The core idea
A letter alone tells you almost nothing
Cup letter is a relative measurement — the gap between your bust and your band, in inches. That means the exact same letter describes a different amount of tissue depending on the band it's attached to. A 30D and a 40D share a letter, but a 40-inch band with a 4-inch cup gap is carrying meaningfully more volume than a 30-inch band with the same gap.
This is the single most common misunderstanding in how people compare sizes, and it explains almost every "wait, that doesn't look like a D" reaction people have when comparing themselves to someone else wearing the same labelled size.
Comparing across bands
Sister sizes: same volume, different label
Because cup letter scales with band, moving the band up or down while adjusting the cup letter in the opposite direction keeps the actual volume almost identical. A 34D, 32DD and 36C are commonly cited as an equivalent trio — none of them is "more" or "less" than the others in terms of what the cup holds, even though the labels look completely different on a shelf.
| Size | Band feel | Approx. relative volume |
|---|---|---|
| 32DD | Tighter band | Same as row below |
| 34D | Reference band | Reference volume |
| 36C | Looser band | Same as row above |
See the full sister size chart for every band and cup combination laid out this way.
Comparing across brands
Why the same label fits differently
Even holding band and cup constant, two brands rarely cut identically. Wire shape, cup depth, fabric stretch and where a brand sets its "true" band-to-cup ratio all shift the outcome. Comparing a size across brands is really comparing two different interpretations of the same rough formula, not two identical garments.
Narrow vs. wide-set
The same cup letter can sit closer together or further apart depending on the wire's shape, changing how "full" it looks.
Shallow vs. deep
Some brands cut a shallower cup for the same letter, better suited to a flatter projection; others cut deeper.
Stretch vs. rigid
More stretch in the fabric gives a size more forgiveness at the edges of its range than a rigid, structured cup.
Where "true to size" starts
Some brands run their whole scale slightly larger or smaller than the standard formula, shifting every size along with it.
Comparing across countries
The same body, different labels
Because band and cup conventions vary by country, comparing sizes internationally is really comparing sizing systems, not bodies. A US 34D, a UK 34D, and a EU/FR 75D describe extremely similar fits, despite looking like three different sizes on paper — while a US 34DD and a UK 34DD describe two different things entirely, since the two countries' cup ladders diverge above a D cup.
For a full breakdown of which countries share a system and which don't, see how regional systems differ on the homepage.
Questions
Comparison FAQs
Not by volume — they're sister sizes and hold roughly the same amount, even though the letters look different. The difference is in how the band feels, not how much the cup holds.
Cup letter only measures a distance in inches between bust and band, not shape, root width or how tissue is distributed, so two people with the same letter can look and fit quite differently.
No. Band and cup are calculated independently from two separate measurements, so any band can pair with any cup letter.
They don't compare precisely — a size label is only ever a starting estimate for a given brand's own cut, which is why trying garments on still matters more than matching labels across brands.
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