How to calculate the length of bias for a neckline 💡

Determining the Bias Length for a Round Neckline

Determining the length of bias tape for stretchy fabric can be tricky: too long, it gaps / too short, it puckers! I'll give you the exact formula – don't panic, it's simpler than it looks (especially with the calculator at your disposal).

🔢 The Calculation Formula

To get the perfect length, you need to shorten the bias relative to the neckline/armhole or any part where you want to sew your bias. Before you start calculating, you need two pieces of information:

1st Information: The Length You Will Sew Your Bias On

Photo 1 — measurement before assembly

Photo 2 — measurement with shoulders already assembled

Here, I'm using the example of the bias for the neckline of the Sarah tank top.

The advantage of taking measurements before assembly is that they will be easier to take, and you will potentially be less approximate in your measurement.

To take your measurements, use a measuring tape to properly follow the curve of your garment. Then, measure the front and back. For my part, I have 23 cm for the back and 32 cm for the front, so my neckline measurement (including seam allowances) is 23 + 32 = 55 cm (photo 1).

The measurement in the second photo is 51 cm (including seam allowances).

2nd Information: If You Have Seam Allowances to Apply

For the assembly of this pattern, my seam allowances are 1 cm (so 4 cm in total here) for photo 1. For photo 2, I have no seam allowance since the shoulders are already assembled.

You have everything you need. I'll explain the formula just below, but you can go directly to the calculator if you prefer.

🧮 Bias Calculator

Why apply this formula and not just measure the perimeter? Simply because bias tape is applied to stretchy fabric, and it will stretch slightly during sewing. If you cut the exact perimeter length, you'll end up with too much bias and a puckered neckline. This formula anticipates the stretch during assembly.

One Bias or Two Pieces of Bias?

On commercially available garments, mainly basic t-shirts, your neckline bias, when round (scoop neck), is in one piece. This means you only have one join seam at the bias.

However, if you're a beginner, it's often difficult to sew on stretchy fabric, to distribute the bias correctly without pulling, and to sew evenly with an elastic stitch. I'll give you my advice, what I applied for sewing the Sarah tank top.

Instead of a single bias length, I used two lengths: one for the back neckline and one for the front neckline. I joined them together to form a circle. What does that change? I now have two reference points to ensure proper bias distribution; my reference points are simply the shoulder seams. This way, I reduce the risk of having a neckline that puckers at the back and gaps at the front (or vice versa).

How do I choose my bias?

For bias tape, you have two options: either you buy it pre-made commercially, or you make it yourself.

Option 1: Buy commercially

The advantage is that you'll get neatly folded bias with precise markings and no discontinuities. This is ideal when you're starting out: you don't burn your fingers with your iron, and you already have your finish ready to use. You also save a lot of time on your sewing; making bias yourself can quickly become a real chore.

Note that depending on the material you're working with, the difficulty level is not the same. Here, this article is specifically dedicated to stretch bias; if you haven't got the hang of it yet, getting an even bias isn't always easy, as you need to be careful not to pull on the fabric, which deforms more easily. However, if you're working with a more rigid material, like cotton, bias is much easier to make.

I would still note two drawbacks: the first is that you have to find the exact fabric matching your garment (unless it's a creative choice), the second is simply that you will pay a bit more.

If you are a beginner or trying for the first time, I recommend buying pre-made bias. Making it yourself might discourage you.

Option 2: Create your own bias

The main advantage is that you'll be able to get bias tape that matches your fabric exactly. Another advantage, which might seem like a drawback, is that you'll practice being meticulous and precise, both in cutting and in folding/pressing. Generally, it's often inattention errors or neglected steps that make finishes less attractive; a 3 mm shift on a 1 cm bias is immediately noticeable!

But don't panic: there are tools to make your own bias tape at home for a few euros. This allows you to have clean, straight folds (the cutting, however, is still all you).

Sewing is a very stimulating creative universe, and perhaps sometimes a little too much; we accumulate scraps "just in case," impulse-buy fabrics without a project in mind, tests, muslins, failures... and then a never-ending mountain of fabric (even though we are often engaged in an upcycling approach, yes, we are beings of contradiction). The hardest parts to use are small scraps, long but narrow, and making your own bias is ideal for recycling them. You don't need very long pieces: you can cut on the bias and join all your pieces together.

What bias width to choose?

If you are using store-bought bias tape (sold folded in 4): use 18 mm or 20 mm for a neckline. This is the standard width, strong enough without being bulky once folded.

For example, for the Sarah tank top, my unfolded bias is 5 cm wide.