Bra Sister Sizing: Cup Volume Mathematics and Band Tension Physics

The short answer: sister sizes are the sizes either side of yours on the same diagonal of a size chart — one band size smaller with one cup letter larger, or one band size larger with one cup letter smaller. A 34C's sister sizes are 32D and 36B. Within one bra style, sister sizes typically share the same underwire, so the cups fit similarly; what changes is the length of the band. That makes them a useful fix when the cups fit but the band does not — and a poor substitute for getting the band right.

A cup letter on its own does not describe a fixed size. A C cup on a 30 band and a C cup on a 40 band are very different cups, because the letter only means something in relation to the band it is attached to. Sister sizing is the practical consequence of that.

How to find your sister sizes

Take your size and move one step either way:

• Sister size down: one band size smaller, one cup letter larger. 34C becomes 32D.
• Sister size up: one band size larger, one cup letter smaller. 34C becomes 36B.

A few worked examples, using UK cup lettering (A, B, C, D, DD, E, F, FF, G):

Two lettering traps are worth knowing. UK sizing runs DD, E, F, FF, G, while US lettering diverges above D — Agent Provocateur's own size chart, for instance, lists UK 38E as US 38DDD/F — so "one cup letter" means one step in the brand's own sequence, not one letter of the alphabet. And a brand with its own numbering — Agent Provocateur's AP 1 to 7 scale is one — needs its own conversion chart before you can work out a sister size at all.

What actually stays the same

The lingerie brand Playful Promises puts it plainly in its sister sizing guide: between sister sizes, "the underwire length is the same, as well as the neck edge. The only thing that changes is the length of the underband."

The fitting community Bratabase makes the same point more carefully in its sister sizing entry: it is commonly said that sister sizes hold the same breast volume, which is an easy way to understand it, but what really happens is that sister sizes use a very similar underwire piece, so the cup fit is similar while the band changes.

That is why the common claim that sister sizes have "exactly identical" cup volume is better treated as a rule of thumb than as a measurement. The wire is shared; the cup sits on a band of a different length, and the cup can feel slightly shallower or deeper as a result.

Why sister sizes feel different on the body

Because the band is the part that changes, the band is where you feel it.

Fitters agree that the band does much of the work of supporting the bust, and Bratabase calls a band that is too big "the most common bra sizing problem". If you sister size up to escape a tight band, you also get a band that grips less. The usual symptoms of a band that is too loose are familiar:

1. The band rides up at the back instead of sitting level.
2. The front lifts away from the body, and the wires stop sitting flat against the ribcage.
3. The straps end up carrying weight they were not designed for, and dig into the shoulders.

Sister sizing down does the opposite: a firmer band, the same wire. That is often the better direction when a bra rides up, provided the band is still comfortable to breathe in. How the wire and its channel take that load is covered in underwire bra engineering and channel wear.

When to use a sister size

• The cups fit, but the band is uncomfortably tight on the loosest hook: try the sister size up — one band larger, one cup smaller.
• The cups fit, but the band rides up or slips when you lift your arms: try the sister size down — one band smaller, one cup larger.
• Your size is out of stock: a sister size in the same style is a reasonable thing to try, knowing the band will feel firmer or looser.

The limits

• Stay close. Playful Promises advises going no more than one size up or down, because the band ends up too loose or too tight; Bratabase advises no more than two steps in either direction.
• Same style only. Bratabase warns against applying sister sizing across brands or models, because grading varies and it will most likely not work as expected. A sister size in a different brand is a new size to try on, not a calculation.
• It does not fix a cup problem. If the cups gap or spill in your usual size, sister sizing keeps the same wire and will not change that. Measure again, starting with how to measure yourself for a bra at home.

Making a new bra last

Buy a new bra so it fits on the loosest hook. As the band relaxes with wear you can move to the tighter hooks — Playful Promises notes its bras come with at least three sets of hooks and eyes for exactly this. Elastic recovers best when it is washed cool and dried flat; the full method is in how to wash and store luxury lingerie, and how fit philosophy differs between the luxury houses is in best luxury lingerie brands.

Brand and fitting-guide statements quoted as read on 16 October 2026. Sizing conventions differ between brands and countries; the brand's own size chart for the specific style is the authority.