Half Square Triangle Calculator — HST Cutting Sizes

HST cutting sizes depend on the construction method. Each method produces a different number of HST units from a pair of squares. The exact metric figures come from the imperial quilting standards (¼" / 6mm seam each side of the diagonal) — the defaults below round up slightly so you can trim each unit down to an accurate finished size.

Cutting formulas

2-at-once: Cut size = Finished size + 2.5cm (exact 7⁄8" = 2.2cm). One light + one dark square → 2 HSTs.

4-at-once: Cut size = (Finished size + ½″ seam allowance) ÷ 0.64, plus a small trim margin. Divide the UNFINISHED HST size (finished + ½″ / 1.27cm) by 0.64 — for a 7.5cm finished HST the unfinished size is ~8.8cm, so cut ~13.7cm squares. One pair, stitched all four edges and cut on both diagonals → 4 HSTs. This is a multiplicative rule, so the cut square grows in proportion to the finished size rather than by a fixed amount.

8-at-once (Magic 8): Cut size = (Finished size × 2) + 4.5cm (exact 1¾" = 4.45cm). One pair of these larger squares → 8 HSTs.