What Is a Sister Size in Bras? The Cup-Volume Swap, Explained
A 34C, 32D, and 36B all hold the same cup volume. They're sister sizes — same breast tissue, different rib cage. Here's the rule, when to use it, and the full sister-size table.
You wear a 34C. The cup fits perfectly — no spillage, no gap — but the band rides up your back, won't stay flat, and feels loose by the end of the day. Or the cup fits but the band feels like a vise. Either way, the answer isn't a different cup size. It's a sister size.
Sister sizing is the most useful concept in bra fitting. It lets you keep the cup volume you already wear (and like) while changing the band to one that actually fits your rib cage. Same volume of breast tissue. Different label.
The One-Sentence Rule
Sister size formula
Worked example with 34C:
- 34C — your starting size
- 32D — sister size (smaller band, larger letter, same volume)
- 36B — sister size (larger band, smaller letter, same volume)
All three bras have an identical cup capacity. The breast tissue takes up the same space. What differs is how tightly the band hugs your underbust.
Why Sister Sizes Exist (The Cup-Letter Trick)
Cup letters are not absolute volumes. They're relative to the band number.
A "C cup" on a 30 band holds less volume than a "C cup" on a 38 band. The cup letter measures the difference between the bust and the band:
| Cup letter | Bust − band difference |
|---|---|
| AA | 1" |
| A | 2" |
| B | 3" |
| C | 4" |
| D | 5" |
| DD / E | 6" |
| DDD / F | 7" |
| G | 8" |
| H | 9" |
So 30D = bust 5" larger than band 30 = 35" bust. 38B = bust 3" larger than band 38 = 41" bust. The cup letter alone doesn't tell you volume — it's always relative to the band.
Sister sizes work because as the band increases, the cup letter decreases by one to keep the volume identical:
| Size | Band | Cup difference | Approx volume |
|---|---|---|---|
| 32D | 32" | 5" | Volume X |
| 34C | 34" | 4" | Volume X |
| 36B | 36" | 3" | Volume X |
| 38A | 38" | 2" | Volume X |
Same volume, four different bands. That's the trick.
Full Sister-Size Table (28 to 44 Band)
| Sister size group (same cup volume) | Smaller band | Original | Larger band |
|---|---|---|---|
| Volume A | 28C | 30B | 32A |
| Volume B | 30C | 32B | 34A |
| Volume C | 32C | 34B | 36A |
| Volume D | 34C | 36B | 38A |
| Volume E (DD) | 32E | 34D | 36C |
| Volume F (DDD) | 34F | 36E | 38D |
| Volume G | 30G | 32F | 34E |
| Volume H | 32H | 34G | 36F |
| Volume larger | 34I | 36H | 38G |
The pattern continues — each row shifts band by 2 (one size) and cup letter by 1 in the opposite direction. For the full sizing range and exact band/cup measurements see our bra size chart.
When to Use Sister Sizing
Band rides up your back
Band digs in, leaves marks, hard to breathe
The cup fits perfectly but the brand doesn't stock your size
Between cycles or weight changes
When Sister Sizing Won't Help
The cup itself is wrong
If the cup spills (too small) or has gap at the top (too big), the cup volume is wrong, not the band-cup ratio. Change the cup letter alone (same band): 34B → 34C if cups are too small. Sister sizing keeps the cup volume the same — that's exactly what you don't want here.
Wires don't lie flat against your sternum
The wires sitting in the middle of breast tissue rather than against the sternum is a cup-too-small problem. Sister up to 34C or 34D, not to 32D — the band size isn't the issue.
Straps fall off your shoulders
Strap problems are usually band problems in disguise. The band is too loose, so the bra hangs forward. Tighten with sister sizing (down a band, up a cup), not with strap shortening.
The bra fit fine new but is loose now
The band has stretched. Replace the bra. The size on the label is still right; the elastic just gave out (typical lifespan: 6–12 months of regular wear).
Sister Sizes and International Sizing
Sister sizing works the same way internationally — the cup-volume relationship is universal. But the labels look different:
| Region | Equivalent of 34C | Sister size (down band) | Sister size (up band) |
|---|---|---|---|
| US | 34C | 32D | 36B |
| UK | 34C | 32D | 36B |
| EU | 75C | 70D | 80B |
| FR / ES | 90C | 85D | 95B |
| IT | 3C | 2D | 4B |
| AU / NZ | 12C | 10D | 14B |
| JP | 75C | 70D | 80B |
For full international bra sizing see our bra size chart and US to EU conversion.
The Two-Step Sister-Size Decision
- Identify what's wrong. Cup spills or gaps → change cup, same band. Band rides up or digs in → use sister sizing.
- Pick the right direction. Loose band → smaller band, bigger cup letter. Tight band → bigger band, smaller cup letter. Always opposite signs.
For the full bra fitting workflow including how to measure for the band and cup from scratch, see how to measure for a sports bra (the technique is identical for everyday bras) or our main bra reference at bra size chart.
FAQs
What is a sister size in bras?
A sister size is a different bra size with the same cup volume. The rule: down a band and up a cup letter, or up a band and down a cup letter.
What is the sister size of 34C?
The sister sizes of 34C are 32D (smaller band, larger cup letter) and 36B (larger band, smaller cup letter).
When should I try a sister size?
Try a sister size when the cup fits but the band is wrong. If the band feels loose, swap to a smaller band and a larger cup letter. If the band digs in, swap to a larger band and a smaller cup letter.
How do you calculate sister sizes?
Change the band number by 2 (one size) and the cup letter in the opposite direction by 1.
Are sister sizes the same fit?
The cup volume is the same. The band tightness and how the bra sits on your rib cage will differ. Pick the one whose band fits.
What if the cup is wrong but the band fits?
Sister sizing won't help. Change the cup letter directly: same band, larger or smaller cup.
Stop Throwing Away Bras That Almost Fit
Sister sizing is the reason most "wrong" bras aren't actually wrong — they just need to slide one notch in either direction. Two wears, no return needed. Three sizes that hold the same cup volume.
Stores running Tailor Size Guide show shoppers their primary bra size plus the two sister sizes — so band-fit issues resolve in one click instead of three returns.
Selling lingerie on Shopify? Show sister sizes, cut returns.
Most shoppers don't know sister sizing exists. Tailor Size Guide adds the two sister sizes to every bra page automatically — so when the band feels wrong, the next size to try is one click away.