Setting Triangles Calculator
Setting a quilt on point? Type the block size, get the side and corner squares to cut, which way to slice each, and, if you fill in your layout, the counts and background yardage.
Type the finished block size. The two squares fill in right away. Add your on-point layout to get triangle counts, the finished quilt size, and the background yardage.
Why on-point needs triangles at all
When blocks are set on point they stand on a corner, so the edges of the quilt come out as a row of triangular notches instead of a straight line. Setting triangles fill those notches to square the quilt back up: the side triangles run along the four edges, and a corner triangle finishes each of the four corners. Get the grain right and the outside of the quilt stays straight and stable while you quilt and bind it; get it wrong and the edges ripple. That is the entire reason the two triangles are cut from different squares in different ways, and it is what the diagrams below show.
Questions people ask
Why are the two triangles cut from different-sized squares?
Because the straight grain has to end up in a different place on each. A side triangle's LONG edge lands on the outside of the quilt, so that long edge must be on grain; you get that by cutting a square on both diagonals, and the square is sized by the block's diagonal. A corner triangle's two short LEGS land on the outside, so the grain runs along the legs instead; you get that by cutting a smaller square on one diagonal. Same idea, mirror-image geometry, two different squares.
These don't match the numbers in my pattern.
Turn off 'Cut a little big and trim down.' With it off you get the exact chart numbers (a 12″ block gives an 18¼″ side square and a 9⅜″ corner square). With it on, the squares are cut slightly larger so you can trim the quilt's edges perfectly straight after the triangles are sewn on, which is the safer way to work because the sewn edge is on the bias. The exact sizes are shown either way.
How do I count how many side triangles I need?
Fill in the on-point layout, the number of whole blocks across and down, and the calculator counts them and the squares to cut. If you'd rather count by hand: side triangles fill the flat notches along all four edges, and there are two per block along each edge minus the corners; corners are always four. When in doubt, cut one extra square of each; setting triangles are cheap insurance against coming up one short.
Does this include the blocks and sashing?
No. The background yardage here is for the setting triangles only. Your block fabrics and any sashing are separate, because they depend on the block design, which this tool doesn't know. TideStitch does the whole cut list, triangles included, when you plan the actual quilt.
Is this the same math as TideStitch?
Yes. The app sizes on-point setting triangles with exactly these formulas, a quarter-square rule on the block diagonal for the sides and a half-square rule on the half-diagonal for the corners, and rounds cuts up to an eighth. This page assumes a ¼″ seam.