What this calculator does
A half-square triangle, or HST, is a square made of two right triangles. It is the most common unit in quilting. There are several ways to make them, and each needs a different starting square. Patterns usually give one method; this calculator tells you the square size for all three, so you can use whichever you prefer.
- 2 at a time: draw a diagonal on the light square, sew ¼ inch either side, cut on the line. Two HSTs.
- 4 at a time: sew all the way around the edge of two squares, then cut on both diagonals. Four HSTs with bias outer edges.
- 8 at a time: sew ¼ inch either side of both diagonals, cut in half both ways, then on the drawn lines. Eight HSTs.
How to use it
- Enter the finished size from your pattern. If the pattern gives an unfinished size, subtract ½ inch.
- Pick the method and enter how many HSTs you need.
- Cut the number of square pairs shown: one light and one dark square per pair.
- Use the trim size to square up each HST.
The math
2 at a time: F + 7/8″ (or F + 1″ to trim)
4 at a time: (F + 7/8″) × 1.414
8 at a time: 2 × (F + 7/8″) (or 2 × (F + 1″) to trim)
trim to F + ½″
All square sizes are rounded up to the next 1/8 inch, the smallest mark on a standard quilting ruler. The 4-at-a-time size uses the square root of 2 (1.414) because the finished HSTs come from the quarters of a square cut on its diagonals.
HST square size chart
Sizes with trimming room. Cut, sew, then trim to finished + ½ inch.
| Finished | 2 at a time | 4 at a time | 8 at a time | Trim to |
|---|---|---|---|---|
| 1 1/2″ | 2 1/2″ | 3 5/8″ | 5″ | 2″ |
| 2″ | 3″ | 4 3/8″ | 6″ | 2 1/2″ |
| 2 1/2″ | 3 1/2″ | 5 1/8″ | 7″ | 3″ |
| 3″ | 4″ | 5 3/4″ | 8″ | 3 1/2″ |
| 3 1/2″ | 4 1/2″ | 6 1/2″ | 9″ | 4″ |
| 4″ | 5″ | 7 1/4″ | 10″ | 4 1/2″ |
| 5″ | 6″ | 8 5/8″ | 12″ | 5 1/2″ |
| 6″ | 7″ | 10″ | 14″ | 6 1/2″ |
Tips
Press toward the dark. Pressing the seam toward the darker fabric stops it shadowing through the light side. For blocks with many HSTs, pressing seams open helps them lie flat.
Trim on the diagonal. Line up the 45° line on your ruler with the HST's seam when you trim, so the diagonal runs exactly corner to corner.
Need to know how much fabric all those squares take? Put the square size into the fabric cutting calculator.
Questions people ask
What does finished size mean for an HST?
The size the square shows once it is sewn into the quilt, without seam allowances. A 3 inch finished HST is trimmed to 3½ inches before it is sewn into a block.
Why add 7/8 inch for the 2-at-a-time method?
Cutting a square on the diagonal loses length along the bias, so the classic rule is finished size plus 7/8 inch. Many quilters add a full inch instead and trim, because a slightly oversized HST trimmed square is more accurate than an exact one.
Which method is fastest?
8-at-a-time makes the most HSTs per seam, but every HST has bias edges, so press carefully. 2-at-a-time is the most forgiving and the best place to start.
Should I starch before cutting?
Starching makes bias edges much less stretchy and is especially useful for the 4- and 8-at-a-time methods.