Calculator
Setting Triangle Calculator
Cut sizes for side and corner setting triangles on an on-point (diagonal set) quilt, plus how many of each you need for your layout.
Setting Triangle Cut Sizes by Block Size
| Finished Block | Side Triangle (cut, X) | Corner Triangle (cut, ↘) |
|---|---|---|
| 6″ | 9-3/4" | 5-1/8" |
| 8″ | 12-5/8" | 6-5/8" |
| 9″ | 14" | 7-1/4" |
| 10″ | 15-1/2" | 8" |
| 12″ | 18-1/4" | 9-3/8" |
| 14″ | 21-1/8" | 10-7/8" |
| 15″ | 22-1/2" | 11-1/2" |
| 16″ | 24" | 12-1/4" |
| 18″ | 26-3/4" | 13-5/8" |
Side triangle squares are cut with two diagonal cuts (an X) — 4 triangles per square. Corner triangle squares are cut with one diagonal cut — 2 triangles per square.
How Many Setting Triangles You Need
| On-Point Grid (rows × cols) | Side Triangles | Corner Triangles |
|---|---|---|
| 3×4 | 10 | 4 |
| 4×5 | 14 | 4 |
| 5×7 | 20 | 4 |
| 6×8 | 24 | 4 |
| 7×8 | 26 | 4 |
Formula: side triangles = 2 × (rows + cols) − 4. Corner triangles = 4, always.
Setting a Quilt On Point
An on-point (diagonal set) layout rotates every block 45°, so the rows run corner-to-corner instead of edge-to-edge. That leaves triangular gaps around the outside of the grid — side setting triangles fill the gaps along the edges, and corner setting triangles fill the four corners.
Both are deliberately cut oversized and trimmed after piecing, for two reasons: it's much easier to trim a slightly-too-big triangle flush than to stretch a slightly-too-small one, and cutting them as quarter-square (side) or half-square (corner) triangles keeps the straight grain running along the quilt's outer edge instead of the stretchier bias grain — a bias outer edge ripples and stretches out of shape over time.
Shop Fabric for Setting Triangles
A background solid or low-volume print is the most common choice — it lets the on-point blocks read as the focal point.
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Frequently Asked Questions
What size do I cut side setting triangles?
Side setting triangles are cut oversized so the outer edge falls on the straight grain: square size = (finished block size × 1.414) + 1.25", cut with two diagonal cuts (an X) to yield 4 triangles per square. For a 12" block, that's an 18-1/4" square.
What size do I cut corner setting triangles?
Corner setting triangles are cut as half-square triangles: square size = (finished block size ÷ 1.414) + 0.875", cut once diagonally to yield 2 triangles per square. For a 12" block, that's a 9-3/8" square. You always need exactly 4 corner triangles, regardless of quilt size.
How many setting triangles do I need for an on-point quilt?
For an on-point grid of R rows × C columns of blocks: side triangles needed = 2×(R+C)−4, and corner triangles needed = 4 (always). A 5×7 on-point grid needs 20 side triangles and 4 corner triangles.
Why are side and corner setting triangles cut differently?
Both methods are chosen so the triangle's outer edge (the one that becomes the quilt's outer edge) falls on the fabric's straight grain instead of the stretchier bias grain. A quarter-square cut (X, 4 triangles) puts the straight grain on the long outer edge for side triangles; a half-square cut (1 diagonal, 2 triangles) does the same for corner triangles.