Fuzzy Quilt Math: The Secret Built-In Error and How to Fix It

|Sponsored| If your quilt pattern includes a quilt block with triangles, it also includes a built-in error. And it’s all because of your rotary cutter.

You probably thought that if your triangles didn’t match perfectly with your squares, there must be something else going on—maybe you didn’t cut as accurately as you thought, or the problem was sewing them together on the bias.

But that’s not the case. The measurements you are working from aren’t precise. They are rounded up.

I didn’t even know this built-in error existed. I discovered it when I was developing a tool to help eliminate complicated math and fractions so that quilters could make blocks any size they wanted without having to line up the little eighth-inch marks on a ruler.

The idea was to make extension bars that could be attached to the edge of an acrylic ruler to automatically add the seam allowance for squares and triangles.

As you may probably know, we are told to add 7/8″ to a square that will be cut in half for Half-Square Triangles, and 1¼” to a square that will be cut in quarters for Quarter-Square Triangles.

When rotary-cutting shapes, we’re told to add these dimensions to account for seam allowances. The problem is, the measurements for Half-Square Triangles and Quarter-Square Triangles have a built-in error.

But here’s the thing: I designed these tools using a Computer-Aided Design (CAD) program, which is an extremely precise type of software used by designers, engineers, and architects.

Again, I wasn’t looking for a built-in error, but I figured that if I drew a perfect, finished 2″ Half-Square Triangle in my CAD program and added a precise ¼” all around, it would show the exact size of the square I should start with.

7/8″ expressed as a decimal is 0.875″, so my measurement should have been 2.875″. So why was my square measuring 2.854″?

“But that can’t be right,” I thought, which is how I discovered the built-in error in all quilt patterns with rotary-cut triangles.

The same happened with the square for my Quarter-Square Triangle. Here was my dead-on accurate CAD program telling me it should be 3.207″, when conventional quilt math told me that I should cut a square that was 3¼” or 3.250″.

The Rotary Cutter

After being developed for the sewing market in 1979, it became clear that the rotary cutter was sparking a revolution in quilting. Once the acrylic ruler appeared on the scene, with measurement lines to cut strips and squares, quilting exploded.

Before rotary cutters, quilters just made templates (some of you may remember cutting shapes from empty cereal boxes). The only seam allowance measurement a quilter ever needed to know was a quarter inch. Making templates and cutting with scissors was tedious, but there was no complicated seam allowance math. You simply drew your triangle, and add a quarter inch all around.

Remember drawing on cereal box cardboard to make templates?

With these new rotary cutters and acrylic rulers, adding the seam allowance for squares was easy enough to understand. You were just adding a quarter inch to both sides of the strip at the same time, which was something you could visualize.

But what about triangles? It was much faster to use a rotary cutter and ruler to cut squares and then cut them diagonally into Half- or Quarter-Square Triangles. But if you did that, you would obviously need to add more than the standard half-inch, because you needed the extra quarter inch on the diagonal edges as well.

And here is where the built-in error was born.

When the clear rulers with measurement lines were being developed, almost every ruler manufacturer decided that 1/8″ increments were the smallest increments they could include without becoming confusing. This worked with 100% accuracy for squares.

The next logical step was to figure out what to add for triangles. They found that 7/8″ and 1¼” were as close as they could get using a ruler with lines every 1/8″.

Accuracy Matters

I think it’s safe to say that we, as quilters, know that accuracy matters.

Here’s the deal. The built-in error only occurs when you are using the rotary cutting method of cutting strips and squares. A die-cutter cuts individual triangles like quilters did when they made those cereal-box templates, with each shape including the correct seam allowance. Or you could foundation-piece using rolls or sheets with pre-printed sewing lines.

The built-in error occurs when you’re cutting a square diagonally to make triangles. Such a shame! Using a rotary cutter is so much faster, and gives you the freedom to cut any size.

So the extension bars developed by Guidelines4Quilting™ were designed to eliminate this built-in error, freeing up quilters to use their rotary cutters with precision. We call them Seam Allowance Additions; one is 0.854″ for Half-Square Triangles and another that is 1.207″ for Quarter-Inch Triangles.

These extension bars, called Seam Allowance Additions, add the perfect seam allowance to your rotary-cut shapes, no math needed.

Attach them to the edge of your ruler, line up to the finished size you want, and they automatically add the exact amount of seam allowance to your strips and squares meant for triangles, with no built-in error.

Whatever size quilt block you need, your measurements work perfectly, and re-sizing becomes a breeze because you are working with finished-size math. All you need to know is the finished patch size. Like this video shows, working with finished-size math allows you to re-size as needed, without the complex math of Half- and Quarter-Square Triangles.

For more information on finished size quilting and math-free quilting, visit Guidelines4Quilting™, and check out our YouTube channel for fabulous instructional videos!

~ John Brady of Guidelines4Quilting™


Patchwork inspiration, techniques, and projects are always available through the Quilting Company!

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