On-Point Quilt Calculator
Set your blocks on point and the quilt grows diagonally. Enter the finished block size and the number of blocks across and down to get the finished quilt size plus the setting triangle cuts.
Tip: use URL parameters to pre-fill, e.g. ?blockSize=12&blocksAcross=5&blocksDown=6.
The diagonal math
A block set on point uses its diagonal instead of its width. A 12-inch block has a diagonal of about 17 inches (12 × 1.414). The quilt width is the diagonal times the blocks across, plus the two corner triangle allowances; the height works the same way with the blocks down. The calculator adds 1/4-inch seams throughout.
Setting triangles
Side setting triangles fill the jagged edges between blocks. Cut a square at the block diagonal plus 1.25 inches, then cut it on both diagonals to make four triangles. A 12-inch block needs 17 + 1.25 = 18.25-inch squares. You need 2 × (across + down) of these triangles, which is 22 for a 5×6 layout.
Corner triangles fill the four corners. Cut a square at half the block diagonal plus 0.875 inches (about 8.9 inches for a 12-inch block), then cut it once on the diagonal. Two squares make the four corner triangles.
How the triangle counts work
Every on-point layout needs the same four corner triangles, and the side triangles depend on the grid. For a grid of A blocks across and B blocks down, the side count is 2 × (A + B). A 5×6 layout therefore needs 22 side triangles. Because each square cut on both diagonals yields four triangles, you'd cut six squares for the sides; the four corner triangles come from two squares cut once. If your layout is square — say 5×5 — the count becomes 20 side triangles plus 4 corners, which is the classic formula 4 × (blocks per side − 1).
The calculator reports finished quilt size as the block diagonal times the grid, plus the corner allowances. That's an approximation through the center rows: the actual size depends on seam allowances and how you trim the setting triangles, so compare the result with your target before cutting.
What each input means
Finished block size: the visible block size after seaming, from your pattern.
Blocks across / down: the grid of full blocks in your layout, not counting the setting triangles. A 5 across by 6 down layout uses 30 full blocks plus 22 side triangles and 4 corners.
Common mistakes to avoid
- Using straight-set triangle math. Setting triangles have their own formulas — half the diagonal plus 7/8" for corners, full diagonal plus 1.25" for sides.
- Cutting side triangles the wrong way. Side triangles must be cut on both diagonals so the straight grain runs along the quilt's outer edge; cutting them once leaves stretchy bias on the edge.
- Miscounting the side triangles. The count is 2 × (across + down), not across × down — the corners are handled separately by four corner triangles.
- Trimming points before sewing. Setting triangle points extend past the block corners by design; trim after the row is assembled.
- Forgetting that on-point grows the quilt. Block diagonals replace block widths, so the finished quilt is larger than the same grid set straight. Check the size before buying fabric.
When to use it. Use this calculator when a pattern says "set on point" or asks for setting triangles, or when you want the diagonal look for a sampler or bed quilt. Run it before buying fabric: the triangle cuts are easy to underestimate, and knowing the finished size first keeps your grid from growing past the mattress.
Common questions
Why cut setting triangles from squares on the diagonal?
Cutting a square on both diagonals makes quarter-square side triangles with the straight grain along the quilt's outer edge, where it belongs. Corner triangles are cut once so the straight grain runs along the two outer short sides; the bias edges face the seams, which is fine because the corners are trimmed and bound.
Do I need sashing for an on-point quilt?
No — sashing is optional. This calculator assumes blocks touch, with only setting triangles filling the edges. Add sashing strips as a separate step if your pattern uses them.
Why does my block look smaller on point?
On-point blocks appear smaller visually because the diagonal is shorter than the block's width plus its neighbor's. That's the trade-off that makes the layout dynamic.
Can I mix block sizes?
This calculator assumes one uniform block size. For mixed sizes, calculate each section separately and add the widths by hand.
Where does 1.414 come from?
It's the square root of 2 — the ratio between a square's side and its diagonal. It appears whenever you set square blocks on point.