Half-Square Triangle Calculator

Choose your finished HST size, how many you need, and the method (2-, 4-, or 8-at-a-time) to get the exact cut-square size, squares to cut, and trim-down size.

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:

2-at-a-time: cut = finished + 7⁄8″
4-at-a-time: cut = finished + 1¼″
8-at-a-time: cut = (finished × 2) + 1¾″

Always trim to size

Cut a hair big, press, then trim every unit to the unfinished size (finished + ½″). It feels fussy, but trimming is what makes the seams line up later. The 8-at-a-time method is the fastest when you need a lot of matching triangles.

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Half-Square Triangle Calculator