Quilt math · Calculator
Setting and corner triangles for on-point quilts
The squares to cut for the side and corner triangles of an on-point quilt, for any block size, and how many you need.
The short answer
For 9″ blocks on point, cut 15″ squares for the side triangles (twice on the diagonals) and 8¼″ squares for the corners (once). That’s 1″ larger than exact (14″ and 7¼″), to trim the edges straight.
Work out yours
Free · no accountEach block’s diagonal: 12.73″
10 side and 4 corner triangles
Cut 3, twice on the diagonals: four triangles each. Exactly 14″; this is 1″ larger, to trim.
Cut 2, once on the diagonal: two triangles each. Exactly 7¼″; this is 1″ larger, to trim.
Cut 6, if the squares between the blocks are plain. Leave them out if every square is a block.
Designing an on-point quilt? In Quarter Inch, choose On point: the setting triangles are drawn, counted and cut for you, with their yardage.
Start designing freeSame sums as the app: @quarterinch/cutting diagonalUp and halfDiagonalUp, exact · SETTING_SIDE and SETTING_CORNER, cut a little large
Setting triangle size chart
The squares to cut for each block size. “Exact” sews edge to edge; “to trim” is the size Quarter Inch cuts, 1″ larger, so you can square up the quilt’s edge after the top is sewn.
| Block | Diagonal | Side: exact | Side: to trim | Corner: exact | Corner: to trim |
|---|---|---|---|---|---|
| 3″ | 4.24″ | 5½″ | 6½″ | 3″ | 4″ |
| 4″ | 5.66″ | 7″ | 8″ | 3¾″ | 4¾″ |
| 5″ | 7.07″ | 8⅜″ | 9⅜″ | 4½″ | 5½″ |
| 6″ | 8.49″ | 9¾″ | 10¾″ | 5⅛″ | 6⅛″ |
| 7″ | 9.90″ | 11¼″ | 12¼″ | 5⅞″ | 6⅞″ |
| 8″ | 11.31″ | 12⅝″ | 13⅝″ | 6⅝″ | 7⅝″ |
| 9″ | 12.73″ | 14″ | 15″ | 7¼″ | 8¼″ |
| 10″ | 14.14″ | 15½″ | 16½″ | 8″ | 9″ |
| 11″ | 15.56″ | 16⅞″ | 17⅞″ | 8¾″ | 9¾″ |
| 12″ | 16.97″ | 18¼″ | 19¼″ | 9⅜″ | 10⅜″ |
How many
| Blocks across × down | Side triangles | Side squares | Corner squares | Plain squares between |
|---|---|---|---|---|
| 2 × 2 | 4 | 1 | 2 | 1 |
| 3 × 3 | 8 | 2 | 2 | 4 |
| 3 × 4 | 10 | 3 | 2 | 6 |
| 4 × 5 | 14 | 4 | 2 | 12 |
| 5 × 6 | 18 | 5 | 2 | 20 |
| 6 × 7 | 22 | 6 | 2 | 30 |
| 7 × 8 | 26 | 7 | 2 | 42 |
How it’s worked out
Set on point, each block stands on its corner, and its diagonal runs along the quilt’s edge. The gaps along the edges are filled with side setting triangles, and the four corners with corner triangles.
- A side triangle’s long side is one block’s diagonal, along the quilt’s edge. That edge must be on the straight grain so it doesn’t stretch, so cut it as a quarter-square triangle: a square the diagonal and 1¼″, cut twice.
- A corner triangle’s long side is one block’s side, against the block. Its short sides are on the quilt’s edges, so cut it as a half-square triangle: a square half the diagonal and ⅞″, cut once.
| The block, finished | 9″ |
|---|---|
| Its diagonal: 9″ × 1.414 | 12.728″ → 12¾″ |
| Side square: the diagonal and 1¼″ | 14″ |
| Corner square: half the diagonal (6⅜″) and ⅞″ | 7¼″ |
| Cut a little large to trim: both 1″ more | 15″ and 8¼″ |
Quarter Inch rounds each diagonal up to the next ⅛″, never down, so a triangle is never short. For a 3 × 4 quilt of 9″ blocks it cuts 3 side squares for the 10 side triangles, and 2 corner squares for the 4 corners.
Why cut them large? Blocks on point have points right at the quilt’s edge. Cut the triangles large, sew the top, then trim the edge ¼″ beyond the blocks’ points: the points stay sharp when the binding goes on, and the quilt comes out square even if the blocks vary a little.
Sewing an on-point top
- Lay out the blocks on point, with the squares between, side triangles at the ends of each row and corners last.
- Sew in diagonal rows: each row starts and ends with a side triangle (the first and last rows with a corner).
- The triangles are cut large, so line up their square corner with the block and let the point hang past the edge.
- Join the rows, matching the seams. Add the last two corners.
- Trim the edges straight, ¼″ outside the blocks’ points, with a long ruler.
What changes the answer
- A floating setting. To leave space between the block points and the border, cut the triangles larger still and trim further out.
- Directional prints. A print with a right way up turns with every triangle. Choose a print that looks the same every way.
- The quilt’s size. On point, the quilt is bigger than the blocks set straight: see the on-point quilt size calculator.
Questions
Short answers.
Something else? A real person replies.
hello@quarterinch.app
How do you calculate setting triangles?
Side triangles: the block’s diagonal (block × 1.414) and 1¼″, cut twice on the diagonals. Corners: half the diagonal and ⅞″, cut once.
What size setting triangles for 9″ blocks?
15″ squares for the sides and 8¼″ for the corners, cut a little large. Exactly, 14″ and 7¼″.
Why are side triangles cut twice and corners once?
So the edge of the quilt is on the straight grain. A side triangle’s long side is on the edge; a corner’s two short sides are.
How many setting triangles do I need?
Two for each block along each edge, less the corners: 2 × (across − 1) + 2 × (down − 1) side triangles, and always 4 corners.