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A quarter-square triangle (QST) is a square split on both diagonals — the hourglass unit in Ohio Star, Card Trick, and a hundred other blocks. Cut the starting square at finished size + 1¼″. Enter your finished size and how many units you need; get the cut size, the squares of each fabric, and the fat quarters or yardage to buy. Or type in a square you already have and it tells you what it finishes at.
For 3″ finished QSTs (3½″ unfinished), cut starting squares at 4½″ and trim each unit down after sewing.
| Fabric | Squares | Per fat quarter | Fat quarters | Or yardage |
|---|---|---|---|---|
| Fabric A | 8 | 16 | 1 | ¼ yd (1 strip) |
| Fabric B | 8 | 16 | 1 | ¼ yd (1 strip) |
Each pair of squares makes 2 hourglass units; 8 pairs → 16 units. Yardage assumes 40″ usable width; fat quarters assume 18″ × 21″ after trimming.
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| Finished QST | Unfinished | Cut square (exact) | Cut oversized | Squares per FQ |
|---|---|---|---|---|
| 1½″ | 2″ | 2¾″ | 3″ | 42 |
| 2″ | 2½″ | 3¼″ | 3½″ | 30 |
| 2½″ | 3″ | 3¾″ | 4″ | 20 |
| 3″ | 3½″ | 4¼″ | 4½″ | 16 |
| 3½″ | 4″ | 4¾″ | 5″ | 12 |
| 4″ | 4½″ | 5¼″ | 5½″ | 12 |
| 4½″ | 5″ | 5¾″ | 6″ | 9 |
| 5″ | 5½″ | 6¼″ | 6½″ | 6 |
| 6″ | 6½″ | 7¼″ | 7½″ | 4 |
| 8″ | 8½″ | 9¼″ | 9½″ | 2 |
Two squares make two hourglass units, so "squares per fat quarter" is also QST units per fat quarter of that fabric when paired with a second fabric.
Finished QST size plus 1¼ inches. A 3 inch finished hourglass starts from 4¼ inch squares. Many quilters cut at finished plus 1½ inches instead and trim the unit down after sewing — the extra ¼ inch is cheap insurance against the bias edges stretching. The calculator shows both.
An HST has one seam on the diagonal, so it needs ⅞ inch (two ¼ inch seams measured across a 45° cut). A QST is cut on both diagonals, so every triangle has bias on two sides and the seam allowance is measured across both — 1¼ inches, which is actually 1.207 rounded to a ruler mark.
Two squares — one of each fabric — make two hourglass (2-color QST) units. Sew them as an HST pair first, then cut across the other diagonal and re-sew. For 16 hourglass units you need 8 squares of each fabric.
Make an HST from fabrics A and B. Place it right sides together with a plain square of fabric C, draw the diagonal that crosses the HST seam, sew ¼ inch on both sides, and cut. You get two split QSTs that are mirror images of each other. Every batch of 1 A + 1 B + 2 C squares makes 4 units.
Finished size plus ½ inch, the same as any pieced unit. A 3 inch finished QST measures 3½ inches before it goes into the block.
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