Bust size and cup size and how they relate
Short answer: Bust size is the measurement around the body, while cup size comes from the difference between the measurement over and under the bust. The two belong together, so you cannot choose one without the other. That also explains why the same letter can look different.
Two measurements and two parts of a size
A bra size is made of a number and a letter. The number is the band around the body under the bust, and the letter is the cup. The bust measurement is used to work out how big the cup should be, so both have to be measured before you can be sure of your size.
How to work out the difference
Measure around the fullest part and then just under the bust. The larger the difference between the two numbers, the larger the cup is usually. Every brand has its own chart, so look it up before you decide, and check whether the measurements are in centimetres.
Why the bust measurement alone is not enough
The bust measurement only says how wide you are at the fullest part. It says nothing about the shape, spacing or position of the breasts. That is why trying on matters if you can, and a bamboo bra without underwire can be a good place to start, because the cup is often softer and more forgiving.
Same letter, different volume
A C on a small band is not the same as a C on a large band, because the volume of the cup follows the band. This confuses many people and is why you can sometimes fit several sizes. Check the chart of the brand you want to buy.
The band carries most of the support
Many people think the cup does the most, but it is the band that keeps the bra in place. If it is too loose, it rides up at the back, and a bigger cup will not help. Always find the right band first, and adjust the cup afterwards.
When the two measurements do not match
Sometimes the measurement points to one size, but the bra feels wrong. Then you can try one size up or down in the cup, or a different band with a matching cup. This adjustment is sometimes called sister sizing, and it is perfectly normal to use it.
Bust size helps with clothes too
Your bust size is not only for bras. You also use it when choosing tops and T-shirts, and there you can look at bamboo underwear for women. Always compare with the size guide of the garment you choose, as fit varies.
Measure, try and trust your own impression
Numbers help, but your body has the final say. If the bra sits well, feels comfortable all day and does not move, it is right for you. Under a women's long-sleeved bamboo T-shirt you can easily see whether the bra sits neatly.
Frequently asked questions
What is the difference between bust size and cup size
Bust size is a measurement around the body, while cup size comes from the difference between the measurement over and under the bust. Bust size is part of the sum, but it is not the whole answer.
Should I choose by bust size alone
No. Also measure under the bust and use both numbers in the brand's size chart. And try on if you can, because fit can vary from style to style.
What are sister sizes
They are sizes with a different band but almost the same cup volume. If the band is too tight or too loose, you can sometimes choose a neighbouring size and get a better fit.
