Shopping for a B cup bra involves more than choosing a cup letter, because bra size depends on both the band size and cup size. A B cup uses the same cup letter in both UK and US sizing, although bra sizing, fit and availability can still vary between brands.
This guide explains how B cup volume changes across different band sizes, which brands make B cup bras, how UK and international sizing compare, and how to find your correct starting size before buying.
Start by measuring your underband and bust and entering the measurements into our Bra Size Calculator. For step-by-step measuring and fit checks, see our Bra Fitting Advice. If you are comparing UK and international sizing or shopping across different brands, use the Bra Size Converter.
You can also see how B cup volume changes visually across different band sizes in our How Big Are B Cups? guide.
A B cup is only one part of a bra size. Your complete bra size combines a band size and a cup size, so not all B cups have the same cup volume.
For example, a 30B, 34B and 40B all use the same B cup letter, but the cup volume increases as the band size increases.
B comes after A and before C in both UK and US bra sizing. The two sizing systems use different cup letters further along the cup sequence, but a B cup is generally labelled B in both systems.
A B cup is not one fixed breast size. How big a B cup is depends on the band size it is paired with, because cup volume increases as the band size increases.
For example, a 30B has less cup volume than a 34B, and a 34B has less cup volume than a 40B. Body proportions and bra construction can also affect how a B cup looks when worn.
For a more detailed visual comparison across different B cup band sizes, see our How Big Are B Cups? guide.
A B cup sits between A and C in both UK and US bra sizing. The diagrams below compare these three cup sizes on the same 34 band, showing how the cup volume increases while the band size remains unchanged.
On the same band size, a B cup has more cup volume than an A cup and less cup volume than a C cup.
34A
34B
34C
These simplified educational diagrams compare A, B and C cups using the same 34 band so the change in cup volume is easier to see.
On a 34 band, 34B sits between 34A and 34C in cup volume. The band size stays the same while the cup volume changes.
To compare B cups across more band sizes, visit our How Big Are B Cups? guide, or use the Bra Size Visual Comparison Tool to compare different bra sizes side by side.
How cup sizes work: Bra cup size is based on the difference between the underband and fullest bust measurements. On the same band size, cup volume increases as you move from one cup size to the next.
UK and US cup lettering begins to differ at larger cup sizes, but B comes after A and before C in both sizing systems. This means a B cup is generally labelled B in both UK and US sizing.
B cup bras are available from a wide range of lingerie brands. The brands below include B cups within their size ranges, with links to our individual brand size guides for more detailed sizing information.
Size note: The band and cup ranges below are shown in UK sizing. Size availability can vary by individual bra style and collection, so always check the sizes available for the bra you are buying.
The sizes below show the confirmed B cup availability for the selected brands in our brand size database. These sizes are shown in UK sizing.
Individual bra styles may not be available in every size listed, so always check the size options for the bra you are buying.
A B cup changes in volume as the band size changes, so choosing your complete band-and-cup size is important when shopping for bras. Use the guides below to go directly to recommendations and sizing information for your B cup size.
You can also browse the complete B Cup Bras by Band Size directory or use our Bra Size Calculator if you are not sure which band size to start with.
In most bra sizing systems, yes. A B cup is generally labelled B in both UK and US sizing, and UK and US band numbers are also generally the same. For example, a UK 34B is generally equivalent to a US 34B.
The main differences between UK and US bra sizing appear at larger cup sizes, where the cup-letter sequences begin to differ. Brand sizing can also vary, so always check which sizing system the brand or retailer is using before ordering.
The example above shows how a UK 34B compares across different international sizing systems. For a more detailed explanation of UK and US cup lettering, see our UK vs US Bra Size Comparison Guide. You can also use the Bra Size Converter when comparing sizes across different brands and sizing systems.
A B cup does not represent one fixed cup volume. As the band size increases, the cup volume also increases, even though the cup letter stays the same.
The cup letter remains B, while the cup volume increases with the band size.
30B
34B
38B
These simplified educational diagrams show how B cup volume changes across different band sizes while the cup letter stays the same.
A 38B has more cup volume than a 34B, while a 30B has less. This is why a bra size needs to be considered as a complete band-and-cup combination rather than by the cup letter alone.
For more visual examples across the B cup range, see our How Big Are B Cups? guide or compare different sizes using the Bra Size Visual Comparison Tool.
