Half-Square Triangle Calculator

How the Calculator Turns a Finished Size Into a Cut Size

A half-square triangle, or HST, is two right triangles joined along their long edge into a square unit, and the whole craft of making them accurately comes down to one question: what size square do you start from? The finished size, what the unit measures once it is sewn into the quilt, is smaller than the square you cut, because the diagonal seam and the quarter-inch seam allowance on the two sewn sides both eat into it. This calculator takes the finished size you want and the construction method you prefer, then returns the exact cut-square size, the unfinished size to trim to, and the fabric the job needs.

The classic figure is the seven-eighths-inch rule: for the two-at-a-time method you cut squares seven-eighths of an inch larger than the finished size, so a 3-inch finished HST starts from a 3⅞-inch square. That seven-eighths is not arbitrary — it is the quarter-inch seam allowance on each of the two sewn sides, half an inch in all, plus the small diagonal allowance the turn of the cloth demands, rounded up. The unfinished size, the measurement you square each unit up to before it joins a block, is always the finished size plus half an inch. A plain square in the quilt block layout calculator simply adds that half an inch all round; an HST square carries the extra diagonal allowance on top.

Two-at-a-Time, Four-at-a-Time, or Magic 8

There are three common ways to make half-square triangles, and they differ in how many units each pair of squares yields and how accurate the result tends to be. The calculator works out the cut size for whichever method you pick, so you can compare the trade-offs before you cut.

• Two-at-a-time cuts two squares at finished plus seven-eighths of an inch, sews a quarter inch either side of one drawn diagonal, and cuts on the line for two units. Only the diagonal is on the bias, so this is the most accurate method and the right default.
• Four-at-a-time sews two squares around all four edges and cuts both diagonals for four units. It is fast, but every one of the four edges ends up on the bias, which makes it the least precise — cut a touch large and trim.
• Magic 8 (eight-at-a-time) draws both diagonals on a larger pair of squares, sews either side of both, and cuts into eight identical units at once. It is the efficient choice when a quilt needs the same HST by the dozen.

The two-at-a-time method is the workhorse for a handful of accurate units; magic 8 earns its place when a design repeats one HST many times over; four-at-a-time is for speed when you will square everything up afterward. If you are starting from pre-cut squares rather than yardage — a charm pack of 5-inch squares, say — pairing two contrasting squares gives you the makings of HSTs with no measuring at all.

Cutting Squares to Hit a Target Number

Most projects need a specific count: four HSTs make a pinwheel, a sawtooth border might want forty, a two-colour throw runs into the hundreds. The calculator turns the number of finished units you need into the number of squares to cut, and it remembers the one rule beginners forget — that an HST is half one fabric and half another, so the squares come in matched pairs.

It divides your target by the units each pair yields for your chosen method, rounds up, and reports the squares to cut from each colour alongside the total across both. Ask for sixteen units by the magic-8 method and it returns two squares of each fabric; ask for the same sixteen two-at-a-time and it returns eight of each. The count of squares falls as the units-per-pair rises, which is the whole appeal of the eight-at-a-time approach for repetitive blocks.

How Many HSTs a Fat Quarter Yields

Once the cut size is settled, the next question is how much fabric to pull. Because the squares are cut on a grid, the yield from a given piece of fabric is simply how many squares fit across it by how many fit down. The calculator reports this for a fat quarter (an 18-by-22-inch cut) and, when you cut from yardage instead, for a width-of-fabric length.

• From a fat quarter of each colour, the yield is the squares that fit in 18 by 22 inches multiplied by the units each pair makes — small HSTs run to dozens from a single pair of fat quarters.
• From yardage, the squares fit across the usable 42 inches of WOF and down the length, so a yard of each colour stretches a long way for small units.
• Two fabrics, always — every yield assumes a matched cut of the contrast fabric, because a half-square triangle is never a single colour.

The fat-quarter figure is a quick sanity check against your stash, and the guide to precut sizes sets out where the 18-by-22-inch fat quarter sits among charm packs, jelly rolls, and layer cakes. For the odd triangle or rectangle a mixed block throws up, the kind that is not a square at all, the general fabric yardage calculator estimates plain yardage that a unit-based tool does not cover.

Trimming, Bias, and the Mistakes That Cost Fabric

The difference between HSTs that nest cleanly and a block that will not lie flat is usually one of three habits, and the calculator's outputs are built to head them off. Read the cut size and the unfinished size together: the first is where you start, the second is where you finish.

• Skipping the trim. Even careful sewing wanders, so square each sewn unit up to the unfinished size — finished plus half an inch — before it goes into a block. The oversize cutting option leaves room to do exactly that.
• Forgetting the bias. The diagonal of every HST is on the bias and stretches if you drag the iron; press by lifting it straight up and down. The four-at-a-time method puts all four edges on the bias, which is why it is the one to cut large and trim hard.
• Confusing an HST with a QST. A quarter-square triangle is a different unit with a different formula (it adds an inch and a quarter, not seven-eighths), so reach for an HST chart only when the straight grain runs on the two short sides.

With the units trimmed and squared, they go into blocks and then into the quilt sandwich. Once the top is pieced, size the layer behind it with the quilt backing calculator so the backing clears the edges with room to quilt.