Half square triangle calculator
Cut squares of 3.875 in from each of the two fabrics. Each unit measures 3.500 in before it is sewn into the quilt, and the squares take up 3.875 in of each fabric.
Result
Square to cut
in
Size of the sewn unit before joining
in
Squares of each fabric
pcs
Units these make
pcs
Strips of each fabric
pcs
Length of each fabric
in
How it works
A half square triangle unit is a square made of two triangles. Two squares, one of each fabric, are sewn a quarter inch either side of a drawn diagonal and cut apart, which gives two units; sewn along both diagonals and cut four ways, a larger pair gives eight. The question of what size square for half square triangles has one answer for each method: the finished size plus seven eighths of an inch, doubled for eight at a time half square triangles. Many quilters cut a little bigger and trim, so the HST cutting size can include that room. The tool also answers how many squares for half square triangles a pattern needs, and how much fabric they take.
Formula
Two at a time: square = finished size + 7/8 in. Eight at a time: square = 2 × (finished size + 7/8 in). With room to trim: add 1/4 in to the square. Sewn unit = finished size + 1/2 in. Squares of each fabric = units ÷ 2 (or ÷ 8), rounded up. Squares per strip = usable width ÷ square, rounded down; fabric of each = strips × square.
Worked example
The calculator opens with this example; the result above is computed from it.
| Input | Value |
|---|---|
| Finished size of the unit | 3 in |
| Method | Two at a time |
| Cutting size | Exact |
| Units needed | 16 pcs |
| Usable fabric width | 40 in |
| Square to cut | 3.875 in |
| Size of the sewn unit before joining | 3.500 in |
| Squares of each fabric | 8 pcs |
| Units these make | 16 pcs |
| Strips of each fabric | 1 pcs |
| Length of each fabric | 3.875 in |
Common cases
| Case | Starting values | Square to cut | Size of the sewn unit before joining | Squares of each fabric | Units these make | Strips of each fabric | Length of each fabric |
|---|---|---|---|---|---|---|---|
| Sixteen small units for a block | Finished size of the unit 3 in; Units needed 16 pcs | 3.875 in | 3.500 in | 8 pcs | 16 pcs | 1 pcs | 3.875 in |
| Pinwheels made eight at a time | Finished size of the unit 2.5 in; Units needed 48 pcs | 7.000 in | 3.000 in | 6 pcs | 48 pcs | 2 pcs | 14.000 in |
| Star points with room to trim | Finished size of the unit 2 in; Units needed 32 pcs | 3.125 in | 2.500 in | 16 pcs | 32 pcs | 2 pcs | 6.250 in |
| Large units for a throw quilt | Finished size of the unit 6 in; Units needed 80 pcs | 6.875 in | 6.500 in | 40 pcs | 80 pcs | 8 pcs | 55.000 in |
Questions and answers
What size square do I cut for half square triangles?
For units that finish at 3 in, made two at a time, cut squares of 3.875 in from each fabric. Each sewn unit measures 3.500 in before it goes into the block.
What size squares do I need for eight at a time half square triangles?
For units that finish at 2.5 in, with room to trim, cut squares of 7.000 in. Trim each unit to 3.000 in; the squares take up 14.000 in of each fabric.
Assumptions and limits
- Sizes assume a quarter inch seam allowance. Decimal inches read as eighths: 0.125 = 1/8, 0.375 = 3/8, 0.625 = 5/8, 0.875 = 7/8.
- Room to trim adds a quarter inch to the starting square, as one of the sources suggests. Trim each unit to the unfinished size.
- Squares are counted for each of the two fabrics. Extra units from the last pair are left over.
- The method is defined in inches. Metric figures are the same sizes converted, so they are not round numbers.
The result is an estimate for planning, not professional advice.
Sources
- Sarah Maker: half square triangle charts (two and eight at a time, finished + 7/8 in, trimming advice)retrieved 2026-10-13
- Omni Calculator: half square triangle (raw square = finished + 7/8 in, size chart)retrieved 2026-10-13
Last updated:
Method in depth: Half square triangles and flying geese from squares
Cite this page: Quilt Math, “Half square triangle calculator”, https://quilt-math.pages.dev/cutting/half-square-triangle-calculator/ (updated 13 October 2026).
Found a mistake? Email handycalc.sites+quiltmath@gmail.com.