Half-Square Triangle (HST) Guide
The half-square triangle, or HST, is one of the most-used units in quilting. Two contrasting triangles sewn into a square become pinwheels, flying geese alternatives, star points, and countless block designs.
The Starting-Square Formula
For the classic two-at-a-time method, cut two squares at the finished HST size plus 7/8 inch. The extra 7/8 inch covers the seam allowance and the diagonal bias edge. Quilters usually round up to the nearest convenient cut and trim later.
The Magic 8 Method
To make eight identical HSTs, place two squares right sides together, draw four evenly spaced lines on the back, then sew a scant 1/4 inch on both sides of each line. Cut along the drawn lines and the center diagonal, then press and trim. It is the fastest way to batch HSTs for a big quilt.
Starting Square by Finished Size
| Finished HST | Exact cut | Practical cut (trim later) |
|---|---|---|
| 2" | 2.875" | 3" |
| 3" | 3.875" | 4" |
| 4" | 4.875" | 5" |
| 5" | 5.875" | 6" |
Trimming for Accuracy
Press the seam to one side, then trim each unit to its exact unfinished size using a ruler with a 45-degree line through the center. Trimming after pressing keeps the diagonal seam centered and every block square, which makes assembly much smoother.
Frequently Asked Questions
What size square do I cut for a half-square triangle?
For the two-at-a-time method, cut two squares that are the finished HST size plus 7/8 inch. So a 4 inch finished HST starts from 4.875 inch squares, which quilters usually round up to 5 inches.
What is the magic 8 method for HSTs?
The magic 8 method makes eight identical HSTs from two squares. Draw four lines on the back of one square, sew on both sides of each line, cut along the lines, then cut the half-square blocks in half once more diagonally to yield eight units.
Should I trim HSTs before pressing?
Press the seams first, then trim each unit to the exact unfinished size using a ruler with a 45 degree line. Trimming after pressing keeps the diagonal seam centered and the block square.
How much extra fabric should I allow for HSTs?
The 7/8 inch rule is exact, but most quilters add a full inch and trim down. If you will trim, cut starting squares at the finished size plus 1 inch for easier handling.
Piecing accuracy
Tools for perfectly squared blocks
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