Half-square triangles (HSTs) are the building blocks behind pinwheels, flying geese, stars, and hundreds of other designs. Tell the calculator your finished size and how many you want at a time, and it returns exactly what to cut.
How half-square triangle math works
A half-square triangle is simply a square made from two right triangles meeting on the diagonal. Because that diagonal is the bias of the fabric, you always cut the starting squares larger than the finished size, sew, cut apart, press, and trim back. Each method adds a different amount of "math allowance" depending on how many diagonals you sew:
- 2 at a time: cut two squares = finished size + 7/8" (0.875"). Draw one diagonal, sew 1/4" on each side, cut on the line. Yields 2 HSTs.
- 4 at a time: cut two squares = finished size + 1.25". Sew all the way around, then cut both diagonals. Yields 4 HSTs.
- 8 at a time (magic 8): cut two squares = (finished size × 2) + 1.75". Draw both diagonals, sew 1/4" on each side of both lines, then cut into 8. Yields 8 HSTs.
- Trim to: finished size + 1/2" (0.5") — this is the unfinished size, leaving a 1/4" seam allowance on all sides.
For example, a 3" finished HST made 2 at a time starts from two 3.875" squares and trims to 3.5". The same 3" finished HST made 8 at a time starts from two 7.75" squares.
HST cutting chart
Here are the cut sizes for the most popular finished HSTs across all three methods. The trim-to size is the same regardless of method.
| Finished | 2 at a time (cut 2) | 4 at a time (cut 2) | 8 at a time (cut 2) | Trim to |
|---|---|---|---|---|
| 2" | 2⅜" (2.875") | 3¼" (3.25") | 5¾" (5.75") | 2½" |
| 3" | 3⅜" (3.875") | 4¼" (4.25") | 7¾" (7.75") | 3½" |
| 4" | 4⅜" (4.875") | 5¼" (5.25") | 9¾" (9.75") | 4½" |
| 6" | 6⅜" (6.875") | 7¼" (7.25") | 13¾" (13.75") | 6½" |
Pro tip: Press the seam toward the darker triangle and trim every HST to the unfinished size with a square ruler. A minute of trimming up front saves a frustrating struggle to match points later.
Tips for accurate HSTs
- Cut a little, gain a lot. The extra fabric in each method exists precisely so you can trim to a perfect square — don't skip it.
- Mind the bias. The diagonal seam is stretchy, so handle pressed HSTs gently and press, don't iron, to avoid distortion.
- Use the 8-at-a-time method for scrappy quilts. It's the fastest way to make a pile of matching units from two fabrics.
- Chain piece. Feed pairs through the machine one after another to save thread and time.
Let Quiltler 3 do it for you: Quiltler 3 builds HST-based blocks, tiles them into full layouts, and calculates the cutting and yardage automatically — then exports a PDF. Learn the unit in our half-square triangle guide or see it in action in the pinwheel block, and plan fabric with the fabric calculator.