Setting Triangle Calculator
The squares to cut for the side and corner setting triangles of an on-point quilt, how many of each, and the size of the finished top.
Result
Cut squares for side triangles: 18 1/4 inEstimate
Cut each square twice on the diagonal for four trianglesCut sizes are rounded up to your ruler, so the triangles run a little large and are trimmed square once the top is sewn.
- Squares to cut for side triangles
- 3
- Side setting triangles needed
- 10
- Cut squares for corner triangles
- 9 3/8 inEstimate
- Cut each square once on the diagonal for two triangles
- Squares to cut for corner triangles
- 2
- Blocks in the layout
- 18
- Finished top width
- 50 15/16 in
- Finished top height
- 67 7/8 in
- Block diagonal, point to point
- 17 in
https://calcgallery.com/crafts/setting-triangle-calculator/
Results are arithmetic on the figures you enter and are estimates. Products, materials and animals vary, so follow the maker's instructions and label, make a test piece where it matters, and ask a vet about feeding. Nothing here is safety or veterinary advice. Read the full disclaimer
How to use the Setting Triangle Calculator
- Enter the finished size of one block, without its seam allowance.
- Count the blocks along the top edge and down one side of the on-point layout.
- Enter the sashing width if the blocks are separated by sashing, or leave it at 0.
- Read the square sizes to cut for the side and corner triangles, and how many squares of each.
How it is calculated
side square = (block + sashing) × √2 + 5 × seam rounded up to your rulercorner square = (block + 2 × sashing) ÷ √2 + 3.5 × seam rounded up to your rulerside triangles = 2 × (across − 1) + 2 × (down − 1) four from each squareblocks = across × down + (across − 1) × (down − 1)top width = √2 × (across × (block + sashing) + sashing)The long side of a side setting triangle runs from the point of one block to the point of the next, which is the block’s diagonal: the block size times the square root of 2, about 1.414. A corner triangle’s long side lies against one side of a block, so its short sides are the block size divided by 1.414.
At a 1/4 inch seam the additions are the 1 1/4 inch and 7/8 inch that Generations Quilt Patterns and Quiltville’s on-point notes give, with the total rounded up to the nearest 1/8 inch. The page does the same, and rounds up to 5 mm in metric by default; change the step under More options.
Counting the layout on point
Count the blocks you see along the top edge of the quilt, not along a diagonal row. A layout 3 across and 4 down has 12 blocks in those rows and 6 more set between them, 18 in all. The finished top is the block diagonal times the count each way, before any borders.
With sashing
When sashing runs between the blocks, the side triangles are sized from the block plus one sashing width and the corners from the block plus two. That keeps the outer cornerstones whole instead of cutting them in half, the method Brenda Hotinger of The Quilting Nook worked out in 2021.
Trimming
These sizes put the triangle edges a little beyond the block points. Once the top is sewn, trim the edge 1/4 inch outside the points so the binding does not cut them off. Borders are not included; the fabric width conversion calculator helps once you know the yardage.
With your numbers
- Corner squares = 12 in ÷ 1.414 + 7/8 in = 9.36 in, cut 9 3/8 in
- Side triangles = 2 × (3 − 1) + 2 × (4 − 1) = 10, from 3 squares
- Side squares = 12 in × 1.414 + 1 1/4 in = 18.221 in, cut 18 1/4 in
Quick reference
Cut sizes are rounded up to your ruler, so the triangles run a little large and are trimmed square once the top is sewn.
| Example | Cut squares for side triangles |
|---|---|
| 12 in blocks, 3 × 4 | 18 1/4 in |
| 10 in blocks, 3 × 3 | 15 1/2 in |
| 6 in blocks, 5 × 7 | 9 3/4 in |
| 8 in blocks, 4 × 5 | 12 5/8 in |
| 6 in blocks, 3/4 in sashing, 3 × 3 | 10 7/8 in |
| 9 in blocks, 5 × 5 | 14 in |
| 30 cm blocks, 3 × 4 | 45.5 cm |
| 20 cm blocks, 4 × 4, 7.5 mm seam | 32.1 cm |
Common questions about the Setting Triangle Calculator
- What size do I cut side setting triangles for a 12 inch block?
- Multiply the finished block by 1.414 and add 1 1/4 inches: 12 × 1.414 = 16.97, plus 1.25 is 18.22, so cut an 18 1/4 inch square and cut it twice on the diagonal. Each square gives four side triangles.
- What size do I cut corner triangles?
- Divide the finished block by 1.414 and add 7/8 inch. For a 12 inch block that is 8.49 plus 0.875, or 9.36, so cut two 9 3/8 inch squares and cut each once on the diagonal. The four corners are always two squares.
- Why cut the side triangles from one square and the corners from another?
- So the long outside edge of every triangle is on the straight grain and the edge of the quilt does not stretch. A square cut into quarters puts the straight grain on the long side; a square cut in half puts it on the two short sides, which is where the corners need it.
- Where do 1 1/4 inch and 7/8 inch come from?
- They are the seam allowances for a 1/4 inch seam on a triangle cut from a square, rounded up to the nearest eighth. Exactly, a quarter-square triangle needs about 1.21 inches and a half-square triangle about 0.85 inches, so the chart figures leave a sliver to trim. At another seam allowance the page scales both: 5 and 3 1/2 times the seam.
- How many side setting triangles do I need?
- Two fewer than twice the blocks along the top, plus two fewer than twice the blocks down a side: 2 × (across − 1) + 2 × (down − 1). A layout 3 blocks across and 4 down needs 10, cut from 3 squares, with 2 triangles spare.
- Can I cut the triangles larger and trim them?
- Yes, and many quilters do. Raise the rounding step to 1/4 inch, or add to the block size, and trim the edge 1/4 inch beyond the block points once the top is sewn.