A sister size is a bra size that holds the same cup volume as your size, on a different band. Move one band step one way and one cup letter the other, and the cups hold the same amount while the band gets firmer or roomier. If you wear a 34C, your sister sizes are 32D (firmer band) and 36B (roomier band).
The short answer: Sister sizes are for one job only. When your cups fit but your band does not, trade band for cup letter, one step each way, and keep the cup volume that fits. When the cups are wrong, a sister size is the wrong tool.
Most of us were taught that a cup letter is a fixed size. It is not, and once you see why, the whole chart below becomes easy to read.
What is a sister size in bras?
A cup letter does not describe a set volume on its own. It describes volume relative to the band it sits on. A C cup on a 32 band holds less than a C cup on a 38 band, because the cup is scaled to the frame. As ThirdLove puts it, bra sizes work like ratios: as the band increases, the cup’s volume increases with it, even when the letter stays the same.
Sister sizes are what you get when you hold that volume steady and let the band change instead. Every size belongs to a small family of equivalents that share one cup volume. Your two nearest sisters are always one step away:
- Sister size down: one band smaller, one cup letter larger. The band gets firmer; the cup volume stays the same.
- Sister size up: one band larger, one cup letter smaller. The band gets roomier; the cup volume stays the same.
Same volume is not the same fit, though. Cup width, strap spacing and band tension all shift a little with the size, which is why sister sizing is a fine-tuning tool, not a whole new fitting. As Shefit puts it, a sister size is a starting point for a different fit, never a promise it will feel identical.
How sister sizes work: the 34C example, step by step
Take the most useful example in bra fitting, a 34C whose cups fit well but whose band feels tight even on the loosest hook.
The tempting move is wrong. Most shoppers reach for a 36C. But a 36C cup is built for more volume than a 34C cup, because the letter grew with the band. In fact, 36C is the sister size of a 34D. The band feels better, the cups now gape or wrinkle, and the bra seems worse than the one you started with.
The sister move keeps the volume. Go up one band step, 34 to 36, and down one cup letter, C to B. The result is 36B: the same cup volume as your 34C, on a longer, roomier band. The cups still fit. The digging stops.
The same logic runs the other way. If your 34C band rides up your back and still feels loose on the tightest hook, go down one band step, 34 to 32, and up one cup letter, C to D. The result is 32D: the same cup volume on a shorter, firmer band that can actually do the supporting. The band, not the straps, is meant to carry most of the support, a point the fitting guide at Sloggi stresses.
So the whole system is one sentence: band and cup move in opposite directions, one step at a time, and volume stays put.
The complete sister size chart
Find your size in the chart. Every size in the same row shares one cup volume. Move left for a firmer band, right for a roomier one. The chart uses US cup letters and covers bands 28 to 44.
| Sister size family | 28 band | 30 band | 32 band | 34 band | 36 band | 38 band | 40 band | 42 band | 44 band |
|---|---|---|---|---|---|---|---|---|---|
| A family | 28D | 30C | 32B | 34A | 36AA | ||||
| B family | 28DD | 30D | 32C | 34B | 36A | 38AA | |||
| C family | 28DDD | 30DD | 32D | 34C | 36B | 38A | 40AA | ||
| D family | 28G | 30DDD | 32DD | 34D | 36C | 38B | 40A | 42AA | |
| DD family | 28H | 30G | 32DDD | 34DD | 36D | 38C | 40B | 42A | 44AA |
| DDD family | 28I | 30H | 32G | 34DDD | 36DD | 38D | 40C | 42B | 44A |
| G family | 28J | 30I | 32H | 34G | 36DDD | 38DD | 40D | 42C | 44B |
| H family | 28K | 30J | 32I | 34H | 36G | 38DDD | 40DD | 42D | 44C |
Three things worth knowing before you use it:
- The diagonal never changes. Any two sizes on the same row are sisters, side by side or three steps apart. Side by side is still the smarter try-on, for reasons covered below.
