Half-square triangle (HST) chart
The single most-used unit in patchwork, and the one people most often cut wrong. Here is what to cut for any finished size, by whichever method you use.
A half-square triangle is one square cut once on the diagonal, giving two triangles — or, far more usefully, two squares stitched either side of a drawn diagonal and cut apart, giving two finished units. The second method is faster, more accurate and keeps the bias inside the seam where it cannot stretch.
HST cutting chart
| Finished HST | 2-at-a-time: cut 2 squares | 8-at-a-time: cut 2 squares | Unfinished (trim to) |
|---|---|---|---|
| 1" | 1⅞" | 3¾" | 1½" |
| 1½" | 2⅜" | 4¾" | 2" |
| 2" | 2⅞" | 5¾" | 2½" |
| 2½" | 3⅜" | 6¾" | 3" |
| 3" | 3⅞" | 7¾" | 3½" |
| 3½" | 4⅜" | 8¾" | 4" |
| 4" | 4⅞" | 9¾" | 4½" |
| 4½" | 5⅜" | 10¾" | 5" |
| 5" | 5⅞" | 11¾" | 5½" |
| 6" | 6⅞" | 13¾" | 6½" |
| 7" | 7⅞" | 15¾" | 7½" |
| 8" | 8⅞" | 17¾" | 8½" |
| 9" | 9⅞" | 19¾" | 9½" |
| 10" | 10⅞" | 21¾" | 10½" |
| 12" | 12⅞" | 25¾" | 12½" |
The formulas
- Two at a time: finished size + ⅞″. Two squares of that size give two HSTs.
- Eight at a time: (finished size × 2) + 1¾″. Two squares of that size give eight HSTs.
- Unfinished size: finished size + ½″. This is what the unit should measure before it is sewn into anything.
The ⅞″ is not arbitrary: a half-inch of it is the two quarter-inch seam allowances on the finished unit, and the remaining ⅜″ covers the diagonal, which needs more room than a straight seam because the seam allowance is measured perpendicular to a 45-degree line.
Two at a time
- Draw a diagonal line corner to corner on the wrong side of the lighter square.
- Layer the two squares right sides together and stitch a scant quarter inch on each side of the drawn line.
- Cut apart on the drawn line.
- Press toward the dark, or open if many HSTs will meet.
- Trim to the unfinished size, squaring on the diagonal.
Eight at a time
- Draw both diagonals on the wrong side of the lighter square.
- Layer right sides together and stitch a quarter inch either side of both drawn lines.
- Cut the square in half vertically, then horizontally — do not move the pieces.
- Cut each quarter apart on its drawn diagonals. Eight units.
The eight-at-a-time method is dramatically faster for a quilt that needs a hundred identical HSTs, but every unit comes out of the same two squares, so an error in the marking multiplies by eight rather than by two.
Why oversize and trim
Cut a quarter inch larger than the chart and trim every unit to exact size. It sounds like a waste of fabric and time, and it is the difference between points that survive and points that vanish. Seam allowance drift, the thickness of the thread, and the fold taken up when you press all conspire to make an untrimmed HST slightly small — and in a Sawtooth Star, eight slightly small units add up to a block that is a quarter inch short.
The half-square triangle block → · Pinwheel → · Sawtooth Star →
Questions
What size squares do I need for a 3 inch finished HST?
Cut two squares at 3⅞″ for the two-at-a-time method, or two squares at 7¾″ for the eight-at-a-time method. The finished unit trims to 3½″ before it is sewn in.
What is the formula for half-square triangles?
Finished size plus ⅞ inch for two at a time. For eight at a time, double the finished size and add 1¾ inches. To trim the finished unit, use finished size plus ½ inch.
Should I press HST seams open or to one side?
To the dark side helps points nest when units join, and is stronger. Open reduces bulk where many HSTs converge, such as a pinwheel centre. Both are standard practice — pick based on what the unit joins next.
Why do my half-square triangles come out too small?
Almost always a seam allowance slightly over a quarter inch, compounded by the fabric taken up in the press. Cutting a quarter inch oversized and trimming every unit to exact size eliminates the problem entirely.
Stop looking things up
FourPatch does this arithmetic for every piece in your quilt, every time you change something — block size, sashing width, fabric width, waste allowance. The cutting list and yardage just update.