40G Bra Size (UK)
Everything about the 40G bra size in United Kingdom sizing: what it means, its measurements, whether it runs big or small, sister sizes, and conversions to US, EU, FR & AU.
What is a 40G bra size?
A 40G in United Kingdom sizing is a bra with a 40 band and a G cup. “40” is the band in inches. UK and US bands are identical. “G” is the cup, from a 9-inch difference between the bust and the band (one inch per cup letter). The same body is written as US 40I, EU 90I, FR 105I, AU 18G, IT 6I.
Someone who measures roughly 39 inches (99 cm) around the ribcage and about 49 inches (124 cm) around the fullest part of the bust lands at 40G 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.
“40” is the band in inches. UK and US bands are identical. It sets how the bra fits around the ribcage and provides about 80% of the support.
G is a full-bust cup: a 9-inch bust-minus-band difference. It reflects breast volume, always relative to the 40 band.
Band and cup are one size. A 40G is not the same volume as a 36G: the cup letter only has meaning next to its band.
40G measurements
A 40G comes from two measurements: a snug underbust that gives the 40 band, and a looser bust. The cup is the gap between the two: for G, that gap is 9 inches.
| Measurement | Range | Sets |
|---|---|---|
| Underbust (snug) | 38.5–40.4″ · 98–103 cm | Band 40 |
| Bust (fullest) | 48.5–49.4″ · 123–125 cm | — |
| Difference | 9″ · ≈ 23 cm | Cup G |
Underbust rounds to the nearest even band; an odd reading (e.g. 39″) can go either way, so try 38GG and 40G and let the band decide.
Measurements vary by body shape, so treat these as the center of a range. UK 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 40G big or small?
40G is a full-bust cup on a fuller band: carried by full-bust specialists more than mainstream ranges. 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 G cups, yet the volume grows with the band. 40G is the highlighted size.
Because the cup is measured relative to the band, a 40G holds more than a 36G and less than a 44G, even though all three are “G cups.” So “big” or “small” only has meaning once you know the band.
If 40G 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 40G is your starting size, the calculator settles it from two measurements.
Were you measured as 40G 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 40 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, 36H 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 40G is not the same 40G as ours. Brasize.ai uses the modern snug-measure standard used by professional fitters.
Which bra styles fit a 40G best?
At this depth the bra is doing real load-bearing work. Prioritise full-cup and side-support constructions from full-bust specialists, firm (often three- or four-hook) bands, and wider straps. Molded single-piece cups rarely fit well here; seamed cups distribute weight and shape 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.
Full-cup and side-support constructions from full-bust specialists.
Look for a three-hook band with firm, wide elastic: the band does the lifting.
Wider, cushioned straps and a wire that fully encloses the breast root.
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.
40G sister sizes
Sister sizes hold the same cup volume as 40G 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 40G | Cup volume | When to try it |
|---|---|---|---|
| 36H | Two bands snugger | Same as 40G | Only if 38GG is still loose; wire will be noticeably narrower |
| 38GG | One band snugger | Same as 40G | 40 band rides up or feels loose |
| 40G | Your starting size | — | Band sits firm and level |
| 42FF | One band looser | Same as 40G | 40 band digs in or feels tight |
| 44F | Two bands looser | Same as 40G | Rarely right: the band gives up too much support |
The rule: go down a band and up a cup (40G → 38GG) for the same cups on a tighter band, or up a band and down a cup (40G → 42FF) 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 40G is too small, or too big
Fix: go up a cup on the same band → 40GG. If the band is also loose, go to 38GG.
Fix: gaping cups → down a cup, 40FF. Band riding up with cups that fit → 38GG.
40G in US, EU, FR & AU sizing
The same 40G 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) | 40I | Band in inches |
| United Kingdom (UK) | 40G | Same band as US; cups diverge above DD |
| Europe (EU) | 90I | US band × 2.5 − 10 = 90 |
| France / Spain (FR) | 105I | EU band + 15 = 105 |
| Australia / NZ (AU) | 18G | US band − 22 = 18 |
| Italy (IT) | 6I | (EU − 60) ÷ 5 = 6 |
| Japan (JP) | 90I | EU band; cups close to EU |
| Germany (DE) | 90I | 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 UK: Panache, Freya, Curvy Kate, Bravissimo.
How to tell if 40G is your correct size
A calculated 40G is an accurate starting point. Confirm it on the body: band, cups, gore, straps.
- Band sits firm and level. The 40 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 G 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 40 band is probably too big: try 38GG.
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 40G 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 40G actually your size?
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Calculate My Bra Size →40G bra size: answered
The questions people ask most about the 40G size: its volume, sister sizes, conversions and fit.
Is 40G a big bra size?
40G is a full-bust cup on a 40 band. A cup letter is a ratio, not a fixed volume: G means a 9-inch difference between bust and band, so a 40G holds more than a 36G and less than a 44G. “Big” only has meaning once you know the band.
What is the sister size of 40G?
The sister sizes of 40G are 38GG (snugger band) and 42FF (looser band). Sister sizes hold the same cup volume as 40G on a different band. Move to one when the G cups fit but the 40 band is too loose or too tight.
What is 40G in European sizing?
A UK 40G is an EU 90I. 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 40G in US sizing?
A UK 40G is a US 40I. UK and US bands are identical; UK E = US DDD, UK F = US G, UK FF = US H.
What measurements are a 40G?
Roughly a 39-inch (99 cm) snug underbust and a 49-inch (124 cm) bust at the fullest point. The 9-inch difference gives the G cup; the underbust gives the 40 band. Treat these as the center of a range.
What is 40G in centimeters?
A 40G corresponds to an underbust of about 98–103 cm and a bust of about 123–125 cm: a difference of roughly 23 cm. In metric-labelled systems the same size is EU 90I, FR 105I and JP 90I.
Which bra styles suit a 40G?
At this depth the bra is doing real load-bearing work. Prioritise full-cup and side-support constructions from full-bust specialists, firm (often three- or four-hook) bands, and wider straps. Molded single-piece cups rarely fit well here; seamed cups distribute weight and shape 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 40G is my correct size?
Check four things: the 40 band sits level and snug on the loosest hook, the G 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 38GG.
Was I sized as 40G using the old +4 method?
If your 40 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 36H, same cup volume, a band that supports.