30B Bra Size Guide: Measurements, Fit, Sister Sizes & Conversions

Last updated: August 2026

The 30B bra size pairs a 30 band with a B cup. This guide breaks down exactly what that means in real measurements, how to tell whether 30B is truly your size, which sister sizes to reach for, and how 30B converts around the world.

Band 30B cupUnderbust 29–30″Bust 32–33″

What a 30B bra actually means

Read a bra size as two parts. The 30 is the band, and it comes straight from your underbust — the snug measurement around your ribcage just beneath the bust. The B is the cup, and it comes from how much bigger your bust is than that underbust: about 2–3 inches more for a B cup. The cup letter is a difference, not a fixed volume — which is why the very same cup looks different on a narrow band than on a wide one.

30B measurements

If your own numbers land inside those ranges, 30B is your calculated size. If you are close but not quite, the sister sizes below will usually have you covered.

How to measure yourself for a 30B

You need a soft tape measure and, ideally, a non-padded bra. Stand up straight and breathe normally — measuring too tight is the most common reason people end up in the wrong size.

• Underbust: wrap the tape snugly around your ribcage directly under the bust, level all the way round. For a 30 band you are looking for roughly 29–30 inches (74–76 cm).
• Bust: measure loosely around the fullest part, tape parallel to the floor. For 30B that is about 32–33 inches (81–84 cm).
• Subtract: bust minus underbust gives you the cup. A gap of about 2–3 inches is the B cup that makes 30B.

How to check the fit is actually right

Numbers get you to the right size on paper; these hands-on checks tell you whether it fits in real life. Do them in front of a mirror, wearing the bra on the loosest hook:

• Band: it should sit level all the way around, in line with the front. Slide two fingers under it at the back — snug, not loose, no riding up when you raise your arms. The band does roughly 80% of the support work, so this matters most.
• Cups: do a quick “scoop and swoop” — lean forward, settle everything into the cup, then stand up. There should be no spillage over the top or sides, and no wrinkling or gaping at the edge. Spillage means size up a cup; gaping means size down a cup.
• Centre gore: the little bridge between the cups should lie flat against your breastbone. If it floats off the body, the cup is usually too small.
• Straps: you should fit a finger under them comfortably. If they dig in or carry most of the weight, the band is too loose — tighten it or size the band down.

30B in international sizing

So a US 30B is written 30B in the UK, 65B across much of the EU and 80B in France. The band number and the cup letter can both change between systems, which is why the same bra can look like two different sizes depending on where it was made.

Sister sizes of 30B

Sister sizes hold the same cup volume as 30B but on a different band — the natural sizes to try when the band feels tight or loose. Here is the 30B sister family:

Common fit problems with a 30B — and how to fix them

• Band riding up at the back: 30 is a narrow band and can creep up if it is even slightly loose. It should sit level with the front; if it rides up, go down a band and up a cup (a sister size).

Best bra styles for a 30B

For a 30B, look to most styles — t-shirt, plunge, balconette and push-up all tend to work well. On a narrow band, seek-out brands that cut small bands generously so the wing still feels supportive. Whatever the style, get the band right first — it is the foundation every other part of the fit sits on.

Where to find 30B bras

30B falls outside the core high-street band, so everyday shops may carry fewer options. Specialist fuller-bust or extended-size retailers will have far more choice, and knowing your sister sizes widens what you can wear when a specific size is out of stock.

Is 30B big or small?

30B sits in the small range for cups on a smaller band. Whether that reads as ‘big’ or ‘small’ depends entirely on the band it sits on: the cup volume of 30B is identical to every sister size in the table above, even though the letters differ. That is the single most useful thing to understand about bra sizing — the letter alone tells you very little without the band.

Related sizes and comparisons

• 30A vs 30B — one cup smaller on the same band.
• 30B vs 30C — one cup larger on the same band.
• 28C guide — a sister size of 30B.
• 32A guide — a sister size of 30B.