Bra Sizes Explained
Bra sizes look cryptic — 34C, 75B, E70 — but every label encodes exactly two facts: the band around your ribcage and the cup built from the difference between your bust and underbust. Once you understand how bra sizes work, you can read any label, in any country, in about a minute.
This guide unpacks bra sizes from first principles: what the number means, what the letter means, why the same letter is not the same volume, and how six sizing systems label one identical body six different ways.
Quick answer
Bra sizes combine a band and a cup. The band (the number) comes from your underbust measurement and carries most of the support. The cup (the letter) comes from the difference between your bust and underbust. Because both parts move together, sister sizes like 34C and 32D hold similar cup volume on different bands.
6
sizing systems (US, UK, EU, FR/ES, AU/NZ, JP)
460+
charted sizes with bust ranges
2
measurements needed (underbust + bust)
The anatomy of bra sizes: 34C
Take the label 34C. The 34 is the band — in inches, around the ribcage. The C is the cup — the space the bra provides for breast tissue. Neither part means anything alone: bra sizes are always a pair, which is why sizing advice that only ever mentions cup letters is incomplete.
In centimetre systems the same body wears 75C; in Japan the same size is written C75. Different notation, identical anatomy: a band dimension and a difference dimension. That is the entire grammar of bra sizes. Learn the pair and foreign bra sizes stop being intimidating.
What the band means
The band anchors the bra and carries roughly 80–90% of the support. It is derived from the snug underbust measurement: inch systems round to the nearest even number, so a 33-inch underbust usually meets a 34 band; centimetre systems round to the nearest five, so a 77 cm underbust meets an EU 75.
A firm band is the sign of a well-chosen size among bra sizes: it should sit level all day, feel snug on the middle hook, and stay put when you raise your arms. A band that rides up is doing no work — the straps are, and your shoulders will tell you about it by evening.
What the cup means
The cup letter encodes the difference between bust and underbust — roughly one letter per inch, or 2.5 cm, in most ladders. Step from B to C in the same band and you add about an inch of difference; the cup volume grows with it. The cup sizes guide covers the full ladder, and the cup size chart shows it per system.
Crucially, a cup letter is not an absolute breast size. A 32C and a 38C share a letter but not a volume: the 38C cup is materially larger, because the same difference over a bigger band is more tissue. This is the single most misunderstood fact about bra sizes.
Same cup letter, different volume
Sister sizes make this concrete. Go down one band and up one cup — 36B to 34C to 32D — and the cup volume stays almost constant while the band tightens. This is why bra sizes near each other are interchangeable in ways that are not obvious from the letters, and why a too-loose band is so often misread as a cup problem.
Practically: if the cups fit but the band is loose, take the tighter sister size. If the band is right but the cups run small, take the next cup up — not the looser band. The calculator exposes sister sizes for every system so you can see the trades instantly.
How six countries write the same size
Bra sizes are local dialects. The body that wears US 34C wears UK 34C, EU 75C, FR/ES 90C, AU/NZ 12C and JP C75. The band arithmetic differs — inches versus centimetres versus dress-size numbers — and the cup ladders differ beyond D, where the UK inserts double letters (DD, FF, GG) that continental systems never use.
So when a chart tells you a UK 32DD is an EU 70E, that is not an approximation; it is two alphabets describing the same difference. The bra size chart on this site lays all six dialects side by side with bust ranges, so you can verify each mapping yourself.
From understanding bra sizes to wearing the right one
Knowledge becomes a size through two measurements. Measure the underbust and bust as shown in the measuring guide, run them through the calculator, and you have your starting label in all six systems at once. Then confirm with the body: our fit guide lists the checks — level band, anchored gore, smooth cups — that turn a label into a fit.
How bra sizes change over time
Bra sizes are snapshots, not identities. Weight shifts of a few kilos, hormonal cycles, training regimes, pregnancy and breastfeeding all move the two measurements, and therefore the label. Most people drift at least a band or a cup across a decade — which is normal, expected, and exactly why bra sizes deserve a re-check twice a year.
Band elasticity ages too. A bra that fit when new may need the tightest hook after a year and still ride up: that is the bra wearing out, not your bra sizes changing. Refresh the wardrobe when the band stops doing its share, and re-measure whenever the fit checklist starts failing.
Frequently asked questions
How do bra sizes work?
Bra sizes pair a band with a cup: the band derives from your underbust measurement and the cup from the difference between bust and underbust. Change either measurement and the recommended size changes. That pairing is the whole grammar of bra sizes.
What comes first in bra sizes — band or cup?
In most markets the band comes first (34C), but Japan writes the cup first (C75). Both encode the same two dimensions. Notation order is the only difference between those bra sizes.
Are 34C and 36B the same?
They are sister sizes — nearly the same cup volume on different bands. Choose 36B for a looser band, 34C for a firmer one.
Why do bra sizes vary between brands?
Labels follow sizing conventions, but each brand cuts its own patterns. Two bras with the same bra size label can fit differently; your measurements are the stable reference. Bra sizes vary; your tape readings do not have to.
Is the band or the cup more important?
The band. It carries most of the support, so fit it first and adjust the cup within that band. A loose band makes even a perfect cup feel wrong. Fit the band first and bra sizes almost resolve themselves.
What is the biggest bra size?
Bands commonly run from 28 to 48 in inch systems and cups continue well past E — but any cup letter only means something together with its band. See our cup sizes guide for the full ladder.
Do bra sizes change with weight?
Yes — both measurements respond to weight, hormones and training, so bra sizes move with them. A five-kilo change is often a full band or cup step; re-measure rather than guessing. Small body shifts move bra sizes faster than people expect.
How many bra sizes should I own?
Your measurements define one starting size, but brand variance means a sister size often fits better in a specific label. Treat bra sizes as a small neighbourhood, not a single number. Shopping gets easier once bra sizes stop feeling like identity.
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 ·