- Letters are not standardised past DD. Some US brands print DDD as F, and UK brands run a different sequence after D (DD, E, F, FF). Shefit advises checking the brand’s own cup sequence before calculating beyond D, since US and UK labels are not interchangeable across brands.
- Your size may sit at a row’s edge. A 28 band has no smaller-band sister in most ranges, and a 44 has no larger one. That is a limit of the size range, not of your body.
If you shop at the larger end of the chart, our guide to the biggest bra size explains how far bands and cup letters actually go.
How to find your sister size in 3 steps
- Start from your best current size, the one whose cups fit you best today. If you do not have one, measure first with our measurement guide; a sister calculated from a wrong starting size just moves the problem.
- Decide which way the band needs to move. Tight on the loosest hook means up. Loose on the tightest hook, or riding up your back, means down.
- Move the cup letter the opposite way, one step. Band up, letter down. Band down, letter up. Then check the fit: band level and snug, cups containing you, straps comfortable.
You can also let our bra size calculator do this step for you: enter your measurements and it returns your size plus both sisters, labelled “Smaller Band, Larger Cup” and “Larger Band, Smaller Cup”. Feed it measurements that produce a 34C and it hands back the pair from the worked example: 32D and 36B.
When to use a sister size
Your band is too tight, but the cups fit. This is the classic case, keyword and all: bra band too tight but cup fits. If the cups sit well and the band digs in even on the loosest hook, take the sister size up. A 34C becomes a 36B. Wacoal describes the same fix: same volume, looser band.
Your band is too loose, but the cups fit. The mirror case: cup size right but band too loose. A band that rides up or only feels secure on the tightest hook is not supporting you. Take the sister size down. A 36C becomes a 34D, and the support moves back into the band where it belongs.
Your size is out of stock. A sister one step away is a reasonable try-on when your exact size is gone, especially in a style you know. ThirdLove lists this among the main everyday uses, alongside a band that feels off and a body that has changed a little.
The same size fits differently in another style. Bands are cut with different firmness, so a size that is perfect in one style can feel a half-step off in another. Treat the sister as a per-bra adjustment, not a new identity: your size has not changed, the bra has.
When a sister size will not help
The cups are wrong. Spilling out of the cups, or cups that gape and wrinkle, is a cup volume problem. Sister sizing holds volume steady by design, so it cannot fix it. Keep the band, change the letter: cups too small on a good band means one letter up on the same band. Both ThirdLove and Shefit are blunt about this: if the cups do not fit, remeasure instead of reaching for a sister.
The shape is wrong. Sister sizes rebalance proportions; they do not change a cup’s shape. If the wires sit on breast tissue at the sides, the gore will not settle, or a moulded cup gaps while a seamed cup in your size fits, that is a shape mismatch. No step sideways fixes shape. Try a different style in your measured size instead.
The bra is simply worn out. A band that has stretched past its elastic life makes any size feel loose. Shefit advises against replacing a worn-out bra with a sister size automatically: remeasure and judge a new bra on its own fit first.
Both band and cups are wrong. If nothing about the bra fits, do not zigzag through sisters. Remeasure, reset to your calculated size, and start again from there.
How far can you go? The one-step rule
Stay one step from your best size in either direction. That is the consistent guidance across fitter sources, and there is a mechanical reason: the further you travel along a row, the more the bra’s frame changes around the same cup volume:
- Wire width grows or narrows, so the wire stops tracing where your breast tissue sits.
- Strap spacing widens or closes, so straps start slipping off shoulders or cutting in.
- Centre gore placement shifts, so the gore floats instead of settling between the breasts.
ThirdLove spells out all three changes as the reason fit breaks down beyond one step. Two steps is a gamble some fitters will try in a fitting room; treat it as the outer edge, not a strategy, and re-check wires, straps and gore carefully if you go there.
Sister sizes in sports bras
Sports bras split into two sizing worlds, and sister sizing only works cleanly in one of them.
