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| From top to bottom these bras have the same cup volume: 38E, 36F, 34FF, 32G, 30GG, 28H |
The relationship between band sizes and cup sizes is something I explain most frequently. The common misconception is that all cup letters are the same size of boob on different band lengths. So, for example, people assume that all women with F cup boobs have the same size boobs regardless of whether they're a 30F or a 40F and this simply isn't true. A lady with 40F boobs is actually five cup volumes bigger than the 30F lady and that's because when the band size gets bigger it increases the volume of the cups.
Sister Sizes is a phrase that refers to bras that all share the same cup volume but have different band lengths. For example, a 34E and a 32F are sister sizes because when you go one band size down from a 34 to a 32 you have to balance the size by going up one cup from an E to an F cup to keep the cup volume the same.
Click Here to read why all D cups aren't the same and Click Here to read why cup volumes increase as the band increases.
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| These bras all have the same cup volume spread out over different length bands. From top down: 28H, 30GG, 32G, 34FF, 36F, 38E |
Because a picture speaks a thousand words I wanted to show you what sister sizes look like and how the proportions change. In the picture above the six bras shown are the following sizes from top to bottom: 28H, 30GG, 32G, 34FF, 36F, 38E. All of these bras have the same cup volume, however, their band lengths and proportions are different.
You can see that the band on the 38E at the bottom is the longest band so the woman who wears this bra needs her band to go around more torso than the woman who would wear the 28H bra at the top which has the shortest band. Proportionally the 38E lady will look like she has a smaller bust because her bust volume is spread out over a wider torso. The 28H lady will look like she has a larger bust because her breast volume projects further forward on a narrower torso. In reality they have the same volume of boob but it is spread out over different areas of chest/torso.
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| From top to bottom these bras have the same cup volume: 38E, 36F, 34FF, 32G, 30GG, 28H |
The Proportions
While the picture above is helpful to show that the cup volume on these sister sizes is the same what it can't show very well is how the proportions differ. You can see that the bands on the 30GG and 28H (bottom two bras) are deeper and have more hooks than the other sizes. This is because the volume of a 30GG and 28H bust projects farther forward than the volume of a 38E bust (which is spread out over a wider area so doesn't project as far forward) so the band has to be stronger to counteract the projection at the front.
Besides the deeper band and more hooks you may also be able to see that the wings are darker on the 28 and 30 bands than the 38-32 bands and this is because it's double layered to give more support. The wing has to be reinforced as there is less length to disperse the weight of the breasts over. The longer the band the greater the area the weight of the breasts can be dispersed over. This is the reason longer bands don't need to be as deep or firm as the shorter bands.
The proportions of the bras differ in the following ways (you'll notice that the gore width decreases as the band length shortens and this is because the breast volume is gradually narrowing in width and increasing in projection).
Band Length | Wing with Hooks Length | Wing with Eyes Length | Wing Depth | Gore Width | |
38E | 29” | 6.25” | 8.25” | 4.5” | 1” |
36F | 28” | 6” | 8” | 4.5” | 1” |
34FF | 26.5” | 5” | 7” | 4.5” | 3/4” |
32G | 24.5” | 4” | 6.25” | 4.75” | 3/4” |
30GG | 23.75” | 4” | 6” | 4.75” | 5/8” |
28H | 22.5” | 3.5” | 5.25” | 5.25” | 5/8” |
Bra Science Vs Bra Reality
I wanted to explain this bit of bra science to further help you understand why cup sizes mean nothing without knowing the band size. In reality good bra fit is affected by things like brand, body shape, age, height and bra style. It's great to know the principles of bra science but remember that when a bra fits you well and you are comfortable that's the most important thing. Leave a comment if you need something explaining more xx
N.B: The bra used in these diagrams is Tango Beige from Panache Superbra