Setting triangles for on-point quilts
Set your blocks on point and you need two kinds of triangle to fill the edges — cut the wrong way round and your quilt edges will stretch. Here are both, for every block size.
An on-point quilt turns every block forty-five degrees. That leaves triangular gaps around the perimeter, filled by side setting triangles along the edges and corner setting triangles at the four corners. They are not the same shape and they are not cut the same way, which is where most on-point quilts go wrong.
Setting triangle chart
| Finished block | Block diagonal | Side triangle square | Corner triangle square |
|---|---|---|---|
| 6" | 8½" | 9¾" | 5⅛" |
| 8" | 11⅜" | 12⅝" | 6⅝" |
| 9" | 12¾" | 14" | 7¼" |
| 10" | 14⅛" | 15½" | 8" |
| 12" | 17" | 18¼" | 9⅜" |
| 14" | 19¾" | 21⅛" | 10⅞" |
| 15" | 21¼" | 22½" | 11½" |
| 16" | 22⅝" | 24" | 12¼" |
| 18" | 25½" | 26¾" | 13⅝" |
The formulas
- Side setting triangles: (finished block × 1.414) + 1¼″, cut twice on the diagonal → 4 triangles.
- Corner setting triangles: (finished block ÷ 1.414) + ⅞″, cut once on the diagonal → 2 triangles.
Why the difference
It comes down to grain. Once the quilt is assembled, the outside edge of both triangle types sits on the quilt's perimeter — and a perimeter cut on the bias will stretch, ripple and refuse to lie flat no matter how carefully you border it.
A side triangle needs its long edge on the perimeter, so it is cut as a quarter of a square — twice on the diagonal — which puts straight grain on that long edge. A corner triangle needs its two short legs on the perimeter, so it is cut as a half of a square — once on the diagonal — which puts straight grain on both legs.
Cut them the other way round and every edge of your quilt is bias. This is the single most common on-point mistake.
How many do you need?
For a straight on-point layout with r rows and c columns of blocks, you always need exactly 4 corner triangles, and the number of side triangles is determined by how many block edges land on the perimeter. FourPatch counts them for you when you switch a layout to on point — it also cuts the number of setting triangle squares you need to buy fabric for.
Cut them big
Every formula above already includes a generous allowance, and many quilters add another half inch on top, planning to trim the quilt edges square after the top is assembled. This is standard practice and worth doing: an on-point top has dozens of seams converging on the edge, and trimming to a straight line afterwards is far easier than trying to make every triangle land perfectly.
On-point changes how many blocks you need
A 12″ block set straight occupies 12″ of quilt. Set on point it occupies its diagonal — about 17″. That is why on-point quilts need markedly fewer blocks to reach the same size, and why they look larger for the same amount of piecing.
Quilt size chart → · Yardage guide →
Questions
What size setting triangles do I need for a 12 inch block?
Side triangles come from 18¼ inch squares cut twice on the diagonal. Corner triangles come from 9⅜ inch squares cut once on the diagonal.
Why are side and corner setting triangles cut differently?
So the straight grain lands on the outside edge of the quilt in both cases. Side triangles need straight grain on their long edge, so they are quartered; corner triangles need it on their two short legs, so they are halved.
What is the formula for setting triangles?
Side: block size × 1.414 + 1¼ inches, cut twice diagonally. Corner: block size ÷ 1.414 + ⅞ inch, cut once diagonally.
How much bigger is an on-point block?
A block set on point occupies its diagonal, which is the finished size times about 1.414. A 12 inch block takes up roughly 17 inches of quilt.
Stop looking things up
FourPatch does this arithmetic for every piece in your quilt, every time you change something — block size, sashing width, fabric width, waste allowance. The cutting list and yardage just update.