Any time you assemble a quilt top in the diagonal onpoint setting, you need setting triangles to fill in the spaces around the sides of the quilt top center. If you create a quilt using a published pattern, the pattern directions include the setting triangle dimensions. However, if you alter the pattern or create your own quilt design, you need to calculate these dimensions for accurate triangles. Two formulas exist to help you with this task, one each for the side and corner setting triangles.
Things You'll Need
 Rotary cutter
 Cutting mat
 Plastic ruler or cutting edge
 Calculator
Side Setting Triangles

Measure the size of your finished blocks in inches using the plastic ruler . If your quilt top incorporates sashing, include that in your measurement. Do not rely on a pattern's statement of the block size as yours may differ slightly.

Multiply the block size by 1.414. Round that number to the nearest eighth of an inch.

Add 1.25 inches to the rounded number. This number is the size of the square from which you cut your setting triangles.

Divide the number of side setting triangles you need by 4 to find the number of squares you need to cut. Side setting triangles are quartersquare triangles, cut this way so the fabric grain runs along the outside edge of the quilt center. Cut each square diagonally twice to make four triangles.
Corner Setting Triangles

Measure the quilt top's finished block to find its size in inches. Include sashing in your measurement if necessary.

Divide the block size by 1.414. Round that number to the nearest eighth of an inch.

Add 0.875 inch to the rounded number. This is the measurement of the square you use for the corner setting triangles at the corners of the quilt top.

Cut two squares with sides equal to the number you calculated.

Cut each square diagonally to make the halfsquare triangles for the corner setting triangles.
References
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