Setting triangles calculator
Cut squares of 18.250 in for the side triangles and two squares of 9.500 in for the corners. Each block measures 16.97 in corner to corner, so the quilt comes to 50.9 in by 67.9 in before borders.
Result
Square for side triangles
in
Square for corner triangles
in
Block measured corner to corner
in
Side squares to cut
pcs
Side triangles needed
pcs
Blocks in the quilt
pcs
Quilt width before borders
in
Quilt length before borders
in
How it works
Blocks set on point stand on a corner, and the zigzag edge is filled with triangles. Side setting triangles are cut from a square sliced twice corner to corner, so the stretchy bias edges stay inside the quilt; the square is the block's diagonal plus an inch and a quarter. Corner triangles for an on point quilt come from a smaller square cut once. This on point quilt calculator works out the side setting triangles size for any block, counts the squares for your layout, and answers how big a quilt set on point turns out: each block is as wide as its diagonal, about one point four times its side.
Formula
diagonal = block × √2. Side square = diagonal + 1 1/4 in, cut twice corner to corner for four triangles. Corner square = block ÷ √2 + 7/8 in, cut once for two triangles; two squares give the four corners. Both are rounded up to the mark you choose. Side triangles = 2 × (across − 1) + 2 × (down − 1). Quilt = across × diagonal by down × diagonal.
Worked example
The calculator opens with this example; the result above is computed from it.
| Input | Value |
|---|---|
| Finished block size | 12 in |
| Blocks across | 3 pcs |
| Blocks down | 4 pcs |
| Round the squares up to | The next quarter inch |
| Square for side triangles | 18.250 in |
| Square for corner triangles | 9.500 in |
| Block measured corner to corner | 16.97 in |
| Side squares to cut | 3 pcs |
| Side triangles needed | 10 pcs |
| Blocks in the quilt | 18 pcs |
| Quilt width before borders | 50.9 in |
| Quilt length before borders | 67.9 in |
Common cases
| Case | Starting values | Square for side triangles | Square for corner triangles | Block measured corner to corner | Side squares to cut | Side triangles needed | Blocks in the quilt | Quilt width before borders | Quilt length before borders |
|---|---|---|---|---|---|---|---|---|---|
| Large blocks, three across and four down | Finished block size 12 in; Blocks across 3 pcs; Blocks down 4 pcs | 18.250 in | 9.500 in | 16.97 in | 3 pcs | 10 pcs | 18 pcs | 50.9 in | 67.9 in |
| Medium blocks for a baby quilt | Finished block size 9 in; Blocks across 3 pcs; Blocks down 3 pcs | 14.000 in | 7.250 in | 12.73 in | 2 pcs | 8 pcs | 13 pcs | 38.2 in | 38.2 in |
| Small blocks for a throw quilt | Finished block size 6 in; Blocks across 6 pcs; Blocks down 8 pcs | 9.750 in | 5.125 in | 8.49 in | 6 pcs | 24 pcs | 83 pcs | 50.9 in | 67.9 in |
| Precut square blocks for a twin quilt | Finished block size 10 in; Blocks across 4 pcs; Blocks down 6 pcs | 15.500 in | 8.000 in | 14.14 in | 4 pcs | 16 pcs | 39 pcs | 56.6 in | 84.9 in |
Questions and answers
What size squares do I cut for setting triangles?
For blocks of 12 in, cut squares of 18.250 in for the side triangles and squares of 9.500 in for the corners. Side squares are cut twice corner to corner, corner squares once.
How big is a quilt set on point?
A block of 10 in measures 14.14 in from corner to corner, and that is the width it takes up on point. This layout comes to 56.6 in by 84.9 in before borders.
Assumptions and limits
- Side squares are cut twice corner to corner and corner squares once, which keeps the straight grain on the outside edge of the quilt.
- Rounding only ever goes up, to a mark you can find on a ruler. Decimal inches read as eighths: 0.125 = 1/8, 0.375 = 3/8, 0.625 = 5/8, 0.875 = 7/8.
- The layout counts the blocks along the top and side edges; the rows between them hold one block fewer and are included in the total.
- Sizes assume a quarter inch seam allowance. The quilt size is the pieced center, before any border.
The result is an estimate for planning, not professional advice.
Sources
- C&T Publishing: quilting tips, setting triangles (formulas and a worked 6 in example)retrieved 2026-10-13
- Phoebe Moon: piece a quilt on point (formulas and a chart for 4 to 12 in blocks)retrieved 2026-10-13
Last updated:
Method in depth: Quilts set on point: setting triangles and size
Cite this page: Quilt Math, “Setting triangles calculator”, https://quilt-math.pages.dev/cutting/setting-triangles-calculator/ (updated 13 October 2026).
Found a mistake? Email handycalc.sites+quiltmath@gmail.com.