Bra Sizing & Underwires

Welcome To The Minefield

When you talk about bra sizing and pattern making in the same topic, it’s likely more difficult than navigating a minefield. This section is written from an Australian perspective so please keep that in mind before you send in American or European corrections. I'm absolutely certain this page will attract criticism anyway!


The first step in determining your bra size is to determine your band size. Thanks to the wonders of non-universal standard sizing, your band size could be assigned any arbitrary number depending on what country your bra is from and who manufactures it. Bra bands are, however, universally divided into 5cm (2inch) increments based on the underbust measurement, even in Australia where all other sizing is divided into 4cm increments. This means in Australia and in some European countries your bra band size may not be the same as your dress size.


Throughout the swimwear section of this website you will see me use the term 10B/C. By this I mean an Australian dress size 10 with a B to C cup breast, not a size 10B to 10C bra. It refers to a swimsuit that fits an average Australian size 10 woman.


The table below shows how manufacturers from around the world label their bands for the Australian market. International manufacturers would label their bands differently again for other overseas markets, the American market for example, would start with an underbust measurement in whole numbered even (units of 2) inches. If all that isn't enough inconsistency, I’m now starting to see swimwear bras come in small, medium and large with absolutely no point of reference to either cup or band.

Cup size is estimated by subtracting the under bust (band) measurement from the bust measurement and comparing the result to the table below. Each cup has a fit range of 2.5cm. Again this table is suited to Australian bras only. International countries use different values to achieve the same task. Be careful when ordering ‘equivalent’ bras from overseas because if they use inches then the larger sizes are not actually equivalent thanks to 1” being 2.54cm and not 2.5cm!

Let’s say, for example, your client has a 91cm bust, a 74cm underbust and a 66cm waist. Assuming an Australian manufacturer made to standard, in most cases she’d best fit a size 14 blouse, a size 12 swimsuit and a 12D bra. Of course, because of a lack of standardisation, mislabelling and imports, she should really try the garment on before purchasing … which is why I’d never recommend buying a bra over the Internet unless they have a good exchange policy.


As far as making patterns goes, the next thing we need to consider are the underwires. What shapes and sizes do they come in? For a single cup size you can get a half coverage wire, a full coverage wire and even an extra-long wire. Their shapes may vary a little but for a single manufacturer they just tend to change length rather than change arch diameter. Unfortunately every manufacturer seems to have their own idea of what is the perfect wire shape, and while fashion may well have a part to play, breasts don’t change with fashion. This means you need to decide on your wire supplier before you start making patterns.


Most wire manufacturers do follow the 1” increment system meaning you will have one wire that suits several different cup sizes. For example, an 8D uses the same wire as a 10C or a 12B or 14A. They will all have their own wire labelling system that says what cup they should fit. Most label their wires by length or diameter rather than cup sizes.

Similar Wires:

  • 10A, 8B

  • 12A, 10B, 8C

  • 14A, 12B, 10C, 8D

  • 16A, 14B, 12C, 10D, 8DD

  • 18A, 16B, 14C, 12D, 10DD

  • 18B, 16C, 14D, 12DD, 10E

  • 20B, 18C, 16D, 14DD, 12E

The small problem Australian swimwear pattern makers face is how to put a device designed for 5cm band increments into a one piece garment that requires 4cm increments. Below is a table showing how I fudge the wire selection for a one piece made to our standard 4cm increments.

Below is a diagram of a complete set of full coverage wires and half coverage wires. These are by no means standard, but they are the ones I will be using to demonstrate how to make patterns with wires. Click on the links below the illustrations to download the files for your CAD software. You will need to select your wires before making your patterns and adjust them accordingly.

Half Wire (Corel Draw)

Half Wire (Adobe Illustrator)

Full Wire (Corel Draw)

Full Wire (Adobe Illustrator)

One thing you will notice if you look at my wire maps is that I use the same wire for A and AA cups (and indeed AAA if one should ever present). This is because they each have the same breast diameter, just a smaller breast volume. All I do is take about 5mm from each side of the seam to make the smaller (flatter) AA cup. That said, while AA cup women do wear lingerie bras (and should), they rarely buy underwired swimwear. Mostly they will buy padded/boosted preformed cups that don't need wires at all.


Fitting a bra is an art in itself. I strongly recommend being professionally fitted but here are a few hints:

  • If your bra is riding up it means the band may be too loose. Start by trying a tighter hook. If that isn’t enough you may need to try a smaller band size (which means you may need to go up a cup size).

  • If your bra is digging into your shoulder you may have the straps too tight so start by loosening them or trying a padded strap. A bra should be supported by the band so you may need to go down a band size. This can be a serious balancing act for women with larger breasts.

  • If you are falling out of a full coverage bra you need to move up a cup size. Any amount of swelling or spilling means the cup is too small. Even push up bras should fit comfortably by creating shape without overflow. Plus size women may need to look at designs created with wire shape in mind because as the breast gets larger it varies in shape a great deal more and what shape is comfortable for one woman may not be for another, even in the same cup size!

  • If your bra is baggy, wrinkled or hollow in any area you may need to look at a smaller cup size.

  • If your wire is poking you, you are probably wearing a band which is too small. Try a bigger band (and hence a smaller cup).


Lastly something you need to consider is litigation. Breasts are sensitive and easily hurt, people are stupid and lawyers are getting cheaper all the time. Simply because you put washing and care instructions into a garment does not mean your client will read them, let alone follow them. Let’s say, for example, your client machine washes her swimsuit in concentrated bleach to get out a stain and she causes deterioration of the wire casing. A few washes later the casing starts to tear. One day she is out in the sun, the wire heats up and then penetrates the casing causing her injury. Don’t be surprised if you get a letter from a lawyer threatening to sue. This is a reality you need to consider. Unfortunately the polycarbonate and plastic wires are not as stiff as traditional wires. Thanks to more advanced preformed cups, underwires in swimwear are slowly becoming a thing of the past.

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