A half-square triangle — HST for short — is two triangles sewn together into a square, with the seam running corner to corner on the diagonal. It's the building block behind pinwheels, flying geese arrangements, sawtooth borders, and about half the quilt blocks ever drafted. The triangles themselves are simple. The interesting question is how to make a lot of them without losing your mind or your points.
The trick all good methods share: you never cut a triangle. You cut an oversized square, sew, slice, and then trim the finished unit down to a crisp, exact size. Cutting squares instead of triangles keeps the stretchy bias edge buried inside the unit until the very end, which is why your points stay sharp. There are three common ways to do it, and they trade speed for control in predictable ways.
2-at-a-time: the workhorse
This is the method most of us learned first, and it's still the best for scrappy quilts where every HST uses a different fabric pairing.
- Cut your square at finished size + 7/8 inch
- Pair one light square with one dark square, right sides together
- Draw one diagonal line corner to corner on the lighter square
- Sew a ¼-inch seam on each side of that line
- Cut apart on the drawn line
Two HSTs, one pairing. Because you're handling one light and one dark square at a time, you have total control over which fabric meets which — perfect when no two units in the quilt are the same. The downside is simply volume: making 200 HSTs this way is 100 trips to the cutting mat.
4-at-a-time: quick matched pairs
The 4-at-a-time method speeds things up when you want four identical HSTs from one fabric pairing.
- Cut your square at finished size + 1¼ inch
- Pair two squares right sides together — no drawn line needed
- Sew a ¼-inch seam all the way around all four edges
- Cut on both diagonals, corner to corner
That single sewn-and-sliced unit gives you four HSTs at once. It's a nice middle gear. The catch is that cutting both diagonals exposes bias on every single edge of all four units, so they want to stretch as you press. Handle them gently and press, don't iron — lift and lower rather than dragging.
8-at-a-time: many matching units, fast
When a pattern calls for dozens of identical HSTs — think a quilt made entirely of one light and one dark — the 8-at-a-time method is a genuine time-saver.
- Cut your square at (finished size × 2) + 1¾ inch
- Pair two larger squares, draw both diagonals
- Sew ¼ inch on each side of both drawn lines
- Cut into quarters, then across both diagonals
Eight HSTs from one pair of squares. The math looks odd until you see it work, and it's worth understanding — the full walkthrough of the 8-at-a-time method breaks down exactly why that magic number lands where it does.
Rule of thumb: 2-at-a-time for scrappy variety, 4-at-a-time for quick small batches, 8-at-a-time when you need a crowd of matching HSTs. Match the method to how many of the same unit you need.
The accuracy trade-off
Here's the honest tension. Every diagonal cut you make turns a stable straight grain edge into a stretchy bias edge. The 2-at-a-time method exposes the least bias per unit, so those HSTs behave best. The 8-at-a-time method exposes the most, which is why those units feel a little floppy until they're pressed and trimmed. None of this is a problem — it's just a reason that trimming is not optional, no matter which method you pick.
They all need trimming
Whichever route you take, every HST comes off the cutting mat slightly oversized on purpose. You trim each one to its unfinished size, which is the finished size plus ½ inch (¼ inch of seam allowance on each side). A square ruler with a 45-degree line you can lay right along the diagonal seam makes this quick and accurate. A good rotary half square triangle ruler is one of the few notions genuinely worth the drawer space.
Let the calculator do the arithmetic
The formulas above are easy to remember for whole-inch sizes, but they get fiddly with finished sizes like 3½ or 4¾ inches. Drop your finished size and how many HSTs you need into the Half-Square Triangle Calculator and it returns the cut square size, how many squares of each fabric to cut, the total units produced, and the trim-to measurement — for all three methods at once. If you'd rather print a reference, the HST size chart lists the common sizes, and once your blocks are done the Backing Yardage Calculator will tell you how much fabric to buy for the back.