If the Nine-Patch is the first block every quilter learns, the half-square triangle — HST for short — is the first unit every quilter learns. It is simply two right triangles sewn together into a square, with a crisp diagonal running corner to corner. From Pinwheels to Friendship Stars to Broken Dishes, hundreds of classic blocks are built almost entirely from HSTs.
On this page
What is a half-square triangle?
A half-square triangle is a square unit made from two right-angle triangles, each cut from a different fabric and joined along their long (hypotenuse) edge. The seam runs diagonally from one corner to the opposite corner. Rotate and combine these little squares and you can create pinwheels, stars, arrows, chevrons, and dozens of other secondary patterns.
Because the diagonal seam sits on the bias, HSTs can stretch out of shape if you are rough with them. Cutting your starting squares a little large and trimming the finished unit down is the reliable way to keep every HST perfectly square.
What makes the HST so important is its flexibility. A single unit looks plain, but the diagonal gives it direction. Place four of them with the points meeting in the middle and you have a pinwheel; turn the diagonals outward and you get an hourglass-style burst; line them up in a row and they form a sawtooth edge. Almost no other unit gives you that much design range from one simple square, which is exactly why the HST appears in more quilt patterns than any other pieced unit.
The half-square triangle math
The most common formula is for the 2-at-a-time method, where two squares produce two HSTs:
Cut size = finished size + 7/8"
So for a 3" finished HST, you cut two 3 7/8" squares. The extra 7/8" covers the two quarter-inch seam allowances created by the diagonal seam. Because trimming a bias unit accurately is tricky, many quilters use an oversized approach instead: add a full 1" (or even 1 1/2" for large units), sew, then trim down to the exact unfinished size. You waste a sliver of fabric but gain perfectly square HSTs every time.
Other methods change the math. The 4-at-a-time method starts from larger squares cut at the finished size plus about 1 1/4", and the magic-8 method starts from squares cut at the finished size times 2 plus 1 3/4". The chart below keeps things simple with the classic 2-at-a-time numbers.
Half-square triangle cutting chart
This chart uses the 2-at-a-time method. Cut one light and one dark square at the listed size to yield two finished HSTs. The "oversized" column is the safer cut-and-trim size.
| Finished HST | Cut size (exact, +7/8") | Cut size (oversized, +1") | Yield per pair |
|---|---|---|---|
| 2" | 2 7/8" | 3" | 2 HSTs |
| 3" | 3 7/8" | 4" | 2 HSTs |
| 4" | 4 7/8" | 5" | 2 HSTs |
| 6" | 6 7/8" | 7" | 2 HSTs |
Need any size instantly? Our half-square triangle calculator gives you the exact cut size for all four methods, and the fabric calculator totals the yardage for your whole quilt.
4 ways to make half-square triangles
- 2-at-a-time. Cut two squares at finished + 7/8". Draw a diagonal on the back of the light square, sew 1/4" from each side of the line, then cut on the line. Press to get two HSTs.
- 4-at-a-time. Place two squares (cut at finished + about 1 1/4") right sides together, sew all four outside edges, then cut both diagonals. You get four HSTs with no marking required.
- 8-at-a-time (magic 8). Cut two large squares at (finished × 2) + 1 3/4". Mark both diagonals, sew 1/4" on each side of both lines, then cut into eight wedges — eight identical HSTs at once.
- Continuous / strip method. Layer two long strips, mark a row of touching diagonals, sew on each side, and cut to chain out many HSTs from a single pass — ideal for scrappy quilts.
Pro tip: The 2-at-a-time method is the most beginner-friendly and gives you the most control over fabric pairing. Reach for the magic 8 only when you need eight of the exact same HST.
Trimming & squaring up
After pressing (toward the dark triangle, or open to reduce bulk), place a square ruler over each HST with the 45-degree line directly on the diagonal seam. Trim the top and right edges, rotate, and trim the remaining two edges to your exact unfinished size. For a 3" finished HST, that target is 3 1/2". Taking the few seconds to trim is the difference between points that match and points that float.
Blocks made from half-square triangles
Once you can make accurate HSTs, an enormous catalog of blocks opens up:
- Pinwheel — four HSTs rotated around a center to spin like a windmill.
- Friendship Star — four HSTs and five squares form a simple, friendly star.
- Broken Dishes — HSTs arranged so the diagonals zig-zag across the block.
- Sawtooth borders — a row of HSTs creates a classic toothed edge, close cousin to Flying Geese units.
Design with half-square triangles digitally
HSTs reward experimentation, because rotating them creates entirely new secondary patterns. With Quiltler 3 you can build a block from HSTs, fill the triangles with your own fabrics, and tile dozens of blocks to watch pinwheels and stars emerge. When you love it, export a PDF with cutting instructions and exact yardage.
New to digital design? Start with our guide to designing quilts or our beginner's guide to quilting.