Quilt Setting Triangle Calculator
Enter the finished block unit, side triangle count, fabric width, and cutting buffer to size grain-safe side and corner setting triangles for an on-point quilt.
Recommended fabric to buy1.75
- Effective finished unit
- 12
- Side-triangle square cut size
- 18.5
- Side squares to cut
- 5
- Extra side triangles
- 0
- Corner-triangle square cut size
- 9.63
- Corner squares to cut
- 2
- Base fabric length
- 1.54
- Cutting plan
- Cut 5 side squares at 18.5 inches and cut each square on both diagonals for 20 side triangles, leaving 0 extra. Cut 2 corner squares at 9.625 inches and cut each square once diagonally for 4 corner triangles. Pack the squares as 1 row with 1 side square and 2 corner squares; 2 rows with 2 side squares and 0 corner squares. For non-directional fabric, the two corner squares can be cut on either diagonal.
Fabric-width row layout
| Row | Side squares | Corner squares | Row height | Width used |
|---|---|---|---|---|
| 1 | 1 | 2 | 18.5 | 37.75 |
| 2 | 2 | 0 | 18.5 | 37 |
| 3 | 2 | 0 | 18.5 | 37 |
Square sizes are inches; fabric lengths are yards and cover setting and corner triangles only.
Usable width should exclude selvages and any edge you do not want in the quilt top.
How to use this calculator
- Enter the finished block size, then add finished sashing only if the setting triangles reach the sashing line.
- Count the side openings around the quilt top, excluding the four corner openings.
- Add any trimming allowance, usable fabric width, fabric buffer, and purchase rounding increment.
- Choose whether the fabric is directional, then read the square sizes, quantities, row layout, and yardage.
How the setting triangles are calculated
On-point quilts use two different triangle types. Side setting triangles fill the edges between corner units, and each is cut from a square that is sliced twice diagonally. Corner triangles fill the four corners, and each is cut from a square that is sliced once diagonally. Those cuts keep the straight grain on the outside edge of the quilt, which helps the top stay stable while it is handled, bordered, and quilted.
The calculator first builds the finished unit used by the setting triangle:
B = F + H
F is the finished block size, and H is the finished sashing width. If the pattern stops the setting triangles at the block edge, leave sashing at zero. If the sashing continues between the blocks and the setting triangles, include the finished sashing width.
The cutting sizes are then rounded upward to the next one-eighth inch:
S = ceil8(B x sqrt(2) + 1.25 + O)
C = ceil8(B / sqrt(2) + 0.875 + O)
- S is the square size for side triangles.
- C is the square size for corner triangles.
- O is the extra trimming allowance entered in the form.
- ceil8 means round up to the next one-eighth inch.
The number of side squares is ceil(T / 4) because one side square makes four side triangles. The calculator always uses two corner squares for a conventional rectangular quilt because two squares cut once diagonally make four corner triangles.
How the fabric length is packed
After the square sizes are known, the calculator enumerates every row of side and corner squares that fits across the usable fabric width. A row may contain side squares, corner squares, or both. Its height is the larger square in that row. The tool chooses the combination of rows that covers the required square counts with the shortest total height, then converts that height to yards.
The base yardage is L / 36, where L is the optimized row height in inches. The recommended yardage is ceil((base yardage x (1 + P / 100)) / Q) x Q, where P is the buffer percentage and Q is the selected purchase increment in yards.
What changes the answer most
The finished unit drives both square sizes. Adding sashing can move the side square by more than an inch because that size is multiplied by the square root of two. Usable fabric width matters when it changes how many large side squares fit in one row. A small width change near a packing breakpoint can add or remove an entire row of fabric.
What this tool leaves out
The estimate covers only side and corner setting triangles. It does not include blocks, sashing strips, cornerstones, borders, binding, backing, labels, test blocks, fussy cutting, repeat matching, or extra yardage for a directional print beyond the corner diagonal instruction. Pattern instructions should override the calculator if they use a nonstandard layout or a different seam allowance strategy.
Worked example
For a quilt with 12 inch finished blocks, no sashing, 20 side triangles, a 1/4 inch trimming allowance, 42 inches of usable fabric width, and a 10% buffer, the effective finished unit is 12 inches.
The side square is ceil8(12 x sqrt(2) + 1.25 + 0.25), or 18.5 inches. The corner square is ceil8(12 / sqrt(2) + 0.875 + 0.25), or 9.625 inches. Twenty side triangles require five side squares, and the four corners require two corner squares.
Across 42 inch fabric, the shortest layout is three rows: two rows with two side squares, and one row with one side square plus two corner squares. The base length is 55.5 inches, or 1.542 yards. With 10% extra and quarter-yard rounding, the recommended purchase is 1.75 yards.
Common questions
What is the difference between side and corner setting triangles?
Side setting triangles fill the open spaces along the four outside edges of an on-point quilt, excluding the corners. They are cut from squares sliced twice diagonally so the long outside edge stays on straight grain. Corner triangles fill the four corners and are cut from squares sliced once diagonally.
Do I include sashing in the finished block size?
Include sashing only when the setting triangle must span the block plus the finished sashing beside it. If your pattern places the setting triangle against the block edge and handles sashing separately, enter zero for sashing. When in doubt, compare the calculator result to the pattern diagram before cutting.
Why are side squares cut twice diagonally?
Cutting a side square on both diagonals makes four quarter-square triangles. In an on-point setting, that method puts straight grain on the outside quilt edge and bias edges inside the seam lines. Cutting those side pieces as half-square triangles would put a bias edge on the perimeter.
How do I count side triangles in an on-point layout?
Count the triangular openings along the top, bottom, left, and right edges, but do not count the four corner openings. If the count is not divisible by four, the calculator rounds up the square count and reports the spare side triangles you will have after cutting.
Should setting triangles be cut oversized?
Many quilters add a small trimming allowance because on-point edges are handled a lot before the top is squared. A quarter inch is a practical starting point, but a tested pattern may already include its preferred oversizing. If you enter zero, cut and sew a test set before cutting the full stack.
How do I cut setting triangles from directional fabric?
Directional fabric needs more attention at the corners because the diagonal direction affects how the print appears. The calculator tells you to cut the two corner squares on opposing diagonals, which mirrors the print direction around the four corners. Strong repeats or fussy placement may still require extra fabric.