When I was in elementary school the very word arithmetic was enough to make
my palms sweat. Changing the name of the class to math in junior high did
not improve my anxiety and by high school all I wanted to know was the
minimum math requirement needed to get into college. I took algebra I and
geometry and never looked back. Surprisingly the geometry was almost
(operative word-almost) fun. I liked the problem solving aspect of geometry
because it seemed so much more concrete than the abstract numbers of most
math classes. The fun of geometry did not translate into taking any more
math classes until the required college course which I finally got through
but just barely. No more maths for me--ever! Then I started quilting.
Shortly after learning to quilt you realize that you need to know what to
add to make cut pieces with seam allowances already included. Then you need
to know how much fabric to buy, how to change the size or setting, and how
to free yourself from the tyranny of using only patterns in magazines or
books. In short you need math. I resisted this idea at first convinced that
I was math impaired and could only use patterns as is. But then I became
interested in Amish quilts and I just knew they must have some way of
figuring out how to make a quilt without advanced algebra. Thus began my
quest for the magic numbers needed to make quilts. Below are my magic numbers
for seam allowances and setting triangles.
The Magic Numbers for getting the correct seam allowances:
Squares: The Magic Number is ½".
Add 1/2" to the finished size of the square.
Rectangles: The Magic Number is ½".
Add 1/2" to both sides of the finished rectangle.
Right angle triangles: These are the triangles
that are 1/2 of a square and the seam allowances are weird. The Magic Number is 7/8"
and is added to the finished side of a square which is then cut in half for the triangles.
(This half square triangle is also used as a corner triangle on a quilt set on point.)
Quarter square triangles: This is the triangle with
the straight of grain on the long edge. It looks like a right angle triangle but
rather than 1/2 of a square it is 1/4 of square. The Magic Number is 1 1/4".
Add 1 ¼" to the finished size of the long edge. Cut a square that size and cut
on the diagonal twice to get four triangles. This triangle is used in Ohio Star
blocks for the star points and many other quilt patterns. (It is also the
triangle used when a quilt is set on point to fill in the outside edges.)
Figuring the size needed for setting triangles:
When a quilt is set on point two kinds of triangles are needed: quarter
square triangles for the outside edges with the straight of grain on the
long edge and half square triangles for the four corners with the straight
of grain on the short edges.
Edge triangles: The finished size of the block
in the quilt x 1.414 + 1.25 = the size to cut the square that you will cut
on the diagonal twice. This will yield four triangles for the edges. Now you
ask yourself what the heck is this.!?! It doesn't matter! This is all you
need to know and of course it will be easy, because you will use a calculator.
Those true math people out there are now saying to themselves "Well this is
nothing but the Pythagorean Theorem plus 1 ¼" translated to fractions and
there is another way to do this." Pay no attention to the Wizard behind
the curtain unless you are one of those math people and want to know
all this. For the rest of us just do it. (We don't have to understand why!)
Corner triangles: The finished size of the
block in the quilt divided by 1.414 + .875 = the size of square to cut
on the diagonal once. You will need to do this two times to yield four
triangles for the four corners so the straight of grain will be on the
short outside edges. The math people immediately recognize that this is
the opposite of the above and the 7/8 " is translated to .875.
For both of these corner and edge setting triangles round up if the fraction
is in-between 1/8 ".
Remember that finished size is the size of the block or unit without seam
Have fun with this math and never be frightened again unless someone tries
to get you to talk about bell shaped curves or logic and then be scared,
be very, very scared.