Putting it "OnPoint"!!
(Click here for printerfriendly version)Figuring the math for diagonal quilt settings!
Here is the math formula for finding out the sizes of triangles you will need for a quilt that is
set "block to block" WITHOUT sashings:
 Corner Triangles
 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 two squares the size determined above. Cut each square in half diagonally for the four corner triangles.
 Example #1:
Finished block size = 12"
12 / 1.414 = 8.486 (Round to 8.5 = 81/2")
81/2" + 7/8" = 93/8" squares to cut for Corner Triangles  Finished block size = 6"
6 / 1.414 = 4.243 (Round to 4.25 = 41/4")
41/4" + 7/8" = 51/8" squares to cut for Corner Triangles
 Side Triangles
 Take the finished block size and multiply by 1.414
Note: Round up to the nearest dimension on the ruler.
Add 11/4" 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#3:
Finished block size = 12"
12 x 1.414 = 16.96 (Round to 17 = 17")
17" + 11/4" = 181/4" squares to cut for Side Triangles Example#4:
Finished block size = 6"
6 x 1.414 = 8.48 (Round to 8.5 = 81/2")
81/2" + 11/4" = 93/4" squares to cut for Side Triangles
Example#2:
Common Sizes for Setting Triangles
 
Finished Block Size

Side Triangles

Corner Triangles

3"  51/2"  3" 
31/2"  61/4"  35/8" 
4"

7"

33/4"

41/2"  7 5/8"  41/8" 
5"

83/8"

41/2"

51/2"  91/8"  47/8" 
6"

93/4"

51/8"

61/2"  101/2"  51/2" 
7"

111/4"

55/8"

71/2"  12"  61/4" 
8"

125/8"

65/8"

81/2"  133/8"  67/8" 
9"

14"

71/4"

91/2"  143/4"  75/8" 
10"

151/2"

8"

101/2"  161/8"  83/8" 
11"  167/8"  83/4" 
111/2"  171/2"  91/8" 
12"

181/4"

93/8"

121/2"  19"  93/4" 
13"  193/4"  101/8" 
131/2"  203/8"  101/2" 
14"

211/8"

107/8"

141/2"  213/4"  111/8" 
15"

221/2"

113/8"

151/2"  231/4"  117/8" 
16"
161/2"
17"

237/8"
24 1/2"
25 3/4"

121/4"
121/2"
13"

*Note* 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 personally would round it up to the 1/4". If the closest fraction is 3/8", then I would round it up to the 1/2". The blocks will float inside the top a bit when pieced with bigger triangles, but I trim it down after the top is together.
Lay out the triangles and corners with your blocks. Begin sewing the quilt into rows starting at one corner. 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 get larger as you get to the center of the quilt. I like two assemble the quilt rows together as I go until there are two halves of the top.....and then sew the two halves together in one long seam.
Lay out the triangles and corners with your blocks. Begin sewing the quilt into rows starting at one corner. 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 get larger as you get to the center of the quilt. I like two assemble the quilt rows together as I go until there are two halves of the top.....and then sew the two halves together in one long seam.
After the top is together, use your ruler and rotory cutter to trim the edge of the quilt 1/4" away from the corners of the blocks at the quilt top edge. This gets rid of all the dog ears from sewing the rows together too.
What if my quilt has SASHINGS?! What sizes do I cut?
The side and corner triangles need to be cut using the same calculations above, only we now add the sashing measurement to the block measurement as one unit. The sashing is considered to be part of the block when calculating the side and corner triangles. Say you have an 8" finished block size, but you are setting the quilt 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.
4 comments:
Thanks for this handy info, Bonnie!! I'm finishing up a UFO with an onpoint setting and your directions here! Thanks!!!
Melissa
You did my math for me. Thank you so much! Every time I make a quilt without using a pattern I swear I'll never do it again. Then... I do it again. You helped me look good. And I gotta say, you helped my quilt look good.
Sally
Thanks for doing my math for me. Every time I make a quilt without a pattern I swear I'll never do it again. Then... I do it again. So thank you, Bonnie, for making me look good. Or more importantly, for making my quilt look good.
Sally
THANK YOU for providing this reference chart. I decided at the last minute to set some blocks on point and had no idea how to begin calculating the math. Then I remembered you mentioned this very thing in a recent post. Your generosity is [seam]ingly limitless.  Gena in Dallas
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