These underwired B bras are available in UK sizing from established lingerie brands including Triumph and Freya. Underwired bras have a wire beneath each cup to provide support and help shape the bust.
If you are shopping from outside the UK, check the sizing system shown by the retailer before ordering. You can compare UK and US sizing in our UK vs US Bra Size Comparison Guide.
Before ordering, confirm your starting size with our Bra Size Calculator and use our Bra Fitting Advice to check the band, cups, wires and straps.
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Start by measuring your underband and fullest bust. Enter those measurements into our Bra Size Calculator to get a recommended UK starting size.
If you are shopping in the US or another country, you can then compare your UK starting size with the sizing system used by the brand or retailer. For B cups, UK and US sizing generally use the same band number and B cup letter, but other international sizing systems use different band numbers.
Once you have a starting size, use our Bra Fitting Advice to check how the band, cups, wires and straps fit when you try on the bra.
If you are comparing sizes between brands or sizing systems, use the Bra Size Converter alongside our individual brand size guides.
For step-by-step fit checks and troubleshooting, see our Bra Fitting Advice.
These non-wired B bras are available in UK sizing from established lingerie brands including Triumph and Glamorise. Non-wired bras, also commonly called wireless bras in the US, do not have an underwire beneath the cups and are available across a range of B cup band sizes.
If you are shopping from outside the UK, check the sizing system shown by the retailer before ordering. You can compare UK and US sizing in our UK vs US Bra Size Comparison Guide.
Before ordering, confirm your starting size with our Bra Size Calculator and use our Bra Fitting Advice to check the band, cups and straps.
As an Amazon Associate, we may earn from qualifying purchases. This does not affect the price you pay.
Sister sizing can be useful when your usual bra size is unavailable in a particular style. Sister sizes use a different band size and cup letter while keeping a similar cup volume.
If you go up in the band size, go down in the cup size. If you go down in the band size, go up in the cup size.
For example, 30C, 32B and 34A are sister sizes because they have a similar cup volume but different band sizes. They are not identical sizes, because changing the band size also changes how the bra fits around the body.
To find the sister sizes for your own bra size, use our Bra Sister Size Calculator or read our Bra Sister Sizes Guide for more examples and fitting advice.
A common B cup sister-size example is:
These sizes have a similar cup volume but different band sizes.
30C
32B
34A
These simplified educational diagrams show how sister sizes can have a similar cup volume while the band size and cup letter change.
If you go up in the band size, go down in the cup size to keep a similar cup volume. If you go down in the band size, go up in the cup size.
For a full explanation of how sister sizing works, see our Bra Sister Sizes Guide or use our Bra Sister Size Calculator to find the sister sizes for your own bra size.
You can also use the Bra Size Visual Comparison Tool to compare different bra sizes side by side.
Explore more B cup sizing information, nearby cup sizes, visual comparisons, sister sizes and international bra sizing with the guides below.
Is B a small bra size?
B is a cup size rather than a complete bra size, so it does not describe one fixed breast size.
Cup volume depends on both the band size and cup letter. For example, a
30B has less cup volume than a 40B, even though
both use the same B cup letter.
What is the difference between A and B cups?
On the same band size, a B cup has more cup volume than an A cup. B comes after A
and before C in both UK and US bra sizing. See our
A Cup Bras guide
and
C Cup Bras guide
for nearby cup-size comparisons.
What does B mean in bra sizing?
B identifies the cup part of a bra size and comes between A and C in both UK and US
sizing. It needs to be combined with a band size, such as 30B,
34B or 40B, to describe a complete bra size.
Is a UK B cup the same as a US B cup?
Generally, yes. B is used as the same cup letter in both UK and US sizing, and the
band numbers are also generally the same. For example, a UK 34B
is generally equivalent to a US 34B. The cup-letter sequences
begin to differ at larger cup sizes. See our
UK vs US Bra Size Comparison Guide
for more detail.
Does band size affect B cup volume?
Yes. B cup volume increases as the band size increases. A 38B
therefore has more cup volume than a 32B, even though both use
the same cup letter. Our
How Big Are B Cups?
guide shows this visually across different band sizes.
What cup size comes after B?
C comes directly after B in both UK and US bra sizing. The two systems use the
same early cup letters but begin to differ further along the cup sequence.
For general background on bra sizing systems and cup-size terminology, see the bra size reference on Wikipedia .
