Half-square triangles
Start with the right square size. Make two blocks at a time, with room to trim.
How we size two-at-a-time HSTs
With a standard ¼-inch seam, the traditional starter size is finished block size plus ⅞ inch. We round up to the next ¼ inch to leave trimming room. Larger custom seams are checked against the diagonal seam geometry and may require a larger starter.
Default starter size = finished size + ⅞ inch, then round up
One square of each color makes two HSTs. Trim each unit to finished size plus two seam allowances before sewing it into the quilt. This calculator uses the two-at-a-time method.
Read the complete HST sizing guideA few things to know
Do I enter finished or unfinished HST size?
Enter the finished size after the unit is sewn into the quilt. The tool adds twice the seam allowance to show the unfinished trim target.
Why does a 4-inch finished HST call for a 5-inch square?
The default calculation gives 4 7/8 inches: 4 inches plus two 1/4-inch seam allowances plus a 3/8-inch starting-square allowance. Rounding upward to the next 1/4 inch gives 5 inches. With custom seams, the tool also checks that the starter is large enough for the diagonal seam.
What happens if I need an odd number of units?
Each pair of starter squares makes two units. The pair count rounds upward, so 33 requested units need 17 pairs and produce 34 units. For one two-fabric pairing, cut 17 starter squares from each fabric.
Can I use this result for eight-at-a-time HSTs?
No. The starter-square rule and pair count here are for the two-at-a-time method. Other construction methods need their own cutting calculations.