QuickCheckTools

Half-square triangle calculator

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Enter the finished size and how many HSTs you need. You'll get the square size to cut, how many squares of each fabric, and how much fabric to buy.

Worked example

Two 3" finished HSTs, 2 at a time (Utah State University Extension's published example):

  1. Add ⅞" to the finished size: 3 + ⅞ = 3⅞".
  2. Cut one 3⅞" square from each fabric.
  3. Draw a diagonal, sew ¼" either side, cut on the line: two HSTs that measure 3½" and finish at 3".

For 8 at a time the same HSTs need (3 + ⅞) × 2 = 7¾" squares. If you prefer to trim, Missouri Star Quilt Co. cuts 4" squares instead, or 10" layer cake squares for eight 4" HSTs.

Questions

What size square do I cut for half-square triangles?

For the 2-at-a-time method, cut squares ⅞" bigger than the finished HST: 3⅞" squares make 3" finished HSTs (3½" unfinished). If you like to trim, cut them 1" bigger instead and square up afterwards.

How does the 8-at-a-time method work?

Add ⅞" to the finished size and double it. For 3" finished HSTs, cut 7¾" squares, draw both diagonals, sew ¼" either side, then cut horizontally, vertically and on both diagonals to get eight HSTs.

What size for 4 at a time?

Sew ¼" around all four edges of two squares and cut on both diagonals. The square size is (finished + ½") × 1.414 + ½", rounded up to the next ⅛": 9¾" for 6" finished HSTs. Some charts give bigger figures; this one is the exact geometry, and the trim option adds ¼" if you want room to square up. The 4-at-a-time HSTs have bias edges, so handle them gently.

How many squares do I need?

Each pair of squares (one of each fabric) makes two HSTs with the 2-at-a-time method, four with 4-at-a-time and eight with 8-at-a-time. The calculator rounds up and tells you how many spares you'll have.

Why ⅞"?

Two ¼" seams take up ½", and the diagonal seam takes up a little more because it runs at an angle. The exact figure is 0.854", rounded up to ⅞" so your HSTs are never too small.

How much fabric do I need?

The calculator lays your squares across 40" usable fabric width, works out how many strips you need of each fabric, and rounds up to the next ¼ yard (or the step you choose).

Assumes ¼" seams (0.75cm in metric) and 40" usable fabric width (102cm). 2- and 8-at-a-time sizes match Utah State University Extension, Sarah Maker and Missouri Star Quilt Co.; every size is also checked against the sewing geometry.