Simple Math for Quilters

I have been teaching quiltmaking for more than 30 years. It quickly became apparent that some quilters were petrified of "math" and didn't think they could make quilts if they had to use math skills. As long as they have a pattern with all sizes explained, they are happy. And that is perfectly OK.

For me, and those like me, it is just the opposite--math skills make it easier for us to make quilts, in the sizes we want to make them. We use our calculators to figure out what sizes to cut our pieces. This gives us freedom to design our quilts as we choose.

Student work: made in a 5 Easy Pieces Workshop I taught in Panama City, FL--isn't this beautiful? It has half square and quarter square triangles, and squares. Once you know the "magic numbers for quilters" you can make anything: 

I always tell beginning students: "if you had known you were going to be a quilter, you would have paid more attention in Geometry class". What we do is pretty much basic geometry--shapes and sizes--all determined by the basic "rules" of geometry.

Rotary Cutting Changed Everything--we used to cut templates from cardboard or plastic, and trace around them. Now, we cut with a Rotary cutter and rulers, getting the pieces quickly cut the exact size we need, usually for machine piecing.

So, let's just look at a few of the Basic Magic Numbers Every Quilter Needs to Know:

1. We need to know the FINISHED size of the piece we need to cut--that is the size it will be AFTER it is sewn into the quilt. 

2. SQUARES and RECTANGLES: add 1/2" to the finished size--2" finished square would be cut 2.5",    2" X 4" finished rectangle would be cut 2.5" x 4.5" 

3. HALF SQUARE TRIANGLES: add 7/8" to the finished size, cut in half ONCE on the diagonal--you may choose to add 1" to create a bit of insurance--you will have to "trim to size" when sewn. The "trim to" size is simply 1/2" larger than the Finished size--just like ALL squares and rectangles.

4. QUARTER SQUARE TRIANGLES: add 1.25" to the finished size, cut in half TWICE on BOTH diagonals. These can also be over-sized--add 1.5" to finished size to oversize. Then "trim to" 1/2" larger than the finished size--it is a square and that is still the rule, just like Half Square triangle squares and plain squares. 

Let's Practice with this simple project, a little table topper, about 17" square:

The Shoo-Fly blocks are 6" finished so the floral print square in the middle is cut 6.5" I like to make HST with a HST ruler--see how to make this block here: Shoo Fly blocks

The Perimeter Triangles are Quarter Square triangles--we want the LONG side of the triangle, which is along the outside edge, to be straight of grain. We need to do a bit more math. 

We know the SHORT side of the quarter square triangle finishes at 6"--it is sewn to the 6" Shoo Fly Blocks. To figure out the length of the LONG side, MULTIPLE 6" x 1.4142 (another magic number, the square root of 2"--don't worry, there will not be a test at the end). 

6" x 1.4142 = 8.4852" --that is pretty darn close to 8.5" so we will use 8.5" as the FINISHED size of the LONG side of the QST. 

From the Magic Numbers above, we now add 1.25" to 8.5" = 9.75" to know what size square to cut that will give you 4 QST the correct size. I like my blocks to "float" just a bit so I cut the square 10":

Now let's figure out what size squares we need for the four corner triangles--they are HST--we want the straight of grain on the SHORT sides of the triangles.

We know the LONG side of the HST will measure 6" finished--it is sewn to the 6" Shoo Fly blocks. To figure out the short side measurement, DIVIDE 6" by 1.4142.

6" / 1.4142 = 4.24268"--that's pretty darn close to 4.25" so we use 4.25" as the FINISHED size of the SHORT side of the HST. 

From the Magic Numbers above, we now add .875" to 4.25" = 5.125" to know what size squares to cut to get the 4 HST needed for the four corners. Again, I like that "float" so I cut two squares 5.5", then cut them in half once on the diagonal. I can always trim them down if I like once the top is sewn.. You could use 5.25" for less "float", you decide.

I designed a sew-a-long a few years back to practice using these numbers. All 5 posts are still there, an a excellent step-by-step project with lots of photos. How to Make a Quilt in 5 Easy Steps

There are more advanced "magic numbers" for more unusual shapes but let's save those for another day.

Let's quilt.

Barbara