Sister Size Calculator
Enter the size you already wear. You get every size that holds the same cup volume, in both directions, with the trade-off each one makes.
Sister sizes of 34Cexample
Those two are the immediate neighbours - the ones to try first when the cup is right and the band is not.
The whole ladder
| Size | What it changes |
|---|
Every row holds the same cup volume. Going down the list loosens the band and shrinks the letter; going up tightens the band and grows it. Beyond one step in either direction the equivalence gets rougher - the cup is the same volume but a distinctly different shape - so treat the far ends as last resorts rather than genuine alternatives.
Why a C cup is not a fixed amount of anything
The thing that makes sister sizes confusing is a false assumption underneath the question: that a cup letter names a volume. It does not. It names a difference - three inches between bust and band is a C, whatever band that is. So a C on a 30 band and a C on a 40 band are three inches of difference measured around very different frames, and they are nowhere near the same amount of tissue.
Once that is clear, sister sizes stop being a trick and become arithmetic. If you tighten the band by two inches but your body has not changed, the difference between bust and band grows by two inches - so the cup letter has to climb to compensate. The volume in the cup is conserved. Only the label moves.
Why sister sizes hold the same amount
Every site repeats the sister-size rule. Few say where it comes from, so: it is not a coincidence or an approximation someone noticed. Bras are built that way. When a manufacturer grades a pattern up a band size, the cup is redrawn to hold more, and dropping a letter takes back the same amount - so a 36D cup and a 38C cup come off the line the same size. The rule describes a manufacturing decision.
Run those four sizes through the breast volume model and they come out identical:
| Size | Modelled cup volume |
|---|---|
| 30DD | 545 mL |
| 32D | 545 mL |
| 34C | 545 mL |
| 36B | 545 mL |
A spread of 0 mL across the four - they are exactly equal. That is not independent proof, and it would be dishonest to dress it up as such: the model is built on the same assumption, so of course it agrees. It is a consistency check, not evidence.
The real limit on sister sizes is not volume. It is the band. Every row here holds the same cup, so travelling along the ladder never costs you cup room - but each step moves the band two inches, and the band is what holds the bra up. Three steps from your measurement is a band six inches off your ribcage, which will ride up your back no matter how well the cup fits. That is the reason to trust one step and distrust three, and it has nothing to do with the arithmetic above.
Diagnose before you move
Which direction to move, and when
Sister sizes only ever solve a band problem. Two of these four symptoms have a sister size as the answer. The other two do not, and reaching for one will hide the fault rather than fix it.
The band rides up your back
Or you are already on the tightest hook, or you can pull the band more than about two inches away from your ribs. Go down a band and up a cup. This is by far the most common fault, and the one the +4 method produces by design.
The band digs in painfully
Or you cannot fasten it on the loosest hook. Go up a band and down a cup. Check the cup first, though: a cup that is far too small drags the band down and can feel like a band that is too tight.
The cup gapes, or you spill over it
A sister size will not fix this and may disguise it, because every sister size holds the same volume by definition. You need a different cup letter on the band you already have.
The wire sits on breast tissue
The wire should sit behind the tissue, in the crease, flat against the ribs. If it rests on the breast, that is a cup problem and frequently a shape problem - a wider or narrower wire, not a different label.
An example, worked all the way through
Suppose you wear a 34C and it nearly works, but the band creeps up over the day and you fasten it on the last hook when it is new. The band is too loose. Drop it to a 32 - and because your body did not change when the label did, the cup has to grow a letter to hold the same volume, giving a 32D.
| Band | Cup | Size |
|---|---|---|
| 30 | DD | 30DD |
| 32 | D | 32D |
| 34 | C | 34Cthe 34C in this example |
| 36 | B | 36B |
| 38 | A | 38A |
Note what happens at the ends of that ladder. A 30DD and a 38A hold roughly the same volume as the 34C in the middle, and neither is a sensible substitute for it - the 30DD puts that volume on a narrow, very firm band and the 38A spreads it across a wide, slack one. The equivalence is real and the garments are not interchangeable. One step is advice; three steps is trivia.
Sister sizes across countries
The rule is identical everywhere because the arithmetic is, but the labels move differently. In UK sizing a 34C's tighter sister is a 32D, same as the US. Above a DD the ladders diverge - a US 32DDD's sister is a US 30G, but written in UK sizing those are a 32E and a 30F. Switch the sizing system in the calculator above rather than converting by eye.
Related
- Bra size calculator - start from measurements instead of a size.
- Breast weight calculator - the model used to check the equivalence above.
- B cup vs C cup - what one letter is worth.
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