Sports Bra Size Sold Out? Try Your Sister Size
Can’t find your usual sports bra size?
Before you give up, it’s worth checking your sister size.
Sounds a bit odd, I know. But sister sizing is simply a way of finding another bra size with a similar cup volume, while changing the band size.
It can be really handy when your usual size is sold out, or when the cups feel about right but the band is either too tight or too loose.
And no — it’s not you. Bra sizing is just annoying.
What is a sister size?
A sister size is a bra size that has a similar cup volume to your usual size, but with a different band size.
The basic rule is:
If you go up in the band, go down in the cup.
If you go down in the band, go up in the cup.
So, for example, if you usually wear a 34E, your sister sizes may be:
32F — smaller band, larger cup
36DD — larger band, smaller cup
The cup letter changes, but the actual cup volume stays closer than you might expect.
Sister Size Sizing Guide
Why does the cup letter change?
This is the bit that confuses everyone.
A D cup is not the same size in every band.
A 32D, 34D, 36D and 38D are all different cup volumes because the cup size is relative to the band size.
So when you change the band, you usually need to change the cup letter too.
That’s why someone might think, “I can’t possibly be an F cup,” when actually, in that band size and brand, that may be exactly what fits.
Bra sizing: not here to make our lives easy.
UK cup sizing can be extra confusing
Most of our sports bras use UK sizing, which includes cup sizes like:
D, DD, E, F, FF, G, GG, H, HH, J
So if you are moving through sister sizes, make sure you follow the UK cup order, not a generic A, B, C, D chart.
32FF/G— smaller band, larger cup
36E — larger band, smaller cup
And if you usually wear a 34G, your sister sizes would be:
32GG/H — smaller band, larger cup
36F/FF — larger band, smaller cup
Note: not all brands offer FF or GG sizing so just pick the next cup up or down for that brand
This is why sister sizing is helpful, but also why it can get confusing quickly.
When can sister sizing help?
Sister sizing can be useful if:
- Your usual size is sold out
- The cup fits but the band feels too tight
- The band feels too loose but the cups are okay
- You’re between sizes
- You’re trying a new brand or style
- Your body has changed and your old size no longer feels right
It’s especially useful when you’re close to the right fit, but something is just slightly off.
A quick sports bra warning
Sister sizing is helpful, but it’s not magic.
This matters even more with sports bras, especially if you’re running, doing HIIT, playing netball, riding horses, or doing anything high-impact.
For bigger boobs, the band does a lot of the support work. If you go too far up in the band just because it feels more comfortable, you may lose support once you actually start moving.
So for a high-impact sports bra, I’d usually only try one sister size either way.
For example:
If your usual size is 34F, try 32FF or 36E.
But jumping too far away from your usual band size may make the bra feel less supportive, even if the cups seem okay.
Comfort is good. Boobs launching themselves during a jog is not.
When sister sizing probably won’t fix the problem
Sister sizing can help with small fit tweaks, but it won’t fix every bra problem.
It may not be the answer if:
- The underwire is sitting on breast tissue
- The cups are cutting in badly
- The cups are wrinkling or gaping a lot
- The band rides up your back
- The straps are doing all the work
- The bra shape just doesn’t suit your boobs
- You still feel unsupported when you move
In those cases, you may need a different style, not just a different size.
Some boobs suit compression styles. Some need encapsulation. Some need a firmer band. Some need more cup depth. Some just need a bra that was designed by someone who has actually met boobs before.
So, should you try a sister size?
Yes, sometimes.
Sister sizing is a good guide when your usual size is unavailable, or when you’re nearly there with the fit but need a small adjustment.
Just remember:
Band too tight? Try going up a band and down a cup.
Band too loose? Try going down a band and up a cup.
And with sports bras, be careful not to go too loose in the band, because that’s where a lot of your support comes from.
Still not sure?
Totally normal.
Sports bra sizing can be confusing, especially if your size has changed after weight changes, pregnancy, breastfeeding, surgery, menopause, or just because bodies like to keep us guessing.
If you’re not sure what size or style to try, use the online fitting form and I’ll point you in the right direction.
Big boobs? We’ve got you.
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