June 20, 2022

How to calculate your bra size


Wondering how to calculate your bra size for the Willowdale Bra? Here’s our guide and bra size chart to help you choose your ideal Willowdale size.

The Willowdale Bra is a full frame underwire bra sewing pattern that’s designed to support and lift big boobs in band sizes 28-54 and cups C-J, all while looking absolutely fabulous.If you have big boobs and you want to start with a bra sewing pattern that’s most likely to fit you right out of the gate, the Willowdale is the bra pattern for you.

The first step to making a bra that fits is to calculate your ideal starting size. Bra sizing varies SO MUCH from country to country and from brand to brand, and your ‘regular’ bra size may not be your Willowdale size. So we’ve put together this guide to help you find your starting size. Let’s get on with it!

Step 1: Take your measurements

Since bra sizing varies considerably, we recommend choosing your Willowdale size based on your measurements. So let’s start by taking our measurements.

There are two key measurements you’ll need to take to select your size: full bust and underbust. It’s important to take these measurements at the start of every project, and try to wear your most supportive bra while doing so, if you have one (if not, just go with your best option!).

1. Your full bust is measured at the bust apex, which is the fullest part of your bust. This is the part that would hit the wall first if you walked directly into a wall, and may or may not be your nipples.

2. Your underbust is measured directly below your breasts, at the narrowest part of your rib cage, where your bra band will sit. Hold the measuring tape taut but be sure to breathe out.

Step 2: Determine your size

Use your two measurements and the bra size chart below to identify your bra size.

Important note: this chart cannot be used to determine sister sizes (more on that below).

If you don’t see a size at the intersection of your measurements, you may still be able to create your ideal size in the Willowdale! Skip ahead to the section titled “What if your measurements put you outside of the size range?”.

Step 3: Determine your wire size

Use your Willowdale size that you identified in Step 2 to figure out what wire size you’ll need. The Willowdale has been designed for use with a rounded, regular underwire such as the regular heavy duty underwire from Porcelynne.

Step 4: Make a muslin

We’ve worked hard to design a bra that will fit you well out of the gate, but boobs can vary so much in shape and size, and it’s impossible to capture the true shape of your bust with two measurements. So once you’ve determined your ideal starting size, the best thing to do is to make a muslin (test version) to check the fit.

Here is where bra fitting is a little more tricky than regular garment fitting. The fabric that you use for a bra can have a radical impact on fit—as in, you could achieve a perfect fit with one fabric, and then you a different one and it barely fits at all; this is much more so than when you’re fitting say a top or a dress. For that reason, the IDEAL approach is to use the exact same fabrics for your muslin as you intend to use for your final bra. By doing that, you’re ensuring that your fitting process will result in a bra that fits at the end.

You need to make an entire bra to get the best sense of the fit, because the various tensions through the fabric and elastic make a big difference, especially for large cup bras like the Willowdale.  If you want to be able to reuse your elastics and notions, you can baste them onto your muslin and remove them once you’ve assessed the fit.

Once you’re able to try on your Willowdale, refer to our bra fitting guide to determine what (if any) adjustments you would like to make.

What if your measurements put you outside of the size range?

If you fall outside the range of sizes included in the Willowdale, it may still be possible to use this pattern to create a bra that fits.

For example, let’s say you have an underbust measurement of 36” and a full bust measurement 48”.

According to the size chart you do not fit into one of the pre-determined sizes. You can use what’s called “sister sizing” and the wire chart (not the size chart!) to modify an existing pattern to fit. Sister sizes are different bra sizes that share the same wire size.

Step 1: Determine your band size

Use your underbust measurement to find your band size in this chart:

Band size 28 30 32 34 36 38 40 42 44 46 48 50 52 54
Underbust measurement (in) 28-29 30-31 32-33 34-35 36-37 38-39 40-41 42-43 44-45 46-47 48-49 50-51 52-53 54-55
Underbust measurement (cm) 72-76 77-81 82-86 87-91 92-96 97-101 102-106 107-111 112-116 117-121 122-126 127-131 132-136 137-142

In our example, the underbust measurement of 36″ would be a 36 band.

What do you do if your underbust measurement falls outside of the chart above?

  • If your underbust is an even number, that is your band size.
  • If your underbust is an odd number, subtract 1 from your underbust. That is your band size.

Step 2: Determine your cup size

To find your cup size, subtract your full bust measurement from your band size and use this chart:

Difference between full bust and band size
Cup size
inches cm
3″ 8 C
4″ 10 D
5″ 13 DD
6″ 15 E
7″ 18 F
8″ 20 G
9″ 23 H
10″ 25 I
11″ 28 J

What do you do if the difference is less than 2″ or greater than 11″?

  • A difference of 1″ would be an A cup size and a difference of 2″ would be a B cup.
  • For differences greater than 11″, continue going up the alphabet.

In our example, a 12″ difference between the band and full bust would be a K cup.

Step 3: Find where your size would be on the wire chart

Use the wire chart to find where your size would be.

The 36K in our example would be the highlighted space in the wire chart:

Step 4: Find the closest sister size

Reading across this chart, you can see that the 36K has sister sizes of 38J, 40I, 42H, etc. because they all share the same wire size. Choose the closest one—in this case, the 38J.

Step 5: Adjust the band size

Once you’ve selected the closest sister size, you can use that size pattern and wire size, but you’ll need to adjust the band size up or down depending on what you need.

To modify the band by one size, add or remove 1/2″ from the side seam of the front bands and 3/8” from the side seam of the back band. To modify the band by two sizes, add or remove 1” from the sides seam of the front band and ¾” from the side seam of the back band.

You will need to check that the length of the two seams is still the same.  The front band seam may now be longer than the back seam. If necessary, extend the top of the back band up to match the length of the front seam.

Once you’ve made your pattern adjustments, you can make up your muslin to check the fit!

Willowdale sizing FAQs

Can I use the Cashmerette size calculator to find my Willowdale size?

The Cashmerette size calculator can help you find your size in Cashmerette garments, but unfortunately it won’t work for your Willowdale bra size.

My Willowdale size is different than my usual bra size. Which should I go with?

Bra sizing varies dramatically around the world and by brands, so go with your Willowdale size (as determined by your measurements) which may not be the size that you’re used to wearing or making.

Do I need to figure out my size before purchasing the Willowdale?

Nope! All of the sizes are included in the pattern, which accommodates full bust from 31″ to 65″ (79 to 165 cm) and underbust from 28″ to 55″ (72 to 142 cm).

Are these US sizes, European sizes, or what?

There’s so much variation between bra sizing from country to country and company to company, so we recommend identifying your size using your measurements, and the instructions we’ve included in the pattern for choosing your size! You will likely find your recommended size is similar to your RTW size, but even if it’s not, don’t worry! Your measurements are the best guide to start with.

I still need help choosing my Willowdale size! What should I do?

We’re happy to help! You can email us directly with your full bust and underbust measurements at hello@cashmerette.com.

2 thoughts on “How to calculate your bra size

  1. Anne says:

    Using only full bust and under bust measures is somewhat deceptive. It doesn’t take armpit and back fat into the calculation. Inevitably, using your formula gives me way more cup than I have breast tissue to fill which then requires much refitting. It also makes it difficult if your “girls” are different sizes.

    1. Ayelet says:

      Hi Anna, you’re right that two measurements can’t fully capture the shape, positioning, etc of your boobs and body, but it’s a good starting point for a lot of people and then you can size up or down from there. We plan to have a tutorial on how to address asymmetry!

      -Ayelet at Cashmerette

Let me know what you think!