Sister Sizes: What They Are, How They Work and Why Your Bra Size Can Change
Have you ever tried on a bra in your usual size only to find that the band feels too tight, the cups gape, or your breasts spill out over the top? Before assuming that the bra is the wrong style—or that your body has suddenly changed—it is worth understanding sister sizing.
Sister sizes are different bra sizes that have approximately the same cup volume, but different band measurements. Understanding sister sizes can make bra shopping much easier, especially if you have a fuller bust, a small frame with a larger bust, or struggle to find bras that fit correctly.
At Lady Things Bra Boutique in Pretoria, we often use sister sizing as part of a professional bra fitting when a client’s measurements fall between standard sizes or when a particular bra style fits differently from another.
What Is Sister Sizing?
Sister sizing means moving up or down in band size while adjusting the cup size in the opposite direction to maintain a similar cup volume.
For example:
- 34DD has a sister size of 36D
- 34DD also has a sister size of 32E
- 36D has a sister size of 38C
- 32E has a sister size of 30F
The important thing to understand is that the cup letter does not represent a fixed physical cup size.
A 34D and a 38D are not the same cup volume. The cup becomes larger as the band size increases, even though both sizes are labelled “D”.
This is one of the biggest reasons women become confused about bra sizing.
How Does Sister Sizing Work?
There is a simple pattern to remember:
If you go UP one band size, go DOWN one cup size.
For example:
34F → 36E → 38DD
Each time the band gets larger, the cup letter gets smaller.
If you go DOWN one band size, go UP one cup size.
For example:
34F → 32FF → 30G
The band becomes smaller while the cup letter increases.
The result is a similar overall breast volume, although the bra will not fit exactly the same way.
Sister Size Chart
Here is a simplified example of how sister sizes work:
Your Size | Smaller Band | Larger Band |
32D | 30DD | 34C |
32DD | 30E | 34D |
34D | 32DD | 36C |
34DD | 32E | 36D |
36D | 34DD | 38C |
36DD | 34E | 38D |
38D | 36DD | 40C |
38DD | 36E | 40D |
This chart is useful as a starting point, but sister sizing should not replace a professional bra fitting.
Why? Because the same nominal size can fit differently depending on the bra’s construction, fabric, cup shape, underwire, stretch and brand.
Is a Sister Size the Same as Your Original Size?
Not exactly.
This is a very important distinction.
Sister sizes have a similar cup volume, but they do not have the same band circumference.
For example, if you normally wear a 34DD but try a 36D, the cup volume may be similar. However, the 36D has a larger band.
That means the bra may feel more comfortable around your ribcage, but the band may not provide the same level of support.
Likewise, moving from a 34DD to a 32E gives you a smaller band with a larger cup letter. This can be useful if your current band is riding up or feels too loose.
When Should You Try a Sister Size?
Sister sizing can be helpful when your bra is close to fitting correctly but one element is not quite right.
Your band feels too tight
If the cups fit well but the band is uncomfortably tight, you might consider going up a band size and down a cup size.
For example:
34E → 36DD
However, don’t immediately change sizes simply because the band feels snug when you first put the bra on.
A new bra should generally fit comfortably on the loosest hook, because the band will naturally stretch with wear.
Your band rides up at the back
A band that rides up can be a sign that the band is too large.
In this situation, going up another band size is usually the wrong solution.
You may need to go down a band size and up a cup size.
For example:
36D → 34DD
The smaller band should provide better support while the larger cup accommodates the same approximate breast volume.
Your cups fit but the band is too loose
This is another classic situation where sister sizing may help.
If you are wearing a 36D and the cups fit well but the band moves around your body, a 34DD could potentially provide a better fit.
However, the cups may not fit exactly the same because changing the band also changes the shape and proportions of the bra.
Sister Sizing Is NOT a Shortcut to Finding Your Correct Bra Size
This is where many women get confused.
Sister sizing is useful—but it shouldn’t become a way of forcing yourself into a size that isn’t actually appropriate for your body.
For example, if a woman normally wears a 38E but cannot find that size in a particular shop, she might be told to try a 40DD.
That may technically be a sister size, but it doesn’t necessarily mean it will provide the same support.
The band is one of the most important structural components of a supportive bra.
If the band is too loose, it cannot properly anchor the bra to your body. The straps may then end up doing too much of the work, which can lead to discomfort and poor support.
Why Cup Letters Are So Misleading
One of the most common bra-sizing myths is:
“A D cup is a D cup.”
It isn’t.
Cup size is relative to band size.
A 30D, 34D, 38D and 42D do not have the same cup volume.
This is particularly important for women who have a larger bust but a smaller frame.
A woman with a 30G bust may have a completely different body shape from a woman wearing a 40G, even though both bras have the same cup letter.
This is why simply saying “I am a D cup” doesn’t tell a professional bra fitter enough information to determine your correct size.
