Cup Sizes Guide
Cup sizes are the most visible and least understood half of a bra size. The letters look absolute — an A sounds small, an E sounds large — but a cup letter only describes how much your bust exceeds your underbust, within one sizing system's ladder. Change the band and the letter changes meaning.
This guide covers how cup sizes work, how the ladder runs from A to E and beyond, why the same letter is not the same volume, and how to find yours in any of six international systems.
Quick answer
Cup sizes measure the difference between your bust and underbust, not breast size alone. Each letter step equals roughly one inch (2.5 cm) more difference. Because the letter is relative to the band, a 32D and a 38D hold different volumes — and sister sizes trade band for cup letter at constant volume.
6
sizing systems (US, UK, EU, FR/ES, AU/NZ, JP)
460+
charted sizes with bust ranges
2
measurements needed (underbust + bust)
What cup sizes actually measure
A cup letter is a difference: bust minus underbust, snapped to a ladder. If your bust is 3 inches larger than your underbust, you are a C cup in any mainstream system. Cup sizes therefore grow when the bust grows, when the ribcage shrinks, or both — three different body stories, one letter change. Three stories, and the cup sizes ladder treats them identically.
This relativity is the point. Cup sizes exist so that a single pattern system can dress many proportions: the letter tunes the volume of the cup to the band beneath it. Treating letters as absolute breast sizes leads directly to the classic error — buying a bigger cup when the real problem is a looser band.
The cup sizes ladder, A to E
Reading up the ladder, each step adds about an inch of difference: A ≈ 1 in, B ≈ 2 in, C ≈ 3 in, D ≈ 4 in and E ≈ 5 in, with AA below A for differences under an inch. Continental systems (EU, FR/ES, JP) use these single letters straight through; UK inserts double letters after D — DD, then E, then F, FF — while US prefers DD and DDD. Same body, three dialects — cup sizes translate, volumes do not.
The practical consequence: when a UK brand says E it means one step beyond UK DD, but when a French brand says E it means the same volume as the UK DD. The cup size chart shows both ladders side by side so you never have to hold this in your head.
Why a 32D and a 38D are not the same
Volume grows with the band even at a constant letter. A 38D holds noticeably more tissue than a 32D: four inches of difference over a 38-inch ribcage is more breast than four inches over a 32-inch ribcage. So cup sizes are scale-dependent — the letter is a proportion, not an amount. Scale is why cup sizes never travel without their band.
The mirror rule also holds: the same breast on a smaller ribcage gets a larger letter. Lose band, gain letter — that is the sister-size mechanic, and it is why fitters say the cup letter of your bra sizes is meaningless without its band. Our cup size chart makes the relationship visible per band.
Sister sizes and the cup ladder
Sister sizes move one step along the band for one step against the ladder: 36B → 34C → 32D all hold similar cup volume. Each system performs this dance inside its own sequences — UK double letters stay in the UK, single letters stay continental. The calculator lists your sister sizes per system automatically.
Use them tactically: bands feel different across brands, and the cup that fits in one label may need a sister in another. Cup sizes give you a neighbourhood of options, not a single address. Cup sizes give you the neighbourhood; fit picks the door.
Finding your cup size
Measure the bust at its fullest and the underbust at its root, keep the tape level, and subtract. Feed both numbers into the calculator for a full size — band plus cup — in six systems, or read the difference directly on the cup size chart. If the difference sits near a letter boundary, try both letters and let the fit decide.
For the measurement technique itself, the cup bra measurement page is the focused tutorial, and the measuring guide covers the general two-tape method.
The A–E cup pages
Each letter has its own practical profile — wire width, projection, gaping risk — so we keep a dedicated page per letter with measurement ranges, band combinations and sister sizes for every system:
- A cup — differences near one inch; gaping is the usual challenge.
- B cup — differences near two inches; the most common starting letter.
- C cup — differences near three inches; the reference letter most charts build around.
- D cup — differences near four inches; wire width starts to matter.
- E cup — differences near five inches; UK DD's continental twin.
Cup sizes in the real world
Surveys that measure rather than ask consistently find fitted cup sizes skew larger than self-reported ones — most people who believe they are a B measure into a C or D. The gap comes from band-driven letter inflation: a correctly snug band usually produces a larger-looking letter than the loose bands people default to. Cup sizes did not change; the arithmetic finally got honest input. Honest inputs are why fitted cup sizes look larger than self-reported ones.
The practical takeaway: treat cup sizes as a system output, not an identity. If a fitted letter surprises you, check the band first — the letter is probably just reporting a difference that was always there. Re-measure both readings before blaming the cup sizes themselves; nine times out of ten the band moved first.
Frequently asked questions
What are cup sizes measured in?
In the difference between bust and underbust — inches in US/UK sizing, centimetres elsewhere. Cup sizes are relationships, so the same letter means different volumes on different bands. That relativity is the first lesson of cup sizes.
Which cup sizes are bigger, DD or E?
In UK sizing DD is smaller than E. In EU, FR/ES and JP sizing there is no DD — E is the first step after D, equal in volume to the UK DD.
Can cup sizes change without my breasts changing?
Yes. Lose underbust and the difference grows: the same breasts take a larger letter on a smaller band. This is the sister-size effect. Cup sizes answer to arithmetic, not to adjectives.
What is the order of cup sizes?
AA, A, B, C, D, then it splits: UK continues DD, E, F, FF; US continues DD, DDD; continental systems continue E, F, G with single letters only. Memorise the steps and cup sizes stop surprising you.
Do cup sizes run the same in sports bras?
Many sports brands compress or simplify the ladder. Check the brand's own chart, but the body measurements — bust and underbust — stay the same regardless of bra type.
How do I know if my cup size is wrong?
Spilling over the edge means try a larger cup; gaping or wrinkling means try a smaller one — but check strap tension and band tightness first, because both can fake a cup problem. Most complaints about cup sizes are band problems in disguise.
What is the most common cup size?
Fitted surveys usually land around C to D in cup sizes, higher than self-reports — but averages vary by study, country and band. Your own measurements matter more than any average. Averages describe populations; cup sizes describe one difference.
Do cup sizes mean anything without a band?
No — a letter alone is a difference without a baseline. Cup sizes only become a volume when the band fixes the scale, which is why size labels always pair the two. Band plus difference is what turns cup sizes into volumes.
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.
By Bra Size Calculator ·