Bra Sister Size report

Choose correct bra size

Take out your piece of bra, and you can see the corresponding size information on the label, which is usually 70A, 75B, 80C, 85D, etc., but what does the size mean? The bra size is composed of two parts, the number is the size of the lower bust, and the letter is the size of our cup. When we choose bras, we should not only listen to the advice from shopping guides. If the bust circumference of the bra is too large for you, it may cause your chest to sag. Choosing the correct bra size will make you feel comfortable and beneficial to the shape and health of your breasts.

Many people have heard "sister size" when they buy underwear, but do you know what sister size means? It is well known that a pair of breasts are not the same, and the two cups of underwear can wrap breasts of similar size. Standard conversion formula for bra sister size: 70D=75C=80B=85A base number +5 (such as 70 75 80 85 90), cup -1 (such as E D C B A), so it is easy to get your bra sister size. Some shopping guides will recommend customers buy an 80A bra if 75B is out of stock.

When choosing a bra, you have to consider the cut and style, your bust and cup size, and even your unique breast shape. It is why bras come in so many different sizes. Unlike other clothing, bra sizes rarely fit across the board, and often, a cup or band size can work differently by brand or label. Luckily, there are bra sister sizes that you can use to find the perfect fit every time. We want to explain more details about finding out the right bra sister size with a conversion chart.

What are Bra Sister Sizes?

Finding a fitting bra seems to be an eternal struggle. Whether you are trying a new brand or just choosing a different style of your favorite brand, the bra size you need may be different, and more importantly, our breast size. The shape will change for various reasons. Knowing your breast measurements is the key to obtaining an accurate bra size, but the specific breast shape may affect the fit and comfort of the bra.

This is the reason why the bra sister size is becoming more popular. This concept is based on the combination of cup and strap sizes. The bra sister sizes provide the exact fit and capacity but with slight differences. The basic idea is that as the size of the cup increases, the width of the strap must be reduced to keep the cup able to accommodate the same length of breast tissue. As far as the sister size is concerned, 34B is equivalent to 32C and 36A.

Doesn't it sound crazy? There is still controversy as to whether the bra sister's size is accurate. Although the cup size is the same, due to the difference in the band size, there are sometimes small but sometimes noticeable differences in fit. Bra sizes vary from style to style and even from manufacturer to manufacturer. Therefore, it doesn't hurt to treat the bra sister size as another possibility. But you'd better choose the most accurate bra size.

Find The Correct Bra Sister Size

Bra Sister Size Chart

Sister sizes can be helpful when tweaking the fit of your bra and if you’re struggling to find the correct size that you need in a bra that you love. Below is a sister size chart from which you can find bra sizes related to your actual size(band+cup). It would be best to start with the size for which you have been measured and the size you are looking for in the lingerie shop.

Band Size

Cup Volume






















30DD (E)





30DDD (F)

32DD (E)





30DDDD (G)

32DDD (F)

34DD (E)





32DDDD (G)

34DDD (F)

36DD (E)






34DDDD (G)

36DDD (F)

38DD (E)





36DDDD (G)

38DDD (F)

40DD (E)





38DDDD (G)

40DDD (F)

42DD (E)





40DDDD (G)

42DDD (F)

44DD (E)





42DDDD (G)

44DDD (F)

46DD (E)


How to Calculate Your Sister Size

If you can’t find your size, use the following closest sister sizes. As you move along the sister sizes, the band will become looser or tighter. How do sister sizes work? Learning how to calculate your bra sister size is easier than you think, although it may seem complicated. If 36C fits you well, but the band feels too tight. If you went up a number size to a 38C, you might find that the cups are too big, so your capacity is closer to a D. Instead, the size you’ll want to try is a 38B, the sister size.

1. Find Your Current Bra Size

The first step to finding your bra sister size is knowing your current bra size. If you don’t know your band and cup size or have never properly learned how to measure bra size, you’ll want to start there to get your most accurate measurement.

2. Know Whether You Should Go Up or Down in Sister Size

Once you have your current bra size and notice that the fit is either too small or too large, you can decide to change a sister size.

  • Your cups fit, but the band doesn’t. Both the band and cup sizes need to be adjusted. If your current size is 36C, a size 38C (if 36 is too tight) or 34C (if 36 is too loose) are not the correct sister size. Your right size should be a 38B or 34D to keep the same cup volume.
  • Your band fits, but cups don’t. Keep the same band size but increase (if cups are too small) or decrease (if cups are too big) the cup sizes.
  • Your band and cups are not fit. Up or down the band size based on your need firstly, then adjust the cup size. Most likely, the cup size stays the same.

If your band size needs to change, then first try your larger or smaller sister size. If your band is too tight, go up a band size and down a cup size. For example, you are going from a 32C to a 34B. If your band is too loose, then go down a band size and up a cup size. For example, you are going from a 34B to a 32C.

3. Try on to Find the Perfect Fit

Once you’ve tried a larger or smaller sister size, re-evaluate to ensure it perfectly fits your unique breast shape and body. To help you remember your sister size.

We hope this information about bra sister size is helpful for your bra shopping. Although the sister sizing is claimed to be accurate, it is possible to have exceptions. Women’s bodies and breasts are not the same, so sister sizing may not always work. The best option to get the best fitting bra is to get fitted and try on the various bras.

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