B Cup Size
A B cup size means your bust measures about two inches — five centimetres — more than your snug underbust. It is the second letter of the ladder and, paired with the right band, one of the most common starting points in fitted reality.
This page covers the B cup size completely: how to confirm it with a tape, how 30B through 38B differ despite sharing the letter, which sister sizes to reach for, and how the B cup is written in six international systems.
Quick answer
A B cup size reflects a bust roughly 2 inches (5 cm) larger than the underbust — for example a 34 in underbust with a 36 in bust gives 34B in US/UK sizing, 75B in EU sizing and B75 in Japan. One more inch of difference moves you to a C cup.
6
sizing systems (US, UK, EU, FR/ES, AU/NZ, JP)
460+
charted sizes with bust ranges
2
measurements needed (underbust + bust)
Measuring your B cup size
Confirming a B cup size takes two level tape readings: the underbust snug against the ribcage just below the bust, and the bust at the fullest point without compression. A difference near two inches is the B signature; near three, you have stepped up to C. Because the gap between letters is only an inch, measure twice and average — small technique errors flip letters at the low end of the ladder.
Run both readings through the calculator below to see your full size — band plus B cup — across all six systems at once. If the result hovers between A and B or B and C, the fit checklist, not the arithmetic, should pick the winner.
B cup size on every band
The panel below pairs a B cup with each common band in your selected system, listing the underbust behind the band and the bust range the finished size is built for. A 32B fits a bust near 34 in; a 36B, a bust near 38 in. The B cup size is constant as a difference, but the volumes differ band to band.
At B volume, push-up and padded styles change the silhouette without changing the B cup size itself — the label describes your measurements, not the bra's interior architecture.
Difference range for this cup
1.5–2.5 in
| Size (band + cup) | US | Bust range for this size |
|---|---|---|
| 30B | 29.5–30.5 in | 31.5–32.5 in |
| 32B | 31.5–32.5 in | 33.5–34.5 in |
| 34B | 33.5–34.5 in | 35.5–36.5 in |
| 36B | 35.5–36.5 in | 37.5–38.5 in |
| 38B | 37.5–38.5 in | 39.5–40.5 in |
| 40B | 39.5–40.5 in | 41.5–42.5 in |
Sister sizes — 34B
tighter band
32C
34B
looser band
36A
A cup letter is relative: the same letter over a different band is not the same volume. Brand cuts vary.
B cup sister sizes
The B cup size follows the standard sister mechanics. In US/UK sizing, 34B trades to 32C (tighter band, same volume) or 36A (looser band); in EU labels, 75B trades to 70C and 80A. The panel above lists sister sizes for whichever system you select, always from that system's own ladder.
Sisters matter at B because band tightness dominates comfort at low cup volumes: if your B cups feel fine but the band rides up, take the tighter sister instead of a smaller cup. The classic trap is the reverse — dropping to an A for "less bra" when the real issue was a loose band all along.
The B cup across six systems
The B cup size translates cleanly across borders: US 34B, UK 34B, EU 75B, FR/ES 90B, AU/NZ 12B and JP B75 all encode the same two-inch difference on the same body. Below D the ladders agree letter for letter, so the only things to watch are the band notation — inches, centimetres, dress sizes — and Japan's cup-first order.
Our worked example on the 34B page unpacks one of these labels in full: measurement ranges, all six equivalents and sister sizes derived from the same engine that powers this page.
How a B cup should fit
A correctly fitted B cup size shows cups that lie smooth against the breast with no gaping at the top edge and no spill at the sides, a gore resting on the sternum, and a band that stays level through the day. Gaping is the B cup's signature complaint — often solved by tightening straps, sizing the band down, or choosing a shallower cup style rather than abandoning the letter. A settled band and honest straps keep a B cup size smooth.
The full checklist lives in the bra fit guide. Measure first, fit second, and let the body confirm what the tape proposed.
B cup size: quick reference
The B cup size in one line each. The difference: about two inches, or five centimetres, between bust and underbust. The band pairings: 30B to 38B in inch systems, 65B to 85B in centimetre systems, B65 to B85 in Japan. The sisters: one letter up on a tighter band, one down on a looser band. The signature issue: gaping, usually fixed by straps, band or cup shape before any letter change.
That is the whole B cup size briefing. Measure both readings, confirm the two-inch difference, and let the calculator print the six-system labels — the rest of this site exists to verify what those numbers already know. Confirm the B cup size twice a year and shop with confidence.
Frequently asked questions
What is a B cup size?
A bust measurement about 2 inches, or 5 cm, larger than the underbust. The B cup size becomes a full size when paired with a band — 34B, 75B or B75. A B cup size is a two-inch difference on any of them.
Is a B cup small?
A B cup size is one step below the statistical middle. Volume still depends on the band: a 36B holds more than a 32B, so the letter alone does not say "small". The B cup size depends on its band for meaning.
What is the difference between A and B cup?
One inch of bust−underbust difference — A is near 1 in, the B cup size near 2 in. If your measurements sit between, try both and let the fit decide. Between letters, the B cup size boundary is the trickiest.
What are the sister sizes of 34B?
32C and 36A in US/UK sizing — the same approximate volume on a tighter or looser band. EU 75B trades to 70C and 80A.
Why does my B cup gap at the top?
Usually shape or strap tension, not necessarily a wrong B cup size: tighten the straps, check the band is snug, or try a shallower cup style before sizing down. Most gaping fixes keep the B cup size intact.
What does 34B mean?
A 34 band — underbust near 34 in (86 cm) — with a B cup, so a bust near 36 in (91 cm). The 34B page unpacks this label in full. Band plus B cup size is the whole decode.
Sources & standards
Every size on this site is cross-checked against these published standards and chart families before it ships.
- 2017
ISO 8559-1:2017 — Size designation of clothes — Part 1: Anthropometric definitions for body measurement
Defines the body measurement points (chest/bust girth, underbust girth) this site measures.
- 2017
EN 13402 (label family) — Size designation of clothes — European size labelling
Basis of the continental EU band ladder (65, 70, 75 …) in 5 cm steps.
- 2023
JIS L 4006:2023 — Sizing systems for foundation garments
Japanese foundation-garment sizing; the label family behind JP cup-first sizes such as C75.
- 2019
KS K 9404 — Sizing systems for foundation garments (Korea)
Korean foundation-garment sizing; referenced while the KR system awaits full verification.
- 2025
Multi-source retail consensus — US, UK, FR/ES and AU/NZ bra size charts (cross-brand average)
Inch-based band rounding, UK double-letter cup ladder, FR/ES +15 offset and AU/NZ dress-size bands.
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