30B Bra Size
30B fits a 29–31 inch (74–79 cm) underbust and a full bust of about 33 inches (84 cm). Its sister sizes are 28C and 32A.
Last reviewed on 25 September 2026.
30B at a glance
| Underbust (band) | 29–31 in · 74–79 cm |
|---|---|
| Full bust | ≈ 33 in · 84 cm |
| Bust minus underbust | ≈ 3 in · 8 cm |
| Sister sizes (same cup volume) | 28C · 32A |
| UK / EU / FR / AU | 30B · 65B · 80B · 8B |
| One cup bigger | 30C |
| One cup smaller | 30A |
| Approx. top size | XS · UK 6–8 / US 2–4 |
Measurements for 30B
30B is built for an underbust ribcage measurement that falls in the 29–31 inch range, with a full-bust measurement around 33 inches. The band number — 30 — is a smaller band that needs to sit firmly without cutting in. The cup letter — B — describes the difference between the two measurements; a B cup means the full bust is about three inches larger than the underbust — most cup shapes are widely available at this volume.
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30B in Other Sizing Systems
EU, FR, and JP sizing read the band in centimetres, which is why 30B maps to 65B (EU), 80B (FR), and 65B (JP) — the same body, expressed in different units. AU sizing uses a small numeric scale (8B for this band). UK sizing shares the band number with the US but parts ways at the cup once you go past D, so 30B reads as 30B on a UK label.
Sister Sizes for 30B
These sizes have the same cup volume as 30B. Try them if the band feels too tight or too loose.
← Tighter band, bigger cup · Looser band, smaller cup →
Sister sizes are useful when the cup of your 30B fits well but the band is wrong — too loose by the end of the day, or too tight to clip up comfortably. Going one band smaller and one cup larger keeps the cup volume the same while tightening the band; going the other way loosens the band without losing cup space. Read the full sister-size matrix if you want to see how this works for every size at once.
Fit Tips for 30B
B cups are among the most widely produced sizes, so style choice is broad. If you see gapping at the top of the cup, a demi-cup or push-up shape often fits better than a full-cup.
If your 30B bra is gapping at the top of the cup, the cup may be too large or the wrong shape — try a demi-cup or balconette before assuming the size is wrong. If the centre piece between the cups (the gore) does not lay flat against your sternum, the cup is almost certainly too small; go up one cup. If the band rides up across your back during the day, the band has stretched out or was too loose to begin with — drop one band size and add one cup to keep the same cup volume.
For more detailed troubleshooting, the Fit Test on the home page covers six common visible problems and the size or shape change that usually addresses each one. The guide to measuring mistakes covers the most common reasons a calculation comes out wrong in the first place.
Related sizes
If 30B is close but not quite right, these neighbouring sizes are the most useful next places to look:
See every size at a glance on the full bra size index, or read how to measure your bra size if you are not sure 30B is right for you.
30B — frequently asked questions
What measurements are a 30B bra in inches and cm?
A 30B bra fits an underbust (ribcage) measurement of roughly 29 to 31 inches (74–79 cm) and a full-bust measurement of about 33 inches (84 cm). The cup letter B reflects the roughly 3-inch (8 cm) difference between those two numbers.
What size is equivalent to 30B? (sister sizes)
The sister sizes of 30B are 28C (tighter band, one cup letter up) and 32A (looser band, one cup letter down). They hold the same cup volume as 30B on a different band. Try a sister size when the cups of your 30B fit but the band feels too loose or too tight.
What is 30B in UK, EU, and other sizing systems?
30B (US) converts to 30B in UK sizing, 65B in EU sizing, 80B in French sizing, 8B in Australian sizing, and 65B in Japanese sizing. The body is the same; only the labelling system changes.
Is 30B a small or large bra size?
In terms of how common it is, this size pairs a small band with a smaller cup. There is no universal "small" or "large" bra size — the same cup letter describes a different volume on every band — but within the full range BraCalc covers (bands 28–48, cups AA–H) this combination sits toward the compact end.
Is 30B a small, medium, large, or XL?
As a rough guide, a 30 band usually matches size XS in tops and bralettes — around UK dress size 6–8 (US 2–4). The band number drives this more than the cup letter, so 30B lands in the same letter size as other 30-band bras. Letter sizing varies a lot between brands, so always check the brand's own chart.
What is the next size up or down from 30B?
Keeping the same 30 band, one size up in the cup is 30C, and one size down is 30A. If you want a bigger or smaller band but the same cup volume, use a sister size instead: 28C or 32A. Remember that the cup letter is relative to the band — 32B has more cup volume than 30B, not the same.
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