Sister Sizing for Fuller Busts
Sister sizing can be especially confusing when you are looking for bras for large busts, fuller bust bras or larger cup sizes.
Many mainstream stores stop stocking larger cup sizes or offer a limited range of band-and-cup combinations.
This can leave women believing that they have to compromise.
They may buy:
- A band that is too large because the cup is available
- A cup that is too small because the band is available
- A bra with insufficient support
- A bra that looks fine under clothing but doesn’t actually fit properly
A professional fitting helps identify whether the problem is your band size, cup size, cup shape, bra style—or a combination of these.
Can You Sister Size Between Every Bra Size?
Sister sizing works mathematically, but that doesn’t mean every sister size will work practically.
The further you move from your original size, the more the proportions of the bra change.
For example, moving from:
34F → 36E
is relatively close.
But moving several sizes away:
34F → 38DD
may produce a very different fit.
The band is substantially larger, and the bra may sit differently on your body.
For this reason, sister sizing is best used one step at a time, rather than moving several band sizes simply to find a cup that is available.
Sister Sizes and Bra Styles
Another important factor is bra construction.
A sister size that works beautifully in one bra may not work in another.
Different styles have different:
- Cup shapes
- Underwire shapes
- Cup heights
- Band elasticity
- Strap placement
- Side support
- Coverage
- Fabric stretch
A push-up bra, full-cup bra, balconette bra, sports bra and strapless bra can all fit differently—even when they carry the same size label.
This is why professional bra fitting involves more than taking measurements and calculating a number.
What If the Cup Fits but the Band Doesn’t?
This is probably the most useful way to think about sister sizing.
Ask yourself:
Is the cup actually fitting correctly?
If the answer is yes, then you can consider adjusting the band and changing the cup letter accordingly.
For example:
Band too loose:
Go down a band → go up a cup.
Band too tight:
Go up a band → go down a cup.
But if the cup is also too small or too large, you may need a completely different size rather than simply choosing a sister size.
How to Tell if Your Bra Really Fits
A properly fitted bra should generally have:
A firm, level band
The back band should sit horizontally across your back rather than creeping upwards.
Cups that contain your breasts
There should not be significant spilling over the top, sides or centre of the cups.
Underwires that surround the breast
The underwire should sit around the breast tissue rather than directly on it.
A comfortable centre gore
The centre section between the cups should sit appropriately against your chest for the bra style and your breast shape.
Straps that provide adjustment—not the main support
Your bra’s band should provide most of the support. Straps are there to help position the cups and provide additional support, not to carry the entire weight of the breasts.
The Most Important Thing About Sister Sizing
Sister sizing is a tool—not a rule.
Your best bra size is not necessarily the size you have always worn, the size shown on a calculator, or the size printed on your previous bra.
Your correct size can change depending on:
- The bra brand
- The bra style
- Your breast shape
- Your band measurement
- Your cup volume
- The elasticity of the fabric
- Changes in your body
- Pregnancy or breastfeeding
- Weight changes
- Hormonal changes
This is why trying on a bra and assessing the actual fit is so important.
Need Help Finding Your Sister Size?
If you’ve been buying the same bra size for years but your bras never seem to fit quite right, it may be time for a professional bra fitting.
At Lady Things Bra Boutique in Pretoria, we specialise in private bra fittings for women who struggle to find the right size, including women with fuller busts, larger cup sizes, small frames with larger busts and plus-size fuller busts.
We stock supportive European bras in a wide range of band and cup sizes, including D cup and larger sizes, and our fittings are designed around your individual body rather than simply putting you into a standard size.
Our private bra fittings are free and available by appointment.
If you’re tired of uncomfortable bras, constantly adjusting your straps, bands that ride up, cups that spill over, or struggling to find your size in mainstream stores, let us help you find a bra that actually fits.
Book Your Personal Bra Fitting Today
Book your private one-on-one bra fitting at Lady Things Bra Boutique and discover how enjoyable bra shopping can be when you have expert guidance, exceptional choice, and an entire hour dedicated just to you.
Supporting Your Uniqueness – One Bra at a Time.
FAQs
A sister size is a bra size with a different band measurement but a similar cup volume. When you increase the band size, you decrease the cup letter; when you decrease the band size, you increase the cup letter.
The sister sizes of 34DD are 32E and 36D.
The sister sizes of 36D are 34DD and 38C.
No. Sister sizes have similar cup volume, but the band circumference and overall proportions are different. The fit can therefore feel and look different.
Potentially. If the cups fit correctly but the band is genuinely too tight, going up one band size and down one cup size may help.
Potentially. If the band is too loose, going down one band size and up one cup size may provide a more secure fit.
No. Sister sizing is a useful fitting technique, but it isn’t a substitute for finding the correct size and bra style for your body.
Because cup size is relative to the band. A D cup on a smaller band has less cup volume than a D cup on a larger band.
