Half Square Triangle (HST) Calculator

How to Use the HST Calculator

Enter the finished size of the half square triangle unit you need and pick a construction method. The calculator supports all three standards — 2-at-a-time, 4-at-a-time, and 8-at-a-time — which trade marking time against batch size: two-at-a-time gives the most control over each unit, while eight-at-a-time turns one pair of squares into eight identical HSTs.

The cutting math differs by method. Two-at-a-time squares are cut at the finished size plus 7/8 inch, and eight-at-a-time doubles that: twice the finished size plus 1-3/4 inches. Four-at-a-time is the special case — its units come off the square on the diagonal, so the square is the finished size times 1.414 plus 1-1/4 inches, rounded to the nearest eighth. A 3-inch finished HST therefore starts from a 3-7/8 inch square two-at-a-time, a 5-1/2 inch square four-at-a-time, or a 7-3/4 inch square eight-at-a-time.

The methods also differ in grain: two- and eight-at-a-time keep the straight grain on the unit's outer edges, while four-at-a-time leaves bias edges all around, so handle those units gently and press before moving them. Whichever method you use, cut slightly generous and trim every HST to its unfinished size — finished plus half an inch — for perfect points at assembly.

Frequently Asked Questions