Quilt Setting Triangles Calculator: Side and Corner Setting Triangle Cut Sizes

How the calculator works

Step 1: Side triangle square cut size

Side setting triangle square cut size equals finished block size multiplied by 1.414, plus 1.25 inches, rounded up to the nearest 1/8 inch. The number 1.414 is the square root of 2, which comes from the diagonal of a square, because blocks are set on point. Cut each side square twice on the diagonal, in an X, to make 4 side triangles per square, so the straight of grain sits on the outer edge of the quilt, not the bias. For finished block size 12 inches, side square cut size equals 12 multiplied by 1.414, which is 16.97, plus 1.25, which is 18.22, rounded up to the nearest 1/8 inch, which is 18.25 inches.

Step 2: Corner triangle square cut size

Corner setting triangle square cut size equals finished block size divided by 1.414, plus 0.875 inch, rounded up to the nearest 1/8 inch. Cut each corner square once on the diagonal to make 2 corner triangles per square. For finished block size 12 inches, corner square cut size equals 12 divided by 1.414, which is 8.49, plus 0.875, which is 9.36, rounded up to the nearest 1/8 inch, which is 9.375 inches.

Step 3: Squares needed and triangles made

Side squares needed equals side setting triangles needed divided by 4, rounded up to a whole number, because each side square makes 4 side triangles, and a partial square cannot be cut. For side triangles needed 20, side squares needed equals 20 divided by 4, which is 5 squares, and side triangles made equals 5 times 4, which is 20 triangles, exactly the number needed, with no extra triangle. Corner squares needed equals corner setting triangles needed divided by 2, rounded up to a whole number, because each corner square makes 2 corner triangles. For corner triangles needed 4, corner squares needed equals 4 divided by 2, which is 2 squares, and corner triangles made equals 2 times 2, which is 4 triangles, exactly the number needed, with no extra triangle.

Assumptions, shown next to the result

This calculator uses the traditional on-point, block to block method only, with no sashings between blocks, with seam allowance 0.25 inch on each side, as is standard for piecing. Cut sizes are rounded up to the nearest 1/8 inch, so cut sizes may be a little larger than the raw formula result, which gives a little room for trimming after the quilt top is assembled. Cut sizes are used as entered in the unit mode you choose, with no extra added for mistakes or shrinkage, beyond the rounding up already in the formulas. Side triangles must be cut twice on the diagonal, not once, or the bias edge will sit on the outer edge of the quilt and may stretch. Corner triangles must be cut once on the diagonal, not twice, or the grain will be wrong for a corner. If your on-point quilt uses sashings, do not use these formulas, because sashing changes the setting triangle sizes.

Setting triangle cut size chart

This chart uses the same formulas as the calculator: side square cut size equals finished block size multiplied by 1.414, plus 1.25 inches, rounded up to the nearest 1/8 inch, and corner square cut size equals finished block size divided by 1.414, plus 0.875 inch, rounded up to the nearest 1/8 inch.

Chart values are a guide for the finished sizes shown. Your own finished size may give a different result. Use the calculator above for your own measurements.

Worked example, checked against the calculator

Finished block size 12 inches, side setting triangles needed 20, corner setting triangles needed 4.

Side triangle square cut size: 12 multiplied by 1.414 equals 16.97, plus 1.25 equals 18.22, rounded up to the nearest 1/8 inch, which is 18.25 inches. Cut each side square twice on the diagonal to make 4 side triangles per square.

Corner triangle square cut size: 12 divided by 1.414 equals 8.49, plus 0.875 equals 9.36, rounded up to the nearest 1/8 inch, which is 9.375 inches. Cut each corner square once on the diagonal to make 2 corner triangles per square.

Squares needed: side squares 20 divided by 4 equals 5 squares, which make 20 side triangles exactly. Corner squares 4 divided by 2 equals 2 squares, which make 4 corner triangles exactly.

Frequently asked questions

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Guide for this topic: How to Calculate Setting Triangles.