Setting Triangles Calculator

The cut-oversized-and-trim approach

Cut your setting triangles generous on purpose. The calculator gives you the mathematically correct size, but real-world blocks always have minor size variations. Add an extra 1/2" to 1" to your cut squares, assemble the entire quilt top, then trim the edges straight with a long ruler. Leave a consistent 1/4" seam allowance beyond your block points. This is far more accurate than trying to cut perfect triangles before you know how your blocks actually measure.

Side triangles and corner triangles are cut differently, and this is non-negotiable. Sides are QSTs (cut on both diagonals) so the long edge is on grain. Corners are HSTs (cut on one diagonal) so both short edges are on grain. Every outer edge of your quilt ends up on straight grain, which is what keeps an on-point quilt flat instead of wavy. The calculator handles the different sizing for each type.

When sewing side triangles to your blocks, match centers — don't try to line up the pointed ends. The triangle is deliberately oversized and the points will extend past the block. That extra fabric is what you trim away at the end. Trust the process.