Half-Square Triangle (HST) Calculator
Need help figuring out your half-square triangles?
You’re in the right place! This handy calculator takes the head-scratching out of the maths so you can focus on the fun part—stitching them all together. Whether you’re making 2, 4, or 8 at a time, just pop in your finished HST size (or another known value), and the rest will fill in automatically. Choose your method, pick your units, and let the calculator do the work for you. It’s quilting maths made easy!
To use this calculator:
- First, decide which method you’ll be using—2, 4, or 8 at a time. (Not sure? 2 at a time is a classic, 4 is great for speed, and 8 gives you maximum efficiency.)
- Next, select whether you prefer working in inches or centimetres.
- Then, simply enter a value into any one of the three fields—Finished HST Size, Unfinished HST Size, or Starting Fabric Square Size—and the other two will calculate automatically.
No formulas, no guesswork—just easy answers at your fingertips.
Trimming Tip: All methods include a bit of extra fabric to give you room for trimming and squaring up your HSTs accurately—so your blocks end up tidy and true.
Two At A Time Half Square Triangles
Step 1: Draw a diagonal line across the back of 1 square. Place the 2 squares right sides together.
Step 2: Stitch ¼” seam on either side of the drawn line.
Step 3: Cut along the drawn line and press open. Be careful when pressing, the stitch is on the fabric bias.
Step 4: Use a squaring tool to square up to unfinished block size.
Four At A Time Half Square Triangles
Step 1: Place the 2 squares right sides of fabric together.
Step 2: Stitch ¼” seam from the edge of each side of the 2 fabrics.
Step 3: Cut diagonally, from corner to corner and press open. Note, the edges of the HST is on the fabric bias.
Step 4: Use a squaring tool to square up to unfinished block size.
Eight At A Time Half Square Triangles
Step 1: Draw 2 diagonal lines across the back of 1 square. Place the 2 squares right sides together.
Step 2: Stitch ¼” seam on either side of the drawn lines.
Step 3:Cut across the block, through the intersection of the drawn lines.
Step 4:Cut along the drawn line and press open. Be careful when pressing, the stitch is on the fabric bias.
Step 5: Use a squaring tool to square up to unfinished block size.