sewing.tools

Half-Square Triangles Three Ways: Which Method to Use When

Two-at-a-time, eight-at-a-time, and oversize-and-trim half-square triangles compared — the cut sizes, the trade-offs, and when each one wins.

By The sewing.tools teamPublished 3 min read

The half-square triangle (HST) is the building block behind pinwheels, stars, sawtooth borders, and half the quilts you’ve ever admired. There are many ways to make them; these three cover nearly every situation.

All sizes below assume ¼” seams. “Finished” is the size of the HST once it’s sewn into the quilt; the unfinished unit is ½” larger.

1. Two at a time — the everyday method

Cut size: finished size + ⅞”

  1. Cut one square from each fabric. For a 3” finished HST, cut 3⅞” squares.
  2. Draw a diagonal line corner to corner on the back of the lighter square.
  3. Place the squares right sides together and sew ¼” on both sides of the line.
  4. Cut on the drawn line, press, and you have two HSTs.

Why ⅞”? The unfinished HST needs the usual ½” for its outer seam allowances. The diagonal seam then takes a little more: a ¼” seam sewn at 45° uses about 0.35” of the square’s side. ½” + 0.35” ≈ 0.85”, which rounds to ⅞”.

Best for: small batches, scrappy quilts where every HST is different, and straight-of-grain outer edges that don’t stretch.

2. Eight at a time — for big batches

Cut size: 2 × (finished size + ⅞”)

  1. Cut one large square from each fabric. For 3” finished HSTs, cut 7¾” squares.
  2. Draw both diagonals on the lighter square.
  3. Sew ¼” on both sides of both diagonals.
  4. Cut horizontally and vertically through the center, then along both drawn diagonals.

You get eight identical HSTs from two squares. It’s fast — but every HST from the batch is the same fabric pair, and accuracy depends on cutting the big square precisely.

Best for: quilts that need dozens of identical units, like a sawtooth border or a two-color star quilt.

3. Oversize and trim — for perfect points

Cut size: finished size + 1”, then trim to finished + ½”

Make them exactly like method 1, but cut your squares ⅛” larger. After pressing, use a square ruler with a 45° line: lay the diagonal on the seam and trim the HST down to the unfinished size (3½” for a 3” finished HST).

It’s slower, and it creates a little waste. But it corrects small sewing and pressing errors, and the trimmed HSTs are dead accurate.

Best for: small HSTs (under 2” finished), where tiny errors show the most, and any quilt where points really matter.

Whichever method you use, press the seam before you trim, and press — don’t iron. Pushing an iron back and forth stretches the bias seam.

Planning the quilt

The Half-Square Triangle Calculator gives the cut size and the number of squares per fabric for any finished size and quantity. Making quarter-square triangles (hourglass blocks) next? They need a different allowance — use the Quarter-Square Triangle Calculator. And when your blocks are ready, the Quilt Size Calculator works out the layout.