A sister size is the same cup volume on a different band. Move the band one step and move the cup letter one step the opposite way, and the bra holds roughly the same amount: 34C, 36B and 32D are all sisters.
Not sure it is your size? The bra size calculator turns two measurements into an answer, sister sizes included.
Why do sister sizes exist?
Because a cup letter is not a fixed quantity. On Indian sizing the band is your underbust rounded up to an even number plus four inches, and the letter is the difference between your bust and that band number. A letter only means something against the band it sits on, which is why a C on a 32 band holds less than a C on a 38.
Run that logic backwards and sister sizes fall out of it. If the letter is relative, one actual cup volume must appear under different letters as the band changes. Add two inches of band and the same volume measures one letter smaller against it; take two inches away and it measures one letter bigger. The volume never moved. The reference point did.
What a single letter holds, band by band, is set out in the A cup, B cup, C cup and D cup guides.
The sister size chart, 30A to 40D
The chart covers the full stocked grid with both neighbours for each size. "Unstocked" marks sizes the arithmetic produces but Indian retail, ours included, rarely manufactures.
| Band down (tighter) | Your size | Band up (looser) |
|---|---|---|
| 28B (unstocked) | 30A | none |
| 28C (unstocked) | 30B | 32A |
| 28D (unstocked) | 30C | 32B |
| 28E (unstocked) | 30D | 32C |
| 30B | 32A | none |
| 30C | 32B | 34A |
| 30D | 32C | 34B |
| 30E (unstocked) | 32D | 34C |
| 32B | 34A | none |
| 32C | 34B | 36A |
| 32D | 34C | 36B |
| 32E (unstocked) | 34D | 36C |
| 34B | 36A | none |
| 34C | 36B | 38A |
| 34D | 36C | 38B |
| 34E (unstocked) | 36D | 38C |
| 36B | 38A | none |
| 36C | 38B | 40A |
| 36D | 38C | 40B |
| 36E (unstocked) | 38D | 40C |
| 38B | 40A | none |
| 38C | 40B | 42A (unstocked) |
| 38D | 40C | 42B (unstocked) |
| 38E (unstocked) | 40D | 42C (unstocked) |
Two patterns are worth noticing. Every A size has "none" in the band-up column, because that move would need a letter below A and the scale has none. And every D size's band-down sister is an E, which mostly exists on paper. Both get their own section below.
When should you switch to a sister size?
When the cup fits and the band does not. That is the entire rule. Take a 36C wearer:
- Band rides up at the back, cup fits: go band down, letter up, to 34D
- Band digs in, cup fits: go band up, letter down, to 38B
- Band comfortable but the cup spills or gapes: not a sister size problem. Stay on 36 and change the letter only.
Written the standard way, with band up on the left and band down on the right, that is 38B ← 36C → 34D. All three sit in the stocked grid, and all three hold about the same. Our 36C guide works through this size in detail.
What is the sister size of 32B?
32B has two: 34A ← 32B → 30C. The working: two inches more band drops the letter one step, giving 34A; two inches less band raises it one step, giving 30C. Both directions are stocked, which makes 32B one of the easiest sizes on the chart to adjust by feel. The same arithmetic runs anywhere on the grid: a 34B sits between 36A and 32C.
Do sister sizes feel identical?
No, and they are not meant to. The cup holds the same volume; everything else shifts. The band is one step different, so the grip around your ribcage, where most of a bra's support comes from, changes noticeably. The cup is also cut to suit its own band, so the same volume sits slightly wider across the chest on the larger band and slightly narrower on the smaller one. You switch to a sister size precisely because you want the band to behave differently. If the band already felt right, stay on it and sort the cup letter instead.
The A cup and D cup edge cases
A cups get one sister, not two. There is no letter below A, so the band-up move has nowhere to go, and a 34A's only sister is 32B. When an A cup band digs in, the honest fix is a looser setting on the same band, or the next band up in A with a cup that will run slightly roomier against it. The pinch is sharpest at the smallest bands: a 30A's only sister is a 28B that Indian rails mostly skip, and the 28 bra size guide covers that band and the sister route out of it.
D cups have the opposite problem. Both sisters exist in the maths, but the band-down one is an E, and most Indian rails, ours included, stop at D. A 34D's band-down sister is 32E, which is correct and rarely buyable; the swap you can actually make is 36C, and a band that fits firmly matters more than landing the exact letter.
The same grid thinking applies when a product's size range has edges. Our strapless adhesive bra is made from 30A to 40D. A 28C sits outside that range, but its sister 30B sits inside it, so the chart above is also how you order when your usual size falls just off the edge of what a product comes in.
The short version
Sister sizes keep the cup volume and change the band: band down means letter up, band up means letter down. Use one when the cup fits and the band does not, and expect it to feel different around the ribcage, because that difference is the point of the move. A cups have a single sister, and a D cup's band-down sister is usually not buyable in India, so the practical direction at D is band up. The full measuring method is in the bra size chart for India, and the bra size calculator prints your sisters next to your size.
