Quilting Math: Tackling Triangles

It’s pretty easy to rely on a pattern for all the measurements and calculations you need to make a quilt. But who do you turn to when you want to make it a little larger or smaller?

We’ve produced hundreds of patterns–which means we use quilting math A LOT! The most common questions we see are about those tricky triangles. So, let’s check out the formulas for the most popular triangle units.

Before we dive in, you might want to have a calculator handy. (It’s definitely one of the most valuable tools in my sewing studio!) No need for anything fancy. These days every phone comes with a calculator app, and that will work just fine. Plus, the math tips and techniques we’re reviewing, are included in our free Fons and Porter’s Sew Easy Lessons ebook.

You can find some of our favorite math tips in this FREE downloadable ebook.

You may also want to check out our Essential Math for Quilters Online Workshop! Have fun learning about how math is used in quilt making. Donna Mae will explain in simple terms how to calculate your fabric needs and how to change the size of blocks. In this 5-lesson online workshop, Donna Mae will show you the 4 fractions quilters use, geometric shapes and angles for quilting, calculating and cutting shapes, and her fool-proof technique for calculating spacer borders for the perfect fit!

Now let’s talk triangles! Do all those nutty angles and sharp points bring back bad memories of your high school algebra class? And what the heck is a hypotenuse anyway?


The most common unit in our quilt patterns is the triangle-square. In my early quilting days, the triangles were always made from squares simply cut in half diagonally. Since that long edge (or hypotenuse) was cut on the stretchy bias, I often made some rather wonky triangle-squares. These days there are much better ways to assemble them. To avoid the wonkiness, squares are sewn together before cutting. We love the Sew Easy techniques for making triangle-squares in multiples of 2 or 8—and you’ll find the math pretty simple once you determine your finished size.

To make 2 at a time, you’ll need 2 squares the finished size of the triangle-square plus 7/8”—or .875 on your calculator.
We cut 2 squares 4-7/8” to make these 2 triangle-squares that will finish at 4”
To make 8 at a time, you’ll need 2 squares double the finished triangle-square size plus 1-3/4”—or 1.75 on your calculator.
We cut 2 squares 5-3/4” and used the Sew Easy lesson to make 8-at-a-time triangle-squares that finish at 2”

Hourglass Units

Let’s take things up a notch with hourglass units. No need to cut all those fussy little triangles. With our Sew Easy technique, the units are quickly-pieced 2 at-a-time and the math is easy peasy.

To make 2 hourglass units at a time, you’ll need 2 squares the finished size plus 1-1/4”—or 1.25
First, use the squares to make 2 triangle-squares…
…then sew the triangle-squares together. Voila!

Flying Geese

You could cut up a bunch of triangles to make Flying Geese. But why not whip up 4 at a time instead with our Quick-Pieced Flying Geese technique?

We’ll begin with the finished size of the Flying Geese. To make the math work, the long side needs to be twice the length of the short side. For our example, let’s construct Flying Geese that finish at 3” x 6”.

You’ll need 1 square equal to the size of the long edge plus 1-1/4”, and 4 squares equal to the short side plus 7/8”. The squares for our example are cut 7-1/4” and 3-7/8”
Join the small squares to the large square as outlined in our Sew Easy lesson and you’ll have 4 identical Flying Geese in no time!

See, that wasn’t so hard. By the way, the coolest thing about our Sew Easy techniques for triangle-squares, hourglass units, and Flying Geese? No trimming needed!

Happy Quilting!