Half-Square Triangle Calculator

How the HST math works

A half-square triangle finishes at a tidy size, but you cut it from an oversized square and trim down — that's how you get crisp points. How much bigger depends on the method:

A worked example: 100 HSTs finishing at 3″

Using the 8-at-a-time method, which is what you want when you need a lot of matching triangles. These are the calculator's defaults:

The four spares are not waste — they are your insurance. HSTs are the unit most likely to come out a thread small, and having four in reserve is much cheaper than cutting two more 7¾″ squares later from fabric you may no longer have.

Where the ⅞″ comes from

It is the one HST number worth understanding, because the rest follow from it. A triangle finishing at 3″ has to measure 3½″ before it is sewn — a ¼″ seam allowance on each side. But an HST's seam runs along the diagonal, and a ¼″ allowance measured square to a 45° line eats about 0.354″ of the square's side (that is ¼ × √2). Add the two together: ½″ + 0.354″ = 0.854″, which rounds up to the nearest mark you can actually find on a ruler — ⅞″. That is the whole derivation. The 4- and 8-at-a-time numbers are the standard published equivalents for cutting several units from one pair of squares.

Always trim to size

Cut a hair big, press, then trim every unit to the unfinished size — finished plus ½″. It feels fussy and it is the step everyone wants to skip, but trimming is what makes the seams line up three rows later. An HST that is 1⁄16″ small is invisible on its own and impossible to ignore once forty of them are sewn into a row.

Which method should I use

2-at-a-time is the most forgiving and wastes the least on small numbers — good for a handful. 4-at-a-time sews around one pair of squares and cuts both diagonals; fast, but every edge of the finished unit is on the bias, so handle them gently. 8-at-a-time is the volume method and what the example above uses. The full comparison is in HST methods compared, and the 8-at-a-time math covers that one in depth.

Common questions

Why are my HSTs coming out small?

Almost always a seam allowance that is a fraction over ¼″. Sew a test seam, measure it, and adjust — a scant ¼″ is what most patterns actually assume. Pressing can also shrink a unit if you push the iron along the bias rather than pressing straight down.

Can I skip trimming if I cut carefully?

Not reliably. The bias edge stretches while you press, so even a perfectly cut square produces a slightly irregular unit. Trimming is the correction step.

What about paper piecing?

Foundation paper piecing gets you exact points without trimming, at the cost of removing paper afterwards. See the paper-pieced HST guide.