Adjusting Your Bra Band (With Math!)

zebra lace and lycra

I’m making a new lingerie set using this funky yellow zebra lace from Merckwaerdigh. I find animal prints hard to resist and this one is so soft, feminine and subtle. But to make a bra from this set I have to make some pattern adjustments to the band. The matching lycra is a super stretchy 4-way lycra/spandex fabric, and if you have ever tried to make bras with new band fabrics you probably figured out that no two band fabrics fit alike!

If you were to take apart a few of your RTW bras and detach the elastic, you’d notice the bands are all different lengths. This doesn’t necessarily mean they are different band sizes, just that they were drafted with the negative ease particularly needed for that fabric. Unfortunately, most commercial patterns aren’t always clear on the stretch percent or type of stretch for which they were drafted, probably to allow for the widest interpretation possible.

There is a way to get around all this without worrying about sizes. I like to adjust my band pieces for each bra using just a little bit of math. It’s easy math, I promise!

{ONE} If you have already made a bra from your pattern, put on your bra and measure from the side seam of your bra around your back to the other side seam:

measuring for band

The line of the tape should be level with your underbust along the elastic hem. Try to hold the tape firmly–you want a firm number.

*If your pattern doesn’t have a side seam, you’ll need to measure from the side of your cup where the underwire is all the way around to the other side.

{TWO} Find the width of your bra back closure, with the closure on the loosest hook. Subtract this number from the back measurement you just took in step 1.

measuring hook & eye closure

My Back Measurement 15 7/8″ – Closure Width 2 1/8 = 13.75″

Divide this number in half. This is the length of your bra band piece with no negative ease. Mine would be 6 7/8″. Now you need to adjust it for the stretch of your fabric.

{THREE} Find out the stretch percent of your fabric. I first find the direction in which the fabric stretches the most, and take a length of about 5″ or 12cm and stretch it to its maximum along a ruler.

lycra at rest

My lycra stretches from 5 to almost 10″, which is 100% stretch. The calculation: (5″ stretched / 5″ original length) x 100 = 100%.

lycra stretch percent

Using your stretch percent, you can now make some logical adjustments to your band. Obviously the more a fabric stretches the shorter you want your band to be.

{FOUR} For band fabrics, I have found that reducing length by 3% for every 10% of stretch generally works. For example, if your fabric has a 75% stretch, then multiply 3 x 7.5 to get the stretch reduction of 22.5%. My lycra stretches almost 100%, so I multiply 3 x 10 to get a stretch reduction of 30%.

I take my measurement from step 2, and multiply it by 70%. (Reducing by 30%.)

6.875 x .70 = 4.8″

That is how long I want my new band piece to be along the hemline, from the stitching line of the side seam to the line where the back closure is sewn. My current band piece is 5.4″, which was a good fit in a heavier powernet.

band original width

To shorten this piece, I slash down the middle and overlap. until the new hem length reaches 4.8″. Then I retrace the band and smooth out the lines. Voila–a new band piece!

slash and spread band piece

*Note that it’s important to take these measurements between the seam lines and not the edge of the seam allowance. Seam allowance doesn’t count toward length!

Of course math isn’t the whole story and there are other factors which will influence the fit, including how tightly you pull your elastic while sewing. But I have found this to be a good starting point so I don’t end up with too-loose bands. As always testing is always your best friend!

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