Every HST starts from squares, the question is how many at once. Two at a time keeps edges on grain with the famous ⅞ rule; the magic eight turns two big squares into eight identical units. Enter the finished size and your count; both cutting lists follow.
Your HSTs
Method
Cut
32squares
16 pairings of 4″ squares · 32 HSTs
Pair squares, draw one diagonal, sew ¼″ each side, cut the line, two per pairing. Trim to 3 ½″.
- Starting square4″ (oversize)
- Squares to cut32, half light, half dark
- HSTs per pairing2
- Trim each unit to3 ½″
The Formulas
Two at a Time
starting squares = finished + ⅞ · one pair of squares = 2 HSTsMagic Eight
starting squares = (finished + ⅞) × 2 · one pair = 8 HSTsThe ⅞ pays for the diagonal seam’s bias corners. Trimmers round it to a whole inch, finished + 1: sew, and square the unit down: a half-minute per HST that buys perfect points.
Worked Example
Thirty-two HSTs at 3 in finished, magic eight, trimming:
- Starting square: (3 + 1) × 2 = 8 in: two squares per batch of eight.
- 32 ÷ 8 = 4 batches: eight 8 in squares in all, four light, four dark.
- Sew ¼ in on both sides of both diagonals, cut the plus and the X: eight units each, trimmed to 3½.
HST Cut Chart
| Finished | 2-at-a-time exact | 2-at, trim | Magic 8 exact | Magic 8, trim |
|---|---|---|---|---|
| 2″ | 2 ⅞″ | 3″ | 5 ¾″ | 6″ |
| 2.5″ | 3 ⅜″ | 3 ½″ | 6 ¾″ | 7″ |
| 3″ | 3 ⅞″ | 4″ | 7 ¾″ | 8″ |
| 3.5″ | 4 ⅜″ | 4 ½″ | 8 ¾″ | 9″ |
| 4″ | 4 ⅞″ | 5″ | 9 ¾″ | 10″ |
| 5″ | 5 ⅞″ | 6″ | 11 ¾″ | 12″ |
| 6″ | 6 ⅞″ | 7″ | 13 ¾″ | 14″ |
Working Each Method
- Two at a time: pair squares right sides together, draw one diagonal, sew ¼ in both sides, cut the line. Edges stay on grain, the units barely stretch.
- Magic eight: same pairing, sew ¼ in beside both diagonals, then cut horizontal, vertical, and both diagonals. Fast, identical, but every edge is bias: press, don’t iron.
- Trimming: line the ruler’s 45° on the seam, trim two sides, rotate, trim to unfinished size (finished + ½).
Assumptions and Defaults
- Trim mode rounds the ⅞ to 1 (two-at) and sizes the magic-eight square from it.
- A pinwheel takes 4 HSTs; the count field times any block’s HST count feeds the batch math.
- Fabric pairs split evenly between the two prints.
HST Questions, Answered
What size squares do I cut for half square triangles?
For half square triangles made two at a time, cut squares at the finished size + ⅞ in, the well-known rule, and for the magic eight method cut squares at that figure doubled. Quilters who prefer to trim add 1 in instead and square the units down afterward, and the chart lists both columns.
How does the magic 8 HST method work?
The magic 8 method places two large squares right sides together, sews ¼ in on each side of both diagonals, and then makes four cuts: across, down, and along both diagonals. One pairing yields eight identical HSTs, so the example’s 32 units take only four pairings.
Why do HSTs add ⅞ of an inch?
HSTs add ⅞ in because the cut square needs ½ in for the two seam allowances on its outer edges plus about 0.35 in for the ¼ in seam running along the diagonal, which is ¼ × √2. Those add up to about 0.85 in, which quilters round up to the ruler’s ⅞, and that small extra is why careful piecers still trim.
How many HSTs do I need for a pinwheel quilt?
A pinwheel quilt needs four HSTs per pinwheel block, so a 48-block pinwheel throw takes 192 HSTs, or 24 magic eight pairings. Batches like this are exactly why the multi-unit methods exist, since they save hours of cutting and sewing.