Calculator
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How the numbers work
A block turned on point is wider than it is square: corner to corner, it measures the finished size times 1.414 (the square root of 2). A 9-inch block is about 12.73 inches across on point. Everything else follows from that diagonal.
| Piece | Cut a square of | Then cut | Gives |
|---|---|---|---|
| Side setting triangles | block × 1.414 + 1¼ in | twice on the diagonal | 4 triangles |
| Corner triangles | block ÷ 1.414 + ⅞ in | once on the diagonal | 2 triangles |
Round both up to the next ⅛ inch. For a layout with A blocks along the top and D down the side, you need 2(A − 1) + 2(D − 1) side triangles, always 4 corners, and A × D + (A − 1)(D − 1) blocks. The quilt center finishes at A × diagonal by D × diagonal.
Why the triangles are cut that way
The outside edge of a quilt takes the most handling, and bias edges stretch. Cutting the side triangles from a square cut twice on the diagonal puts the straight grain on their long side, which becomes the edge of the quilt. Corner triangles come from a square cut once, which puts the straight grain on both short sides, again on the outside. Cut them the other way around and the edges wave.
Floating the blocks
Exact triangles bring the block points right to the binding. Many quilters prefer the points to float a little inside the edge. Add 1 to 2 inches to both the side and corner cuts, sew the top, then trim it a quarter inch beyond the block points all the way around. See how to square up a quilt for trimming a top straight.
Sewing an on-point top
- Lay everything out on the floor first: blocks in diagonal rows, side triangles at the ends of each row, corners last.
- Sew each diagonal row, adding the side triangles at the ends. Their long edge faces out; line up the right-angle corner with the block so the tips overhang evenly.
- Join the rows, pinning at every seam, then add the four corner triangles.
- Handle the top by the straight edges and stay-stitch an eighth of an inch from the edge if it will sit a while before quilting.
Blocks that work well on point include the Ohio star, churn dash and a plain nine-patch. To change a block’s size first, use the block resizer.
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