Sister Size Calculator
Same cup volume, different band.
Find bra sister sizes: the same cup volume on a larger or smaller band, for when the cup fits but the band does not.
Snug around the ribcage directly under the bust.
Loosely around the fullest part.
Your size, with its sisters
32D
4 equivalent cup volumes
Sister sizes hold the cup volume and change only the band, because a cup letter is the difference from the band rather than an absolute size. Use them when the cups fit and the band does not. One or two steps stays wearable; beyond that the wires stop sitting correctly.
How the Sister Size Calculator works
A cup letter is not an absolute volume. It is the difference between the bust and the band, which means a 34D and a 32DD hold the same cup while fitting quite differently around the ribcage. That relationship is what sister sizing is, and it is the fix when the cup is right and the band is not.
Also known as: bra sister size chart · equivalent cup volume calculator
Why a cup letter is not a fixed volume
A cup letter records the difference between the bust and the band, not an absolute size. A D is four inches of difference whether the band is a 30 or a 40, which means the actual cup volume of a 30D and a 40D are nothing alike — the 40D holds considerably more, because the same four inches is measured around a much larger ribcage.
That relationship is what sister sizing exploits. Move the band down by one size and up a cup letter, and the difference stays constant while the band tightens: a 34C, a 32D and a 36B all hold approximately the same volume.
It follows that there is no single answer to 'what does a D cup look like'. The letter is only meaningful attached to its band, which is why bra sizes are always written as a pair.
A worked example
Someone measuring a 34 band with a four-inch difference is a 34D. Their sister sizes one step out are a 32DD, with a tighter band, and a 36C, with a looser one. Two steps gives a 30E and a 38B.
If the cups fit but the band rides up at the back, the 32DD is the size to try: it holds the same volume on a firmer band. Simply going up a cup to a 34DD would add volume without fixing the band, which is the wrong direction for the actual complaint.
Where the answer misleads
Sister sizing works over one or two steps and degrades beyond that. The wire width and cup shape are designed around a particular band, so a 30E and a 38B may hold similar volume while sitting quite differently on the ribcage — the wires on the larger band are wider and will not follow the same root.
It also assumes the band is the problem. If the cup is gaping at the top or cutting in at the side, that is a shape mismatch rather than a volume one, and no sister size fixes it; a different style will.
Finally, bands stretch. A bra that needed a sister size when new may fit correctly a year later for the wrong reason, which is why fitters check on the tightest hook when new and the loosest when worn.
Frequently asked questions
What is a sister size?
The same cup volume on a different band. Going down a band size means going up a cup letter to keep the volume, so 34C, 32D and 36B are all sisters. Only the band changes in fit.
When should I use a sister size?
When the cups fit but the band is loose or tight. It is also useful across brands, since a band that runs small in one label can be corrected by moving a sister size rather than guessing at a whole new size.
Is a 34B the same as a 32C?
In cup volume, yes; those are sister sizes. The 32C has a tighter band and will support more, which is why fitters usually move people down a band and up a cup rather than the other way.
How far can I go in sister sizes?
One or two steps in each direction stays wearable. Beyond that the cup shape stops matching the band width and the wires sit wrongly on the ribcage, so it is a fitting adjustment rather than an unlimited conversion.