Getting it "On-Point"!!
Figuring the math for diagonal quilt settings! Easy set in (or also known as setting) triangles.
All you need to remember is 1.414 and 7/8". OK, now for the math.
Blocks Without Sashings
Simple calculations for figuring the size of the set-in triangles for a quilt that is set together on point - WITHOUT sashings. (We'll get to that part later).
Corner Triangles (CT)
Take the finished block size and divide by 1.414
Note: Round up to the nearest dimension on the ruler.
Add 7/8" to that number
Cut each square in half diagonally for the four corner triangles.
Example CT1:
Finished block size = 12"
12 / 1.414 = 8.486 (Round to 8.5 = 8-1/2")
8-1/2" + 7/8" = 9-3/8" squares to cut for Corner Triangles
Example CT2:
Finished block size = 6"
6 / 1.414 = 4.243 (Round to 4.25 = 4-1/4")
4-1/4" + 7/8" = 5-1/8" squares to cut for Corner Triangles
Side Triangles(ST)
Take the finished block size and multiply by 1.414
Note: Round up to the nearest dimension on the ruler.
Add 1-1/4" to that number for the correct size to cut squares for side triangles.
Cut this square in half diagonally twice with an X to produce four Side Triangles.
Cut one square for every four side trinagles needed for the quilt setting.
Example ST1:
Finished block size = 12"
12 x 1.414 = 16.96 (Round to 17 = 17")
17" + 1-1/4" = 18-1/4" squares to cut for Side Triangles
Example ST2:
Finished block size = 6"
6 x 1.414 = 8.48 (Round to 8.5 = 8-1/2")
8-1/2" + 1-1/4" = 9-3/4" squares to cut for Side Triangles
Common Sizes for Set In Triangles |
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| Finished Block Size | Side Triangles | Corner Triangles |
| 3" | 5-1/2" | 3" |
| 3-1/2" | 6-1/4" | 3-5/8" |
| 4" | 7" | 3-3/4" |
| 4-1/2" | 7- 5/8" | 4-1/8" |
| 5" | 8-3/8" | 4-1/2" |
| 5-1/2" | 9-1/8" | 4-1/8" |
| 6" | 9-3/4" | 5-1/8" |
| 6-1/2" | 10-1/2" | 5-1/2" |
| 7" | 11-1/4" | 5-5/8" |
| 7-1/2" | 12" | 6-1/4" |
| 8" | 12-5/8" | 6-5/8" |
| 8-1/2" | 13-3/8" | 6-7/8" |
| 9" | 14" | 7-1/4" |
| 9-1/2" | 14-3/4" | 7-5/8" |
| 10" | 15-1/2" | 8" |
| 10-1/2" | 16-1/8" | 8-3/8" |
| 11" | 16-7/8" | 8-3/4" |
| 11-1/2" | 17-1/2" | 9-1/8" |
| 12" | 18-1/4" | 9-3/8" |
| 12-1/2" | 19" | 9-3/4" |
| 13" | 19-3/4" | 10-1/8" |
| 13-1/2" | 20-3/8" | 10-1/2" |
| 14" | 21-1/8" | 10-7/8" |
| 14-1/2" | 21-3/4" | 11-1/8" |
| 15" | 22-1/2" | 11-3/8" |
| 15-1/2" | 23-1/4" | 11-7/8" |
| 16" | 23-7/8" | 12-1/4" |
Note - (don't tell the quilt police) but I like to round my numbers up a bit when cutting the squares for side triangles. For instance, if your closest fraction is 1/8", I round it up to the 1/4". If the closest fraction is 3/8", then I round it up to the 1/2". I figure it is easier to trim it down to the correct size afterwards than trying to see those 1/8" markings on my ruler.
Lay out the triangles and corners with your blocks. You'll lay these out on the diagonal. There will be a triangle on the end of each row. Begin in one corner, sewing the quilt into rows. Your first row will have one corner block, two large setting "wing" triangles, and the corner triangle. Your second row will have three blocks, two wing triangles...etc. Each row will have one more block as you progress to the center of the quilt. To keep the quilt managle, put rows together in segments, then sew the segments together.
Once you have the top together, use your long ruler and rotary cutter to trim the edge of the quilt 1/4" away from the corners of the blocks at the quilt top edge. Make sure you leave enough for seam allowance without cutting into the actual block.
Blocks with Sashings
The side and corner triangles need to be cut using the same calculations above, EXCEPT we need to add the sashing measurement to the block measurement as one unit. So, if you have an 8" finished block size with 2" finished sashing. You would figure that you need setting triangles for a 10" finished block size (8" block + 2" sashing) to set your quilt.
