How to draft a loop of bias binding

A quick bonus tutorial for how to draft a loop of bias binding with an angled seam. One very easy formula and lots of diagrams included!

Hello friends! How are you doing? It has been a busy season for me with lots of freelance work and a quick birthday vacation to Palm Springs last week. I’m also trying to channel all of my disappointment over the election results into sewing and designing. Maybe I’ll get the house a bit cleaner? Or, start exercising? In reality, I'm just really ready to hunker down for the winter, with a pile of cookies, some knitting and loads of movies. I will also be announcing our next drafting project very soon, but first I'm sharing a bonus tutorial, inspired by the construction of this sweatshirt.

Today's post is a quick tutorial for how to draft a loop of continuous bias binding, specifically a loop with a 45 degree angle joining seam. When I was sewing this sweatshirt, I used binding to finish the interior seams of the collar and sleeve cuffs. For the cuffs, I needed a loop of binding that equaled the cuff opening. So, I thought it was the perfect opportunity to play around and figure out the formula for cutting the length of bias binding.

We can, of course measure the length, add a seam allowance and sew a straight seam in our bias binding. But, the straight seam is a little bulkier than an angled seam. The angled seam spreads out the seam allowance over a wider length, so that it's not layered on itself. If you are sewing with thick fabrics, you will really appreciate the reduced bulk of an angled seam.

Because of the angled seam, calculating this length is not as straightforward as simply adding a seam allowance. In the past, I have just cut my bias binding longer than needed and then when I get close to the gap, I fiddle around to figure out where to join the two ends. But, I've seen some videos of people using the width of the bias binding to determine where to cut the quilt binding. The videos make it look like magic but I thought, "ooh there has got to be a mathy formula here".

So, I did a little playing around, made some drawings, tested, and figured it out. It was a little tricky for me to visualize at first and I found it hard to describe verbally. But, I think the diagrams are really helpful. And, the calculation is SUPER easy. Woohoo!

For this method, the binding can be cut to any height desired. For the formula, I'm calling the height of the unfolded binding X. In the diagrams, I show a solid line for the center fold because that's how it's most commonly used. But, the binding can be used folded or unfolded. The desired finished circumference of the loop is called C.

To figure out the formula, I’m reverse engineering. We’ll start by establishing how the fabric is cut and sewn and then evaluate how much fabric is used in the seam allowance to determine the formula for how long to cut the binding.

First, we have to establish how the ends of the binding will be sewn together. (Be forewarned, if sewn differently, the final circumference will be different.) For this method, the ends of the binding need to be cut straight at a 90 degree angle. To stitch the strip of fabric into a loop/circle, position the ends with right sides together at a 90 degree angle and align the raw edges. Then, starting at the top inside corner of the top fabric, stitch at a 45 degree angle to the opposite side. See the diagram above for a visual. The red dashed line is the stitching line. The light blue dot indicates where NOT to start the stitching line.

Next, let’s evaluate what happens to the fabric when we stitch the two ends together in this way. Above are diagrams of the fabric after stitching, after pressing and then shown flat, as if we marked the seam allowance and disassembled the binding strip.

Because of how we’ve aligned the ends of the binding, when we stitch them together at a 45 degree angle, a right triangle that is X wide is removed from each end of the binding. This triangle is essentially the seam allowance, just in a different shape than usual. (In the diagrams above, I've colored the triangles pink.) When the two right triangles are added together they equal an X × X square.

The illustration at the bottom of the graphic above shows a disassembled version of the binding fabric. This is the visualization that really clarified for me how it works. As Oprah would say, it was the “ah-ha moment”. The blue part of the fabric will be visible on the right side and the pink is the seam allowance. (Obviously, our seam allowance is larger than necessary. But, it’s useful both for alignment when stitching and for ease of drafting and cutting.)

So, now we can create our formula. To create a loop that has a Finished Circumference of C, we need to add X—the height of the binding—to the desired finished length.

The cutting length is the finished circumference C plus the binding height X, or simply C + X. It's deceptively simple, don't you think? And I love that it works for any binding height.

If you want the binding to be a little snug, you can cut the binding slightly smaller than the finished circumference. (When the fabric is cut on the bias, it can stretch to fit.) Many of the tutorials I saw for finishing the binding ends on a quilt, remove about 1/8" from the last 12” of binding to create that snugger fit.

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I hope that you find this helpful or maybe even insightful. Let me know in the comments if you have any questions. Mostly, I have used continuous bias binding on the edge of quilt but it can also be used on clothing or accessories. I’m sure you’ll find a use for it soon!

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Happy mathing!

Beth

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