Compare Bra Sizes
Drag the sliders to set two sizes. Both are drawn to the same scale from the side, with what each cup holds - because the letters on their own will mislead you.
The difference
A 34D cup holds about 69% more than a 34B.
Same band, two cup letters apart: the 34D stands further out and sits wider on the chest.
| Measure | 34B | 34D |
|---|---|---|
| Cup volume, each | 405 ml | 685 ml |
| Weight, each | 250-519 g | 423-879 g |
| Stands out from the chest | 1.5" | 2.1" |
| Width across the chest | 6.1" | 6.6" |
| Ribcage the band fits | 34" | 34" |
Volumes and weights are estimates from the site's breast volume model; the range on the weight is the honest uncertainty, not rounding.
Why the letter is not the size
Most confusion about bra sizes comes from reading the cup letter as an amount. It is a difference: a D is four inches between your bust and your band. Four inches on a small ribcage and four inches on a large one are very different amounts of breast.
So a cup letter cannot be compared without its band. A 30D cup holds about 405 ml on this site's model. A 38D holds about 965 ml. Same letter, and not remotely the same size.
Same band, different cup
This is the comparison that works the way people expect. On one band, each letter up is one more inch of difference and a larger cup: a 34C holds about 545 ml and a 34D about 685 ml, around 26% more.
On this model each letter adds the same amount - about 140 ml - so one letter is a much bigger share of a small cup than of a large one. On a 34 band, going from a B to a C is about 35% more; one letter up from a G is about 13%. There is more on one specific step in B cup vs C cup.
Same letter, different band
Keep the letter and change the band, and the cup changes with it. A 32D (545 ml) is a smaller cup than a 34D (685 ml), which is smaller again than a 38D (965 ml). Anyone who has gone up a band, kept their letter, and found the cups suddenly too big has met this.
Sizes that are secretly equal
Put the two together and you get the result that surprises people. Go up a band and down a letter and the cup stays the same: a 32D and a 34C both come out at 545 ml. They are sister sizes.
What changes between them is the band - the 32D holds that cup on a ribcage two inches smaller. So "which is bigger?" has two answers: the cups are equal, and the frames are not. Set the tool to any sister pair: the volumes are identical and the two outlines land almost exactly on top of each other - the larger frame carries the same cup a touch wider.
Related
- Bra size calculator - find your own size from two measurements.
- Sister size calculator - every size equal to yours.
- Bra size converter - the same size in other countries.
- Breast weight calculator - the model behind the volumes here.
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