A B cup means your bust measures about 2 inches more than your band — so a 34 band with a 36 bust is a B.
Most bra-fit confusion traces back to one wrong assumption: that a B cup is a fixed size. It isn’t. A B cup is a relationship between two measurements — the band under your bust and the fullest part of your bust — and a 2-inch gap between them is what makes it a B in US sizing. That’s why a 30B and a 38B fit completely different bodies even though both carry the same letter.
What Does a B Cup Actually Mean?
A B cup is the 2-inch difference between your band and bust measurements — not a fixed breast volume. Cup letters can’t be compared across band sizes.
- Cup size depends on the band. The letter describes a difference, so it only makes sense paired with a band number.
- Cup naming isn’t fully standardized across countries and brands. A US B, UK B, and EU B can sit on slightly different grids, so check the brand’s own chart.
For a ready-made starting point across bands, a roundup of tested B cup bras with sizing notes is worth a look before ordering.
How Do You Measure for a B Cup?
Measure the band snugly under your bust, measure the fullest part of your bust with the tape level, then subtract band from bust — a 2-inch result is a B.
- Measure the band. Wrap the tape snugly under your breasts, parallel to the floor. That number is your band size.
- Measure the bust. Run the tape around the fullest part of your bust, level and not pulled tight — a tight tape shrinks the number and throws the cup off.
- Subtract band from bust. A 2-inch difference is a B cup; 1 inch is an A; 3 inches is a C.
- Match to the brand’s chart. Sizing varies by brand and region, so take your two measurements to the specific chart for the bra you’re buying — Nike’s women’s bra fit guide and adidas’s women’s sports bra chart both use their own band-to-cup grids.
Some guides still instruct you to round the band to the nearest even number or add inches after measuring underbust. That’s an older convention and produces a different result than the modern snug-underbust method. Treat any single number as a starting point, not a verdict.
B Cup Size Chart and Common Mistakes
The chart below shows how a B cup scales across band sizes. The bust range shifts with every band — that’s the point.
| Band Size | B Cup Bust Range | Fits Best When |
|---|---|---|
| 65B | 79–81 cm | Your band and bust land in this range |
| 70B | 84–86 cm | Band sits snug, bust is ~2 in larger |
| 75B | 89–91 cm | You measure a 2 in band-to-bust gap |
| 80B | 94–96 cm | Wider band, same 2 in difference |
| 85B | 99–101 cm | Larger frame, B cup letter |
| US 34 band example | 36 in bust | 2 in difference equals B |
| US 30B vs 38B | Same letter, different volume | Never assume equal cup volume |
Where people go wrong:
- Measuring in a push-up bra, which inflates the bust number and bumps you up a cup.
- Pulling the tape too tight around the bust, which shrinks the result.
- Assuming 30B and 38B hold the same volume — they don’t.
- Mixing US, UK, EU, and Japan conversions without a chart.
A poor fit creates pressure points and gives inaccurate support, which is why brand guides recommend rechecking your size after any body change.
Trust the 2-inch rule and verify against the brand chart you’re buying from. For a clear reference point, this NHS bra measurement guide walks through the band-and-bust method with worked examples.
Your B Cup Starting Point
Measure your band snug under the bust, measure your bust at its fullest, subtract the two — a 2-inch difference is your B cup. Take both numbers to the brand’s chart before buying, because the letter alone tells you nothing without the band beside it.
FAQs
Is a B cup considered small or average?
Neither label is meaningful alone, because cup size only describes the 2-inch difference between band and bust. A 32B and a 40B look completely different on a body. Comfort, coverage, and support tell you whether a bra is right, not whether the cup is “small.”
Why does a B cup fit differently across brands?
Cup naming isn’t fully standardized across countries and brands, so the same B can sit on slightly different measurement grids. Match your two measurements to the specific brand’s own chart rather than assuming your usual size carries over.
Can I measure for a B cup without a bra on?
Yes, and it’s usually more accurate. Measuring with no bra, or a thin unpadded one, keeps the bust measurement honest. A push-up or padded bra inflates the number and can bump you up a cup. Keep the tape level and snug, never tight.
References & Sources
- Nike. “Women’s Bras Size & Fit Guide.” Brand band-to-cup sizing grid for women’s bras.
- adidas. “Women’s Sports Bras Size Chart.” Separate sports bra chart with B-cup entries tied to band and apparel size.