Cup size is one of the most misunderstood parts of bra fitting. Most women know their size as a letter — A, B, C — but have no idea what that letter actually measures. Here's everything you need to know, explained simply.
What Does Cup Size Actually Measure?
Your cup size is NOT a fixed measurement. It represents the difference in inches between your bust measurement and your band measurement. The bigger the difference, the larger the cup letter.
- 1 inch difference = A cup
- 2 inch difference = B cup
- 3 inch difference = C cup
- 4 inch difference = D cup
- 5 inch difference = DD / E cup
- 6 inch difference = F cup
- 7 inch difference = G cup
Cup Size A — What It Means
A cups have the smallest volume difference (1 inch). Often described as "small" or "petite," but this says nothing about how a woman looks — it's purely about proportional fit. A-cup women need bras with light structure, minimal padding bulk, and thin cups that don't gap.
Best Lumisensy pick for A cups: Lumisensy® Breathable Bra Seamless Comfort — $37.99 — seamless thin cups that sit flush with no gaping.
Cup Size B — What It Means
B cups are the most commonly purchased size in the US. A 2-inch difference means a moderate volume. B-cup women generally have the most options available, but often still struggle with bras that are too padded or too structured for their natural shape.
Best Lumisensy pick for B cups: Lumisensy® Seamless Wireless Comfort Bra — $37.99 — natural shape, no padding, adjustable straps.
Cup Size C — What It Means
C cups are considered a "medium" volume. Women in this size often need more structure in the cup to prevent spillage at the top or sides. A well-fitted C cup bra should have no overflow and lay completely flat against the chest at the center gore.
Best Lumisensy pick for C cups: Lumisensy® Cloudbound Wireless Jelly Bra — $35.99 — jelly structure holds C cups smoothly without underwire.
Cup Size D — What It Means
D cups are often thought of as "large" but they're actually very common — and very misunderstood. A 32D has far less volume than a 38D, even though both are "D cups." Women in D cups need bras with stronger bands and more cup depth to avoid the dreaded "quad boob" effect.
Best Lumisensy pick for D cups: Lumisensy® Classic Seamless Wireless Jelly Support Bra — $38.99 — deeper cups with jelly support for fuller D shapes.
Cup Size DD / E — What It Means
DD and E are the same size — just named differently by different brands. At a 5-inch difference, this cup needs significant structure and a wide, firm band. Many women in this range wear the wrong size because they don't realize DD is not "extra large" — it's simply one step above D.
Best Lumisensy pick for DD/E cups: Lumisensy® Seamless Cloud Cotton Anti-Sagging Bra — $36.99 — anti-sagging technology designed for fuller cup volumes.
Cup Size F — What It Means
F cups require full coverage, strong side panels to prevent side spillage, and a band that does most of the lifting. Many women in F cups are actually wearing a DD and wondering why their bra never fits — sister sizing is key at this range.
Best Lumisensy pick for F cups: Lumisensy® Plus Size Seamless Lumi Bra — $39.99 — full coverage, wide band, designed for larger cup volumes.
Cup Size G — What It Means
G cups are a 7-inch difference and represent a very full bust. At this size, support comes almost entirely from the band — the straps are secondary. Women in G cups need bras with a very firm band, deep cups, and wide straps to distribute weight comfortably all day.
Best Lumisensy pick for G cups: Lumisensy® Plus Size Seamless Lumi Bra — $39.99 — maximum support, seamless finish, built for larger busts.
The Most Important Thing About Cup Sizes: Sister Sizing
Cup size only makes sense in relation to the band size. A 34C and a 36B have almost identical cup volume — they're "sister sizes." If a bra fits in the cup but the band is too tight, go up one band size and down one cup size. If the band is too loose, go down one band and up one cup.
| If you wear | Sister size down | Sister size up |
|---|---|---|
| 34C | 32D | 36B |
| 36D | 34DD | 38C |
| 38DD | 36F | 40D |
Quick Cup Size Reference Chart
| Cup Letter | Inch Difference | Description |
|---|---|---|
| A | 1 inch | Petite / small volume |
| B | 2 inches | Small-medium volume |
| C | 3 inches | Medium volume |
| D | 4 inches | Medium-full volume |
| DD / E | 5 inches | Full volume |
| F | 6 inches | Very full volume |
| G | 7 inches | Extra full volume |
Not sure what size you are? Every Lumisensy bra is designed to fit a wide range of cup volumes with seamless, wireless technology that adapts to your shape — not the other way around.
