100B Bra Size (FR)
Everything about the 100B bra size in France & Spain sizing: what it means, its measurements, whether it runs big or small, sister sizes, and conversions to US, UK, EU & AU.
What is a 100B bra size?
A 100B in France & Spain sizing is a bra with a 100 band and a B cup. “100” is the French/Spanish band: the EU band plus 15 (85 + 15 = 100). “B” is the cup, from a 2-inch difference between the bust and the band (one inch per cup letter). The same body is written as US 38B, UK 38B, EU 85B, AU 16B, IT 5B.
Someone who measures roughly 37 inches (94 cm) around the ribcage and about 40 inches (102 cm) around the fullest part of the bust lands at 100B 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.
“100” is the French/Spanish band: the EU band plus 15 (85 + 15 = 100). It sets how the bra fits around the ribcage and provides about 80% of the support.
B is a modest B-cup: a 2-inch bust-minus-band difference. It reflects breast volume, always relative to the 100 band.
Band and cup are one size. A 100B is not the same volume as a 90B: the cup letter only has meaning next to its band.
100B measurements
A 100B comes from two measurements: a snug underbust that gives the 100 band, and a looser bust. The cup is the gap between the two: for B, that gap is 2 inches.
| Measurement | Range | Sets |
|---|---|---|
| Underbust (snug) | 36.5–38.4″ · 93–98 cm | Band 100 |
| Bust (fullest) | 39.5–40.4″ · 100–103 cm | — |
| Difference | 2″ · ≈ 5 cm | Cup B |
Underbust rounds to the nearest even band; an odd reading (e.g. 37″) can go either way, so try 95C and 100B and let the band decide.
Measurements vary by body shape, so treat these as the center of a range. FR 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 100B big or small?
100B is a smaller-volume cup on a fuller band: one of the most widely stocked sizes. 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 B cups, yet the volume grows with the band. 100B is the highlighted size.
Because the cup is measured relative to the band, a 100B holds more than a 90B and less than a 110B, even though all three are “B cups.” So “big” or “small” only has meaning once you know the band.
If 100B 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 100B is your starting size, the calculator settles it from two measurements.
Were you measured as 100B 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 100 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, 90D 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 100B is not the same 100B as ours. Brasize.ai uses the modern snug-measure standard used by professional fitters.
Which bra styles fit a 100B best?
Shallow-to-modest cups suit plunge, demi and balconette shapes that create lift and shape without excess fabric; wireless and bralette styles also work because there's little weight to support. Avoid full-coverage cups cut for projection: they tend to gape at the top on a shallow profile.
Shallow, lifting shapes: plunge, demi, balconette.
A standard two-hook band is fine; make sure it's firm on the loosest hook.
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.
100B sister sizes
Sister sizes hold the same cup volume as 100B 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 100B | Cup volume | When to try it |
|---|---|---|---|
| 90D | Two bands snugger | Same as 100B | Only if 95C is still loose; wire will be noticeably narrower |
| 95C | One band snugger | Same as 100B | 100 band rides up or feels loose |
| 100B | Your starting size | — | Band sits firm and level |
| 105A | One band looser | Same as 100B | 100 band digs in or feels tight |
The rule: go down a band and up a cup (100B → 95C) for the same cups on a tighter band, or up a band and down a cup (100B → 105A) 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 100B is too small, or too big
Fix: go up a cup on the same band → 100C. If the band is also loose, go to 95C.
Fix: gaping cups → down a cup, 100A. Band riding up with cups that fit → 95C.
100B in US, UK, EU & AU sizing
The same 100B 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) | 38B | Band in inches |
| United Kingdom (UK) | 38B | Same band as US; cups diverge above DD |
| Europe (EU) | 85B | US band × 2.5 − 10 = 85 |
| France / Spain (FR) | 100B | EU band + 15 = 100 |
| Australia / NZ (AU) | 16B | US band − 22 = 16 |
| Italy (IT) | 5B | (EU − 60) ÷ 5 = 5 |
| Japan (JP) | 85B | EU band; cups close to EU |
| Germany (DE) | 85B | 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 FR: Chantelle, Simone Pérèle, Aubade, Etam.
How to tell if 100B is your correct size
A calculated 100B is an accurate starting point. Confirm it on the body: band, cups, gore, straps.
- Band sits firm and level. The 100 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 B 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 100 band is probably too big: try 95C.
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 100B 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 100B 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 →100B bra size: answered
The questions people ask most about the 100B size: its volume, sister sizes, conversions and fit.
Is 100B a big bra size?
100B is a modest B-cup on a 100 band. A cup letter is a ratio, not a fixed volume: B means a 2-inch difference between bust and band, so a 100B holds more than a 90B and less than a 110B. “Big” only has meaning once you know the band.
What is the sister size of 100B?
The sister sizes of 100B are 95C (snugger band) and 105A (looser band). Sister sizes hold the same cup volume as 100B on a different band. Move to one when the B cups fit but the 100 band is too loose or too tight.
What is 100B in European sizing?
A FR 100B is an EU 85B. 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 100B in UK sizing?
A FR 100B is a UK 38B. 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 100B?
Roughly a 37-inch (94 cm) snug underbust and a 40-inch (102 cm) bust at the fullest point. The 2-inch difference gives the B cup; the underbust gives the 100 band. Treat these as the center of a range.
What is 100B in centimeters?
A 100B corresponds to an underbust of about 93–98 cm and a bust of about 100–103 cm: a difference of roughly 5 cm. In metric-labelled systems the same size is EU 85B, FR 100B and JP 85B.
Which bra styles suit a 100B?
Shallow-to-modest cups suit plunge, demi and balconette shapes that create lift and shape without excess fabric; wireless and bralette styles also work because there's little weight to support. Avoid full-coverage cups cut for projection: they tend to gape at the top on a shallow profile.
How do I know if 100B is my correct size?
Check four things: the 100 band sits level and snug on the loosest hook, the B 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 95C.
Was I sized as 100B using the old +4 method?
If your 100 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 90D, same cup volume, a band that supports.