Band-and-cup sports bras (many encapsulation and high-support styles) follow the same rule as everyday bras. If the cups contain you but the band chafes or rides up, one sister step is a legitimate try.
Alpha-sized and brand-system sports bras (S, M, L, or a brand’s own size names) have no letters to trade. Shefit, whose bras use proprietary size names rather than band-and-cup labels, directs wearers to that bra’s own measurement chart instead of substituting a conventional size. The honest limit: a sister size cannot be calculated for a bra that was never sized that way. Use your bust and ribcage measurements against the brand’s chart, and lean on adjustable bands and straps to personalise fit after the size is chosen. For fuller-bust options that do use band-and-cup sizing, see our sports bras for large bust guide.
Sister sizes in wireless bras and bralettes
The volume principle still applies to wireless bras sized by band and cup, and ThirdLove confirms sister sizes follow the same inverse rule for wireless bras and bralettes. Two honest limits:
- Stretch forgives, and hides. Wireless fabrics stretch more, so a sister step can feel fine on day one and sloppy by month three. Judge the band on the loosest hook with the same strictness you would use on a wired bra.
- S/M/L bralettes have no true sister. The nearest equivalent is trying the adjacent letter size while checking that the band stays snug and the cups still cover. Each brand maps S/M/L to different band and cup combinations, so the brand’s own chart is the only reliable translator.
Sister sizes for teens
Once a teen is wearing band-and-cup sizes, sister sizing works as it does for adults, and it is useful during growth, when the band often needs to move first. Two cautions keep it honest:
- Remeasure rather than stretch the system. If a teen’s bra is wrong in the cups and the band, that is growth, not a sister-size problem. Fresh measurements beat a chain of sister steps.
- Comfort is the test, not the label. A sister size that digs, rides up or needs constant adjusting fails, whatever the chart says. The chart proposes; the wearer disposes.
A quick word on US and UK labels
Everything above uses US cup letters. UK sizing follows the same logic, but its letter sequence differs after D (DD, E, F, FF, G), so a lookalike size may not be a sister across systems. Converting between systems is a label conversion, not sister sizing, and Shefit notes the two are often confused. If you shop UK brands, convert first using that brand’s chart, then apply the sister rule inside the new system.
Common questions about sister sizes
What is a sister size in bras?
A sister size is a band-and-cup combination that holds the same cup volume as your size on a different band. Move one band step in one direction and one cup letter in the other: a 34C, a 32D and a 36B all hold the same volume.
What’s the difference between bra sizes 32B and 34A?
In cup volume, almost nothing: 32B and 34A are sister sizes. The difference is the frame. The 32B has a shorter, firmer band and a narrower frame; the 34A carries the same volume on a longer band. Which one fits depends on your ribcage, not on which letter sounds bigger.
What are the sister sizes for 34C?
32D if you need a firmer band, 36B if you need a roomier one. Our bra size calculator returns the same pair when your measurements produce a 34C.
My band is too tight but the cups fit. What should I try?
Take the sister size up: one band larger, one cup letter smaller. A tight 34C becomes a 36B. If it is still tight on the loosest hook, the style itself may run firm; try another style rather than travelling further along the row.
My cup size is right but the band is too loose. What should I try?
Take the sister size down: one band smaller, one cup letter larger. A loose 36C becomes a 34D. Check that the wires still sit behind your breast tissue, since the frame narrows along with the band.
Do sister sizes work for sports bras?
Yes for sports bras sized by band and cup, using the same one-step rule. No for sports bras sized S/M/L or in a brand’s own size names, because there are no letters to trade. For those, measure your bust and ribcage and use the brand’s own chart, as Shefit advises.
Sources
- ThirdLove: definition, inverse rule, hook test, one-step limit, wire/strap/gore changes, wireless applicability.
- Shefit: definition and chart, when cups do not fit, US/UK label caution, brand-system sports bra sizing, worn-out bra caution.
- Sloggi: definition, when to use, band support role.
- Wacoal: band too tight and band too loose solutions.
