34B Bra Size Guide: Measurements, Fit, Sister Sizes & Conversions

Last updated: August 2026

The 34B bra size pairs a 34 band with a B cup. This guide breaks down exactly what that means in real measurements, how to tell whether 34B is truly your size, which sister sizes to reach for, and how 34B converts around the world.

Band 34B cupUnderbust 33–34″Bust 36–37″

What a 34B bra actually means

Read a bra size as two parts. The 34 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.

34B measurements

If your own numbers land inside those ranges, 34B 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 34B

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 34 band you are looking for roughly 33–34 inches (84–86 cm).
• Bust: measure loosely around the fullest part, tape parallel to the floor. For 34B that is about 36–37 inches (91–94 cm).
• Subtract: bust minus underbust gives you the cup. A gap of about 2–3 inches is the B cup that makes 34B.

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.

34B in international sizing

So a US 34B is written 34B in the UK, 75B across much of the EU and 90B 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 34B

Sister sizes hold the same cup volume as 34B but on a different band — the natural sizes to try when the band feels tight or loose. Here is the 34B sister family:

Common fit problems with a 34B — and how to fix them

• Between sizes: if a 34B feels close but not perfect, its sister sizes shift the band without changing cup volume.

Best bra styles for a 34B

For a 34B, look to most styles — t-shirt, plunge, balconette and push-up all tend to work well. Whatever the style, get the band right first — it is the foundation every other part of the fit sits on.

Where to find 34B bras

34B sits in the most widely-stocked part of the size range, so you will find it across high-street and online retailers in almost every style. That is good news for choice — but cut still varies by brand, so use your sister sizes to fine-tune if one label runs small or large.

Is 34B big or small?

34B sits in the small range for cups on an average band. Whether that reads as ‘big’ or ‘small’ depends entirely on the band it sits on: the cup volume of 34B 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

• 34A vs 34B — one cup smaller on the same band.
• 34B vs 34C — one cup larger on the same band.
• 32C guide — a sister size of 34B.
• 36A guide — a sister size of 34B.