How it works
Block diagonal = finished × 1.414
Side triangles: cut square = finished × 1.414 + 1¼" → cut twice diagonally (4 per square)
Corner triangles: cut square = finished ÷ 1.414 + ⅞" → cut once diagonally (2 per square)
Turned on point, a block is as wide as its diagonal, which is the side × 1.414 (√2). Side setting triangles fill the gaps along the edges. Their long side is on the outside of the quilt, so they are cut from a square sliced into four. That keeps the straight grain on the quilt edge and stops it from stretching.
Corner triangles are smaller. Their long side sits against a block, so they are cut from a square sliced once, which puts straight grain on the two outer edges.
All sizes are rounded up to the next ⅛". Many quilters add another ½–1" to both squares so the blocks “float” and the edges can be trimmed straight later. If you do, the quilt will be slightly larger than shown.
Counting
With a blocks across the top and d down the side, you need 2(a − 1) + 2(d − 1) side triangles, always 4 corner triangles, and a×d + (a−1)(d−1) blocks. The quilt center is a × diagonal wide and d × diagonal long.
Last checked October 2026 against standard quilting and knitting math. Found a mistake? Tell us and we’ll fix it.