Karma Half Cup Bra — Simone Pérèle, available at BraTopia
Fit & Sizing Fundamentals
Understanding Sister Sizing: What to Try On When Your Size Is Sold Out
Nothing is more frustrating than finally finding "the one" — and discovering your size is sold out. Before you give up on the style entirely, there's a trick professional fitters use every day: sister sizing. Understanding how sister sizing works means you'll always know exactly what to try on next, instead of guessing.
What Is Sister Sizing?
Sister sizes are bra sizes that share roughly the same cup volume but come in a different band-and-cup combination. Every time you move the band one size in one direction, you move the cup one letter in the opposite direction to keep the same amount of room in the cup.
In practice, that means a bra one band size smaller with the cup one letter bigger — or one band size bigger with the cup one letter smaller — should hold roughly the same amount of breast tissue as your true size. The fit won't be identical, but the volume will be close enough to try on.
The Sister Size Chart
| Snugger Band / Bigger Cup | Your Size | Looser Band / Smaller Cup |
|---|---|---|
| 30DD | 32D | 34C |
| 30E | 32DD | 34D |
| 30F | 32E | 34DD |
| 32DD | 34D | 36C |
| 32E | 34DD | 36D |
| 32F | 34E | 36DD |
| 34DD | 36D | 38C |
| 34E | 36DD | 38D |
| 34F | 36E | 38DD |
Not sure which row is actually you? Find your size first, or check our full size chart — most of BraTopia's styles run in UK sizing, which this chart reflects.
How to Use Sister Sizing When Your Size Is Sold Out
Once you know your true size, finding what to try on is simple:
Example: You Wear 34D
If 34D is sold out, try 32DD first — same cup volume, slightly snugger band, often the closer feel to your original size since bands provide most of the support.
If 32DD also isn't available (or the band feels too tight on you), try 36C instead — same cup volume again, but a looser, more relaxed band.
Shop full cup styles →
The same logic works in any direction and with any style — it's just the band and cup moving in opposite steps to protect the cup volume.
Why Sister Sizing Isn't a Perfect Substitute
A sister size is an approximation, not a duplicate. The band is what provides most of a bra's support, so a looser or snugger band changes how secure the bra feels even if the cup volume is technically the same.
Cup shape also isn't identical across band sizes — a 36D and a 32DD hold similar volume, but the cup is wider and shallower on the larger band, and narrower and deeper on the smaller one. That can matter more for projected shapes or asymmetry. Read more in Top Signs Your Bra Doesn't Fit if either sister size feels off.
Shop balconette styles →
Sister Sizing Across Brands
The sister-size rule holds across brands, but cup depth and shape don't. European labels in particular can run differently from what you're used to. Empreinte is the classic example: their cups are cut deeper and fuller than most brands, so many customers find they need to size down a cup letter rather than sizing by sister size alone. Full details in our Empreinte Sizing Guide.
This is exactly why we recommend treating any sister size — and any new brand — as a starting point to try on, not a guaranteed fit.
Shop full-bust styles →
Common Sister Sizing Mistakes
Avoid these when sizing between sister sizes:
- Changing only the band or only the cup, instead of both together
- Assuming a sister size will feel identical to your true size — it won't, especially in the band
- Sticking with a sister size long-term instead of it being a stopgap
- Applying the same sister size math across every brand without accounting for cup shape differences
- Never checking against your measurement guide to confirm your actual true size in the first place
When to Get Refitted Instead of Guessing
Sister sizing is a great stopgap when your size is sold out — it's not a replacement for knowing your real size. If you're regularly bouncing between three or four "close enough" sizes, or a sister size never quite feels right, that's a sign it's time for a proper fitting rather than more guessing. See Six Essential Tips to Ensure Your Bra Fits Perfectly for the fundamentals, or book a fitting below.
Skip the Guesswork — Book a Fitting
Our certified fitters can confirm your true size and its sister sizes on the spot, and pull styles and brands suited to your shape — no guesswork, no judgment.
Book Your Free FittingFrequently Asked Questions
What is sister sizing?
Sister sizing describes bra sizes that share the same cup volume but come in a different band-and-cup combination. Moving one band size down and one cup letter up (or the reverse) keeps the cup volume roughly the same while changing how snug the band feels.
How do I find my sister size?
Start from your true size, then move the band one size in one direction and the cup one letter in the opposite direction. For example, a 34D's sister sizes are 32DD (snugger band) and 36C (looser band).
Is a sister size the same as my real size?
No. The cup volume is similar, but the band circumference is different, which changes how the bra feels and how much support it gives. A sister size is a close approximation, not an identical fit.
Can I wear my sister size permanently?
You can, but it's meant to be a short-term substitute when your true size is unavailable, not a long-term replacement. Over time the band-support difference can affect comfort and how well the bra holds its shape.
Do all brands use the same sister sizing rules?
The core rule is consistent, but cup shape and depth vary by brand and country of origin. European labels in particular can run differently — for example, Empreinte's cups are cut deeper, so many customers size down a cup letter compared to other brands.
What if my sister size still doesn't fit?
Sister sizing is an approximation, not a guarantee. If neither your true size nor its sister sizes feel right, it's a sign to get professionally refitted rather than keep guessing between sizes.
Does sister sizing work for band size too, or just cup?
Sister sizing always involves both measurements moving together — the band and cup adjust in opposite directions at the same time. Changing only one without the other will change the cup volume, not preserve it.
Where can I get measured for my exact size in Calgary?
BraTopia offers free 1:1 fittings with certified fitters at our Bridgeland, Calgary location. Book an appointment online and we'll confirm your true size and its sister sizes in person.
