How to Calculate Side and Corner Setting Triangles for an On-Point Quilt

Setting triangles fill the spaces around blocks rotated 45 degrees. Side setting triangles fill the gaps along all four edges, including the top and bottom. Corner setting triangles complete the four corners. Their cutting methods place straight grain along the quilt’s outer perimeter. Generations Quilt Patterns explains the two triangle types and their grain direction.

Triangle type	Cut the starting square	Yield per square	Straight-grain edges in the quilt
Side setting triangle	Cut along both diagonals, making an X	Four triangles	The long outside edge
Corner setting triangle	Cut along one diagonal	Two triangles	The two short outside edges

A side triangle is therefore a quarter-square triangle, while a corner triangle is a half-square triangle. Begin with squares whose edges follow the fabric grain. Cutting every square just once diagonally would put a bias edge along the perimeter where a side triangle belongs. The cutting method matters as much as the measurement. Generations Quilt Patterns’ rotary-cutting guidance illustrates these arrangements.

Use the block’s finished side length, measured along one side rather than across its diagonal. Designed to Quilt specifies the block dimension as it will measure in the finished quilt top when calculating the starting squares. Setting Triangles for On-Point Quilts + How to Calculate Them explains this input.

With ¼-inch seams, a block measuring 12½ inches before assembly is intended to finish at 12 inches: the two seam allowances together account for ½ inch. Enter 12 in the formulas. Do not enter 12½, and do not add another seam allowance afterward—the formula already includes its cutting allowance.

Before calculating, write down three separate pieces of information: the finished block size, the required number of side triangles, and the required number of corner triangles. Keeping size and quantity separate makes the resulting cutting list easier to check.

These block-only calculations cover a layout without sashing. If your design includes sashing, use its pattern’s setting instructions or establish the geometry of the complete layout before cutting. A calculator that accepts block size alone may exclude sashing; Quiltler’s setting-triangle documentation explicitly states that limitation.

For a finished block side length of B inches, calculate: side-triangle starting square = (B × 1.414) + 1.25 inches. Round the completed result upward to an eighth-inch ruler mark, then cut the square along both diagonals. Each square produces four side triangles. Quilter’s Cache provides the formula and cutting method.

For the illustrative 12-inch finished block:

1. Multiply: 12 × 1.414 = 16.968 inches.
2. Add the allowance: 16.968 + 1.25 = 18.218 inches.
3. Round upward: 18.218 becomes 18¼ inches.
4. Cut an 18¼-inch square, then cut it corner to corner in both directions.

The result is the side length of the starting square. It is not a measurement to mark along an individual triangle. Label your cutting note “18¼-inch squares; cut twice diagonally” so the measurement stays connected to the correct operation.

Using the same finished block size, calculate: corner-triangle starting square = (B ÷ 1.414) + 0.875 inch. Round upward to the next eighth inch. Cut each square along one diagonal to produce two corner triangles. Designed to Quilt documents this formula and its two-triangle yield.

For the illustrative 12-inch finished block:

1. Divide: 12 ÷ 1.414 ≈ 8.4866 inches.
2. Add the allowance: 8.4866 + 0.875 ≈ 9.3616 inches.
3. Round upward: 9.3616 becomes 9⅜ inches.
4. Cut two 9⅜-inch squares, then cut each once diagonally.

Those two squares produce four corner triangles. The smaller starting-square measurement is expected: corner triangles fill a different space from the larger triangles along the sides.

Round upward after completing each formula. “Up to the next ⅛ inch” means selecting the first eighth-inch measurement equal to or greater than the calculated result. Fat Quarter Shop specifies this rounding rule for both formulas in its On-Point Quilt Setting Cheat Sheet.

For example, the side-square calculation for a 10-inch finished block gives 15.39 inches. A 15⅜-inch square measures 15.375 inches, which is smaller than the result. The upward-rounded cutting size is therefore 15½ inches.

The following compact reference uses the stated formulas and rounds only the final results upward. Every entry is a starting-square size, in inches.

Finished block	Side square: cut twice diagonally	Corner square: cut once diagonally
6 inches	9¾ inches	5⅛ inches
8 inches	12⅝ inches	6⅝ inches
9 inches	14 inches	7¼ inches
10 inches	15½ inches	8 inches
12 inches	18¼ inches	9⅜ inches
14 inches	21⅛ inches	10⅞ inches
16 inches	23⅞ inches	12¼ inches

Choose your cutting approach before preparing the fabric. The rounded formula gives the traditional starting-square size. For extra trimming room, Fat Quarter Shop’s “What Kimberly Cuts” method adds ½ inch to each rounded starting-square measurement. Its chart lists both approaches side by side. Fat Quarter Shop’s On-Point Quilt Setting Cheat Sheet supplies these instructions.

For 12-inch finished blocks, that choice gives:

• Traditional: 18¼-inch side squares and 9⅜-inch corner squares.
• Oversized: 18¾-inch side squares and 9⅞-inch corner squares.

The additional ½ inch applies to the starting square. It does not specify a finished margin around the blocks. Oversized triangles require trimming after assembly; American Patchwork & Quilting instructs quilters to retain a ¼-inch seam allowance outside the pieced block corners. Its diagonal-row tutorial explains the trimming step.

Count the triangular gaps around your actual layout. A conventional rectangular on-point setting has four corner triangles; the number of side triangles depends on the arrangement. Generations Quilt Patterns describes this layout-based counting method.

Convert the triangle count into squares using the cutting yields:

• Side squares: divide the required side-triangle count by four, then round up to a whole square.
• Corner squares: divide the required corner-triangle count by two, then round up to a whole square.

For an illustrative layout requiring 14 side triangles and four corner triangles, with 12-inch finished blocks, the traditional cutting list is:

• Four 18¼-inch squares, each cut twice diagonally: 16 side triangles, with two unused.
• Two 9⅜-inch squares, each cut once diagonally: four corner triangles.

This example assumes the triangle count has already been established from the layout. When making your own list, mark each perimeter gap on a sketch as you count it. If fabric placement matters, label the intended pieces before sewing rather than relying on the total alone.

Lay out the blocks and triangles in diagonal rows before sewing. American Patchwork & Quilting’s assembly sequence joins blocks and side triangles into rows, presses adjacent rows in opposite directions, joins the rows, and then adds the corner triangles. Follow your pattern’s sequence if it specifies a particular construction order. How to: Piece Diagonal Blocks and Rows provides the full procedure.

After assembly, check the outer edge before trimming. Leave ¼ inch beyond the pieced block points for the next seam. If rows differ substantially in length, check block dimensions and seam allowances before joining them; the same tutorial recommends correcting those discrepancies during assembly. American Patchwork & Quilting’s row-matching and trimming instructions explain both checks.

Your final cutting note should identify four things for each triangle type: starting-square size, number of squares, diagonal cutting method, and expected triangle yield. With those recorded, you can move from the calculation to cutting without having to reconstruct which measurement belongs to which piece.

• How to Make Setting Triangles — Quilter’s Cache, Marcia Hohn
• Setting Triangles for an On-Point Quilts — Generations Quilt Patterns
• Setting Triangles for On-Point Quilts + How to Calculate Them — Designed to Quilt
• Cut setting triangles for an on-point quilt — Quiltler Help
• On-Point Quilt Setting Cheat Sheet for Side Setting and Corner Setting Triangles — Fat Quarter Shop
• How to: Piece Diagonal Blocks and Rows — American Patchwork & Quilting