18C Bra Size (AU)
Everything about the 18C bra size in Australia & New Zealand sizing: what it means, its measurements, whether it runs big or small, sister sizes, and conversions to US, UK, EU & FR.
What is a 18C bra size?
A 18C in Australia & New Zealand sizing is a bra with a 18 band and a C cup. “18” is the Australian band, a dress-size-style number: US band − 22 = 18. “C” is the cup, from a 3-inch difference between the bust and the band (one inch per cup letter). The same body is written as US 40C, UK 40C, EU 90C, FR 105C, IT 6C.
Someone who measures roughly 39 inches (99 cm) around the ribcage and about 43 inches (109 cm) around the fullest part of the bust lands at 18C using the modern snug-measure method. The band carries most of the support; the cup describes the breast volume the bra needs to hold: measured relative to that band.
“18” is the Australian band, a dress-size-style number: US band − 22 = 18. It sets how the bra fits around the ribcage and provides about 80% of the support.
C is a moderate C-cup: a 3-inch bust-minus-band difference. It reflects breast volume, always relative to the 18 band.
Band and cup are one size. A 18C is not the same volume as a 14C: the cup letter only has meaning next to its band.
18C measurements
A 18C comes from two measurements: a snug underbust that gives the 18 band, and a looser bust. The cup is the gap between the two: for C, that gap is 3 inches.
| Measurement | Range | Sets |
|---|---|---|
| Underbust (snug) | 38.5–40.4″ · 98–103 cm | Band 18 |
| Bust (fullest) | 42.5–43.4″ · 108–110 cm | — |
| Difference | 3″ · ≈ 8 cm | Cup C |
Underbust rounds to the nearest even band; an odd reading (e.g. 39″) can go either way, so try 16D and 18C and let the band decide.
Measurements vary by body shape, so treat these as the center of a range. AU band labels are converted from the US band by convention rather than read directly in centimeters, which is why a converted size can feel a step different from a bra drafted natively in this system.
Is 18C big or small?
18C is a mid-range cup on a fuller band: a commonly stocked size. But the honest answer is that a cup letter is not a fixed amount of volume: it grows with the band. Tap a size below to see how.
All five are C cups, yet the volume grows with the band. 18C is the highlighted size.
Because the cup is measured relative to the band, a 18C holds more than a 14C and less than a 22C, even though all three are “C cups.” So “big” or “small” only has meaning once you know the band.
If 18C cups fit well but the band feels off, you don't need a different cup: you need a sister size. And if you're unsure 18C is your starting size, the calculator settles it from two measurements.
Were you measured as 18C correctly?
Most people wearing the wrong size were sized by the old “+4 rule”: adding four inches to the underbust to get the band. It inflates the band and, because the cup is the difference from that inflated band, understates the cup by about two letters.
Same body, same volume: the old rule pushes it into a looser band and a smaller cup.
The tell-tale signs you were sized with the old method: the 18 band rides up at the back, you fasten it on the tightest hook from day one, and the straps do most of the work. If that sounds familiar, 14DD is the size to try first: it holds the same volume on a band that actually supports.
Key takeaway: if a calculator adds inches to your underbust, its 18C is not the same 18C as ours. Brasize.ai uses the modern snug-measure standard used by professional fitters.
Which bra styles fit a 18C best?
Mid-depth cups have the widest style range: t-shirt, balconette, plunge and demi all work. Molded cups give a smooth line; seamed cups give better shape. If you gape at the top in a full-coverage style, you're likely fuller at the bottom: a balconette will fit better. On a 40 band, prioritise firm, wide band elastic and a taller back; a soft, narrow band will roll and ride up regardless of the label. Longline and three-hook styles help.
Any shape works; choose seamed for shape, molded for a smooth line.
Look for a three-hook band with firm, wide elastic: the band does the lifting.
Standard wires; make sure the wire sits on the ribcage at the side, not on tissue.
Sister sizes share cup volume but not cup shape: the wire on a smaller band is narrower and more projected, the wire on a larger band is wider and shallower. If you have a narrow breast root, the snugger sister will often fit better; if your root is wide, the looser sister may: even when the band tension of the original was right.
18C sister sizes
Sister sizes hold the same cup volume as 18C on a different band. They're the fix when the cups fit but the band is too loose or too tight.
| Size | Band feel vs 18C | Cup volume | When to try it |
|---|---|---|---|
| 14DD | Two bands snugger | Same as 18C | Only if 16D is still loose; wire will be noticeably narrower |
| 16D | One band snugger | Same as 18C | 18 band rides up or feels loose |
| 18C | Your starting size | — | Band sits firm and level |
| 20B | One band looser | Same as 18C | 18 band digs in or feels tight |
| 22A | Two bands looser | Same as 18C | Rarely right: the band gives up too much support |
The rule: go down a band and up a cup (18C → 16D) for the same cups on a tighter band, or up a band and down a cup (18C → 20B) for the same cups on a looser band. Sister sizes share cup volume but not cup shape: the wire on a smaller band is narrower and more projected, the wire on a larger band is wider and shallower. If you have a narrow breast root, the snugger sister will often fit better; if your root is wide, the looser sister may: even when the band tension of the original was right.
