A few days ago I was asked how to calculate the size of setting triangles for an on-point setting. I thought a post focused on that would help others, too.
[For information to help decide whether to set your center block on point, see this post.]
Have you ever made a block-format quilt with a diagonal setting? The math for setting triangles is the same as that for the setting corners on that block quilt.
How large will your new block be?
finished center x 1.414 = finished size of on-point block
ex: 20″ x 1.414 = 28.28″.
finished center + 0.5″ = unfinished size of center
ex: 20″ + 0.5″ = 20.5″
(finished center/1.414) + 0.875 = size of square to make TWO corners
This assumes PERFECTION. I do NOT assume perfection! Round up at least a half inch.
ex: 20/1.414 + .0875 = 14.144 + 0.875 = 15.019, or about 15 1/8″.
Now round up to 16″.
CUT two squares 16″. Cut two because you need four corners.
CUT each square in half on the diagonal.
To attach the setting triangles,
PINCH the center of the long side to make a crease.
PINCH the edge of the block at the center.
MATCH the creases and pin.
SMOOTH the triangle across the edge, careful not to stretch it.
REPEAT for the opposite side.
SEW with a 1/4″ seam allowance.
PRESS toward the triangles on both sides, and trim the dog ears.
REPEAT for the remaining sides.
TRIM, leaving at least a 1/4″ past the center block’s corner.
It is over-sized before trimming, because these instructions make it that way. You can either leave it over-sized, letting your center block “float” in it, or you can trim to a more exact size, with 1/4″ seam allowance to match the points of your original block. Either way you will need to square it after attaching setting triangles.
Also, the larger your block, the harder it is to manage trimming. It IS doable! But it does get more difficult.
Here is my finished block, turned on point and ready for borders. So far it finishes at 28.25″.