How to Figure Out a Set of Triangles for Quilts

Triangles are a common shape used in quilting.

When figuring out how to cut triangles in quilting, make sure the bias-cut edges do not fall along the outside of the block or outer edges of the quilt center. You can cut squares into half-square or quarter-square triangles, both of which have a 90-degree corner. You can figure out how to cut these triangles to construct a quilt with a little planning and a few calculations.

Things You'll Need

• Paper
• Pencil
• Calculator

Instructions

Half-Square Triangles

1. Determine where the bias edge will fall in the block. Make sure the bias-cut edges do not fall along the outside of the block or the outer edges of the quilt center. When you cut a square in half diagonally once, you call it a half-square triangle. These triangles have their bias edge along their longest edge. If the long edge of your triangle does not lay along the edge of the quilt or quilt block, then it's appropriate to use a half-square triangle.

2. Determine the size of the finished triangle needed. When drafting the block, or measuring the block of a finished quilt, measure the two short edges of the triangle. Each short side should be equal, and they should meet at a 90-degree corner for a half-square triangle to be appropriate to use. If the only measurement you have available is that of the long edge, divide this number by 1.414 to obtain the length of the short edges.The measurement of the short edge of the triangle is the measurement that you will use to determine the size of the square you need to cut.

3. Add the seam allowance. A quarter-inch seam allowance is generally used in quilting. Adding a half-inch seam allowance to a square does not account for the diagonal seam. You need to add a total of 7/8 inch to the length of the short edge of the triangle to account for the outside edge and diagonal seam allowances. For instance, if a finished triangle's short edges need to be 3 inches, then the square needs to be cut 3 7/8 inches. You then cut this square in half diagonally once to yield two half-square triangles.

Quarter-Square Triangles

1. Determine where the bias edge will fall in the block. When you cut a square in half diagonally twice, thus creating four triangles, you call those triangles quarter-square triangles. These triangles have their bias edge along their two shortest edges. If the short edges of your triangle do not lay along the edge of the quilt or quilt block, then you can use a quarter-square triangle.

2. Determine the size of the finished triangle needed. When drafting the block, or measuring the block of a finished quilt, measure the longest edge of the triangle. Each short side should be equal, and they should meet at a 90-degree corner for a quarter-square triangle to be appropriate to use. If the only measurement you have available is a short edge, multiply this number by 1.414 to obtain the length of the long edge. The measurement of the long edge of the triangle is the measurement that you will use to determine the size of the square you need to cut.

3. Add the seam allowance. Adding a half-inch seam allowance to a square does not account for two diagonal seams. You need to add a total of 1 1/4 inches to the length of the long edge of the triangle to account for the outside edge and two diagonal seam allowances. For instance, if a finished triangle's long edge needs to finish at 3 inches, then you need to cut the square 4 1/4 inches. You then cut this square in half diagonally twice to yield four quarter-square triangles.

Tips & Warnings

• You cut bias-cut edges diagonally across the weave, or grain, of fabric. These edges stretch easily. Use caution when handling, sewing and pressing bias-cut edges to prevent distortion of the fabric.

References

• Lucy A. Fazely: Seam Allowance Guide