CraftM8 › QuiltM8 › Calculators › Half-square triangles
What do I cut for half-square triangles?
Half-square triangles by the 2-, 4- or 8-at-a-time method: the square to cut from each fabric, how many squares, and the size to trim to when pressed. Three sizes for one triangle — finished, cut, trimmed — each labelled.
Sets: 20 sets · Spare: 0 units
Show the maths
| 1 | Cut square — 4" + 7/8" = 4 7/8" → round up to 1/8" | 4 7/8" |
| 2 | Sets — 40 units ÷ 2 per set → round up | 20 |
| 3 | Squares per fabric — 20 squares of EACH fabric (20 light + 20 dark) | 20 |
| 4 | Units made — 20 × 2 = 40, needed 40 | 0 spare |
| 5 | Trim to — 4" + 1/2" seam allowances | 4 1/2" |
Units are made slightly oversize and trimmed to size. The long edge of each triangle is on the bias. Cut this many squares of each of the two fabrics, not in total.
Where the formula comes from
For two-at-a-time, the classic 7/8″ oversize on the finished measurement, trimmed back after pressing. The count of squares rounds up in sets, and the tool reports the surplus rather than hiding it.
- 2-at-a-time. Two squares cut at finished size + 7/8", sewn 1/4" either side of the diagonal, cut apart: the 7/8" is two 1/4" seams plus the 3/8" the diagonal consumes.
- 4-at-a-time and 8-at-a-time. Published cut-size formulas (finished × √2 + 1 1/4"; finished × 2 + 1 3/4"). Charts vary by about 1/8" between sources, so this tool rounds up and reports a trim-to size — the standard practice of making units slightly oversized and squaring them up.
This is the app's own arithmetic running in your browser, put through the same worked examples every time this site is built.
A worked example
The worked example is the how-to quilt’s pinwheels: 120 triangles finishing at 6″, two at a time — cut 6 7/8″ squares, 60 of each fabric, trim to 6 1/2″.
| 1 | Cut square — 6" + 7/8" = 6 7/8" → round up to 1/8" | 6 7/8" |
| 2 | Sets — 120 units ÷ 2 per set → round up | 60 |
| 3 | Squares per fabric — 60 squares of EACH fabric (60 light + 60 dark) | 60 |
| 4 | Units made — 60 × 2 = 120, needed 120 | 0 spare |
| 5 | Trim to — 6" + 1/2" seam allowances | 6 1/2" |
Works with
- How many strips, and how much fabric? — Turn the squares into strips and yardage.
- What does a fat quarter really give me? — Or cut them from precuts you already have.
In the app, this answer joins a quilt
Save it to a project, add the other tools' answers, and the shopping list totals every fabric with one rounding — then prints a cut sheet for the table. Offline, no account, one payment, yours forever.
In development A closed test on Google Play is the next step.