marked Patterns

If measuring seems tedious, consider using pre‑marked HST patterns. These paper or fabric templates already include the diagonal and seam lines. Pin the pattern onto layered fabric, stitch along the marked lines, cut the diagonals, press open, and tear away the paper. This approach eliminates most measurement work, though it may restrict you to the sizes supplied with the pattern. The HST calculator is still useful for checking that the pattern’s dimensions match your target finished size.

Final Thoughts

Mastering half square triangles opens up a world of quilting possibilities. By using a dedicated quilting triangle calculator, a half square triangle cutting chart, or a full‑featured HST calculator, you remove guesswork from the cutting stage and achieve accurate results every time. Whether you’re working on a small sampler or a large bed quilt, these tools help you waste less fabric and enjoy the process more.