Signs 18C is too small, or too big
Fix: go up a cup on the same band → 18D. If the band is also loose, go to 16D.
Fix: gaping cups → down a cup, 18B. Band riding up with cups that fit → 16D.
18C in US, UK, EU & FR sizing
The same 18C body measurements are written differently around the world. Bands and cup letters diverge by region, so convert deliberately when buying from an international brand.
| Region | Size | How the band is derived |
|---|---|---|
| United States (US) | 40C | Band in inches |
| United Kingdom (UK) | 40C | Same band as US; cups diverge above DD |
| Europe (EU) | 90C | US band × 2.5 − 10 = 90 |
| France / Spain (FR) | 105C | EU band + 15 = 105 |
| Australia / NZ (AU) | 18C | US band − 22 = 18 |
| Italy (IT) | 6C | (EU − 60) ÷ 5 = 6 |
| Japan (JP) | 90C | EU band; cups close to EU |
| Germany (DE) | 90C | EU sizing |
Cup letters agree across systems up to D. Above D they diverge (a US DDD is a UK E and an EU F) which is where the size converter earns its keep. Brands that size natively in AU: Berlei, Bendon, Bras N Things, Triumph AU.
How to tell if 18C is your correct size
A calculated 18C is an accurate starting point. Confirm it on the body: band, cups, gore, straps.
- Band sits firm and level. The 18 band should be snug around your ribcage with room for two fingers, not riding up at the back. It gives most of the support.
- Cups fully contain the breast. No spillage over the top or sides, and no gaping or wrinkling in the C cups. Gaping usually means down a cup; spillage means up a cup.
- Center gore lies flat. The panel between the cups should sit against your sternum, not float away.
- Straps stay put. If you're tightening straps to feel supported, the 18 band is probably too big: try 16D.
If the cups fit but the band is wrong, move to a sister size rather than changing cup letters. There's no universal standard between brands: a 18C in one label can fit like the size on either side in another, so use the fit checks, not just the label. Full symptom-by-symptom guide: bra fit guide.
Educational resource, not medical advice. Read our editorial policy.
HauteFlair labels cups in UK letters, so these links already convert your size by position (a US DDD lands on a UK E, a US G on a UK F). HauteFlair is owned by the same company as Brasize.ai; that relationship never changes the sizing math, charts, or fit advice on this page. How this site makes money.
Is 18C actually your size?
Don't guess from a label. Enter two measurements and get your accurate size in eight systems, plus your sister sizes.
Calculate My Bra Size →18C bra size: answered
The questions people ask most about the 18C size: its volume, sister sizes, conversions and fit.
Is 18C a big bra size?
18C is a moderate C-cup on a 18 band. A cup letter is a ratio, not a fixed volume: C means a 3-inch difference between bust and band, so a 18C holds more than a 14C and less than a 22C. “Big” only has meaning once you know the band.
What is the sister size of 18C?
The sister sizes of 18C are 16D (snugger band) and 20B (looser band). Sister sizes hold the same cup volume as 18C on a different band. Move to one when the C cups fit but the 18 band is too loose or too tight.
What is 18C in European sizing?
A AU 18C is an EU 90C. EU bands follow US × 2.5 − 10; cup letters match up to D, then EU continues the alphabet (E, F, G) where the US repeats D.
What is 18C in UK sizing?
A AU 18C is a UK 40C. US and UK share inch bands and agree on cups through DD; above that US DDD = UK E, US G = UK F, US H = UK FF.
What measurements are a 18C?
Roughly a 39-inch (99 cm) snug underbust and a 43-inch (109 cm) bust at the fullest point. The 3-inch difference gives the C cup; the underbust gives the 18 band. Treat these as the center of a range.
What is 18C in centimeters?
A 18C corresponds to an underbust of about 98–103 cm and a bust of about 108–110 cm: a difference of roughly 8 cm. In metric-labelled systems the same size is EU 90C, FR 105C and JP 90C.
Which bra styles suit a 18C?
Mid-depth cups have the widest style range: t-shirt, balconette, plunge and demi all work. Molded cups give a smooth line; seamed cups give better shape. If you gape at the top in a full-coverage style, you're likely fuller at the bottom: a balconette will fit better. On a 40 band, prioritise firm, wide band elastic and a taller back; a soft, narrow band will roll and ride up regardless of the label. Longline and three-hook styles help.
How do I know if 18C is my correct size?
Check four things: the 18 band sits level and snug on the loosest hook, the C cups are filled without spillage or gaping, the center gore lies flat against the sternum, and the straps stay put without carrying the weight. If the band is wrong but the cups fit, try a sister size such as 16D.
Was I sized as 18C using the old +4 method?
If your 18 band rides up, fastens on the tightest hook from new, and the straps carry the weight, probably yes. The +4 rule adds four inches to the underbust; the same body measured snugly is closer to 14DD, same cup volume, a band that supports.