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Quilt Math: Magic Numbers posted: 1/23/2003
by Alice Kish Printable Page
Category: Tips Method: All
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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 ". Add 1/2" to the finished size of the square.

Rectangles: The Magic Number is ". 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 " 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 " 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.

Please note:

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 allowances.

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.

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