HandyToolsHub
Sewing & quilting

On-Point Quilt Setting Triangle Calculator

Enter the finished block size and the layout to get the squares to cut for the side and corner triangles, how many of each, and how big the quilt comes out.

Your quilt
in
One side of the square block as it will be in the quilt, without seam allowances. A block that measures 12 1/2" before it is sewn in finishes at 12". Fractions like 9 1/2 work.
Layout

Count the blocks whose points touch the top edge, and those whose points touch one side. The blocks in between are added for you.

in
Leave empty if the blocks are sewn straight to each other. With a width here, the sashing goes round every block with cornerstones, and the triangles are sewn to the outside of it.
Round cut sizes up to the next

The published formulas work in 1/8"; choose 1/4" if that is the smallest mark you cut to.

Cutting
Square for side triangles
Square for corner triangles
Side squares to cut
Corner squares to cut
Blocks to make
One block, point to point
Quilt before borders
Before rounding up
Your layout, to scale. Sizes are finished sizes, edge to edge at the seam lines, before any border.
The two squares at the same scale. Cut along the dashed lines.

For square blocks and 1/4" seam allowances. The cut sizes are the exact size of each triangle plus its seam allowance, rounded up to the ruler. They are not oversized: if you like the blocks to float and to trim the top square afterwards, cut the squares larger than this.

Based on: Quiltville · Tested against 19 worked examples, 13 printed in a published source · How this is worked out →

Common cases

Questions people ask

What size do I cut setting triangles for an on-point quilt?
For the side triangles, multiply the finished block size by 1.414, add 1-1/4 inches, round up to the next 1/8 inch, cut a square that size and cut it on both diagonals to get four triangles. For the corner triangles, divide the finished block size by 1.414, add 7/8 inch, round up, cut two squares that size and cut each once on the diagonal. For 12-inch blocks that is 18 1/4" and 9 3/8" squares.
Why are side triangles cut from a square cut twice, and corner triangles from a square cut once?
It puts the straight grain of the fabric on the outside edge of the quilt. A square cut on both diagonals gives triangles whose long side was an edge of the square, and the long side of a side triangle is the one on the quilt edge. A square cut once gives triangles whose two short sides were edges of the square, and those are the two sides of a corner triangle that sit on the quilt edge. The sources note that straight-grain edges stretch less than bias edges.
How many side setting triangles do I need?
One for every gap between two blocks along the edge of the quilt. A layout with 3 blocks across the top and 4 down the side has 2 gaps on the top, 2 on the bottom and 3 on each side, so 10 side triangles, which takes 3 squares because each square gives four. Every on-point quilt needs 4 corner triangles, cut from 2 squares.
Should I cut setting triangles oversized and trim?
That is a matter of preference. The sizes on this page are the exact sizes plus seam allowance, rounded up to the ruler, and one source describes its chart as the minimum to cut. One author rounds the side squares up further and trims the top 1/4 inch outside the block points once it is sewn; another suggests adding 1/4 inch to each square. If you cut larger, the blocks sit slightly inside the edge until you trim.
How does sashing change the size of the setting triangles?
The side triangles are worked out as if the block were one sashing width larger. For the corner triangles this tool adds two sashing widths, because the long side of a corner triangle runs past the cornerstone at each end of the sashing strip. One source says to add a single sashing width for both; with whole cornerstones on the outside, the layout drawn on this page needs two for the corners.