A square made from two triangles
A half-square triangle unit, often shortened to HST, is a square with a diagonal seam joining two fabric triangles. Two-at-a-time construction makes two matching units from a pair of squares; eight-at-a-time makes eight. One square in each pair is fabric A and the other is fabric B.
Enter the unit’s finished size: the side that will show after the unit is sewn into a quilt. The tool also gives the unfinished trim size, which includes ¼ inch at each outer edge.
Cut a little large, then trim
This tool deliberately uses oversized starting squares and standard ¼-inch seams. In inches:
- Two at a time: starting square side = finished size + 1.
- Eight at a time: starting square side = 2 × (finished size + 1).
- Trim each sewn unit to finished size + ½ before joining it to other pieces.
For a 4-inch finished unit, trim to 4½ inches. Start with 5-inch squares for two at a time, or 10-inch squares for eight at a time. These are trim-to-size methods, rather than minimum-cut formulas. Changing to centimeters converts the same construction and keeps the 0.635 cm seam allowance.
Work in whole pairs
If you need 17 units, two at a time takes nine pairs and makes 18; eight at a time takes three pairs and makes 24. Spare units are shown explicitly. The quantity counts whole square units, not individual triangles, and is for one two-fabric combination. Calculate different color combinations separately.
Follow the on-screen cutting sequence and test a pair before cutting everything. Use a ruler’s 45-degree line on the diagonal seam when trimming. The diagrams show the construction; they are not full-size templates. If calculating fabric with the fabric yardage tool, use the starting-square size and calculate each fabric separately.