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Setting triangle calculator for on-point quilts

Cut sizes for side and corner setting triangles, plus block counts, triangle counts and finished size for an on-point quilt layout.

Finished block size (in)

Blocks across the top

Blocks down the side

Extra on side triangles (in)

Usable fabric width (in)

The formulas

Side setting triangles: cut square = finished block × 1.414 + 1¼″, cut twice diagonally (4 triangles per square).

Corner setting triangles: cut square = finished block ÷ 1.414 + ⅞″, cut once diagonally (2 triangles per square).

An on-point block measures block × 1.414 from point to point, so the finished quilt centre is blocks across × diagonal by blocks down × diagonal. Cut sizes are rounded up to the next ⅛″.

How to use it

Enter the finished size of your blocks (without seam allowance).
Enter how many blocks run across the top and down the side of the on-point layout.
Read the cut sizes for the side and corner setting triangles, how many squares to cut, and the finished quilt size.
Cut side triangles from squares cut twice diagonally and corner triangles from squares cut once diagonally, so the outer edges of the quilt stay on the straight grain.

Questions

How do I calculate setting triangles?

For side setting triangles, multiply the finished block size by 1.414 and add 1 1/4 inches, then cut that square twice diagonally. For corner triangles, divide the finished block size by 1.414 and add 7/8 inch, then cut that square once diagonally.

Why cut side triangles from a square cut twice diagonally?

Cutting a square into four triangles puts the long side of each triangle on the straight grain. Those long sides form the outer edge of the quilt, so the edge does not stretch.

How many setting triangles do I need?

For an on-point layout with A blocks across and B blocks down you need 2 × (A + B) − 4 side triangles and always 4 corner triangles.

Can I make the setting triangles larger?

Yes. Many quilters add an extra 1/2 to 1 inch to the side triangles so the blocks float away from the border, then trim the quilt square after piecing.