Snug and level, directly under the bust.
Loose and level across the fullest point.
Enter both measurements to continue.
A schematic fit diagram, scaled to your cup size.
What actually sets the letter
Subtract your rounded band number from your rounded bust number. Whatever that gap is in inches is your cup index — and the index maps directly onto a letter.
Notice that your band never appears in this formula on its own — cup letter is entirely about the relationship between two numbers, not either one in isolation. That's why the same letter can look completely different from one band to the next.
Beyond the letter
Why the same letter isn't the same size
Letters scale with band
A 30D and a 40D share a letter but not a volume — the same 4-inch gap means more actual tissue on a bigger frame.
Letters ignore projection
Two people with an identical gap can have very differently shaped busts — the letter only ever describes a distance, not a shape.
Letters aren't standardized above D
Once you pass a D cup, brands choose their own labelling — DD, DDD and E can all mean the same thing depending on who made the bra.
Letters shift with your body
Cup volume tends to move first with weight change, pregnancy or nursing, often before the band budges at all.
Questions
Cup size FAQs
Cup size is the difference between your bust measurement and your band measurement, not an absolute number on its own. Every inch of difference moves the letter one step up the scale.
Only relative to the same band. A 30D and a 40D share a letter but hold noticeably different amounts, since cup size is always read alongside its band.
Cup volume responds to weight change, hormonal shifts, pregnancy and nursing faster than the band typically does, so the letter can shift while the band stays the same.
They match up to a D cup, then diverge — a US DD is a UK E, and the gap widens slightly further up the scale.
Yes. Cup letter only encodes an inch difference, not shape, so breast shape and projection still vary widely at any single letter.
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