B cups sit between A and C in both common UK and US bra sizing sequences. However, a B cup does not represent one fixed breast size because the overall cup volume changes with the band size.
This guide explains how big B cups are, including the bust difference, measurements and approximate cup volume. It also includes simplified visual diagrams showing how B cup volume changes across different band sizes and how B cups compare with nearby A and C cup sizes.
To see how B cups compare across the wider cup-size range, view our Bra Cup Size Guide.
Is a B cup small? There is no single answer based on the cup letter alone. A 30B has less overall cup volume than a 40B, for example, because both the band size and cup size determine the complete bra size.
A B cup sits between A and C in both common UK and US bra sizing sequences. In traditional bra sizing, it represents approximately a 2-inch difference between the band size and the fullest bust measurement.
A B cup is not one fixed breast or cup volume. The complete bra size matters: for example, a 30B has less cup volume than a 40B, even though both use the same B cup letter.
The cup progression around B is: A → B → C → D.
These simplified diagrams show how a B cup changes across different band sizes. A B cup does not represent one fixed breast or cup volume, so a 30B, 34B, 38B and 42B increase in overall cup volume as the band size increases.
30B
34B
38B
42B
These images are simplified educational diagrams designed to show relative cup volume. Individual breast shape, body shape and bra style can all affect how a bra size looks when worn.
The B cup letter stays the same, but the overall cup volume increases as the band size increases. This is why a 42B has more cup volume than a 30B.
Comparing a B cup with the nearby A and C cup sizes can make the difference in cup volume easier to understand. The diagrams below use the same 34 band size so only the cup size changes.
34A
34B
34C
These simplified educational diagrams show the relative change in cup volume between A, B and C cups on the same 34 band.
On the same 34 band, 34B has more cup volume than 34A and less cup volume than 34C. The band size stays the same while the cup size changes.
Generally, yes. UK and US band sizes use the same numbers, and B is also used as the same cup letter in both common sizing systems. This means a UK 34B is generally equivalent to a US 34B.
The main differences between UK and US bra sizing appear further along the cup sequence, where the cup letters begin to differ. Brand labelling can also vary, so it is always worth checking which sizing system a brand or retailer is using before ordering.
For a full comparison, see our UK vs US Bra Size Comparison Guide or use our Bra Size Converter.
These underwired B bras are available in UK sizing from established lingerie brands including Triumph and Fantasie. Underwired bras use a wire beneath the cups to provide structure and are widely available across B cup band sizes.
These product recommendations are currently based on UK availability. If you are shopping from the US or another country, check the sizing system and current size availability shown by your local retailer before ordering.
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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For more B cup sizing and brand information, see our B Cup Bras Guide.
The bust measurement for a B cup changes with the band size. In traditional bra sizing, a B cup represents approximately a 2-inch difference between the band size and fullest bust measurement. The examples below show how this works across several B cup sizes.
| Bra Size | Approx Bust Measurement |
|---|---|
| 32B | 34 inches |
| 34B | 36 inches |
| 36B | 38 inches |
| 38B | 40 inches |
Cup volume also changes with the band size. For example, a 34B has a similar cup volume to 32C and 36A. These are known as sister sizes.
If you go down in the band size, go up in the cup size to keep a similar cup volume. If you go up in the band size, go down in the cup size.
You can check the sister sizes for your own bra size with our Bra Sister Size Calculator or read our Bra Sister Sizes Guide for more examples.
A B cup cannot be described as one fixed small or large size because cup volume changes with the band size. A 30B, 32B, 34B and 36B all use the same B cup letter, but they do not have the same cup volume.
The same principle continues across other B cup band sizes. As the band size increases, the corresponding B cup volume also increases. This is why the complete bra size is more useful than the cup letter alone when comparing breast or bra cup size.
To explore B cup sizes from 28B to 56B, including available brands and individual size guides, see our B Cup Bras Guide.
These non-wired B bras, also commonly called wireless bras, are available in UK sizing from established lingerie brands. Non-wired bras do not use an underwire beneath the cups and are available across a range of B cup band sizes.
These product recommendations are currently based on UK availability. If you are shopping from the US or another country, check the sizing system and current size availability shown by your local retailer before ordering.
Before ordering, confirm your starting size with our Bra Size Calculator and use our Bra Fitting Advice to check the band, cups and straps.
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A B cup does not have one fixed breast volume or weight. As the band size increases, the corresponding cup volume also increases, which means a 30B and a 40B do not represent the same overall cup volume.
The table below provides approximate illustrative estimates across B cup band sizes from 28B to 56B. These figures are intended to show the relative increase in volume across band sizes rather than predict an individual woman's breast volume or weight.
| Bra Size | Estimated Cup Volume | Estimated Weight (Both Breasts) |
|---|---|---|
| 28B | ~300 cm³ | ~0.5 kg / 1.1 lb |
| 30B | ~340 cm³ | ~0.7 kg / 1.5 lb |
| 32B | ~390 cm³ | ~0.8 kg / 1.7 lb |
| 34B | ~440 cm³ | ~0.9 kg / 1.9 lb |
| 36B | ~500 cm³ | ~1.0 kg / 2.2 lb |
| 38B | ~560 cm³ | ~1.1 kg / 2.5 lb |
| 40B | ~630 cm³ | ~1.3 kg / 2.8 lb |
| 42B | ~710 cm³ | ~1.4 kg / 3.1 lb |
| 44B | ~790 cm³ | ~1.6 kg / 3.5 lb |
| 46B | ~880 cm³ | ~1.8 kg / 3.9 lb |
| 48B | ~980 cm³ | ~2.0 kg / 4.3 lb |
| 50B | ~1080 cm³ | ~2.1 kg / 4.8 lb |
| 52B | ~1190 cm³ | ~2.4 kg / 5.2 lb |
| 54B | ~1310 cm³ | ~2.6 kg / 5.7 lb |
| 56B | ~1440 cm³ | ~2.9 kg / 6.3 lb |
The estimates illustrate the general principle that B cup volume increases with band size. A B cup on a smaller band therefore has less cup volume than a B cup on a substantially larger band.
These figures are illustrative estimates, not measurements of an individual woman's breasts. Actual breast volume and weight can vary considerably between people wearing the same bra size because of differences in breast shape, tissue density, body proportions and bra fit.
A B breast size is an informal way of referring to breasts fitted into a B cup bra. However, B is only the cup part of a bra size and does not describe one fixed breast size or volume.
In traditional bra sizing, a B cup represents approximately a 2-inch difference between the band size and the fullest bust measurement. The complete bra size still matters because cup volume changes with the band size.
For example, a 30B, 34B and 40B all use the B cup letter but have different cup volumes. This is why breast size cannot be accurately compared using the cup letter alone.
You may also see searches or informal references to B boob size. This means the same general thing, but in bra fitting it is more useful to consider the complete band-and-cup size rather than B as a standalone breast size.
For B cup brand availability, individual band-size guides and bra recommendations, see our B Cup Bras Guide.
A B cup is often described as a smaller cup size because it appears near the beginning of the bra cup sequence. However, the cup letter alone does not tell you how large someone's breasts are.
Band size makes a significant difference. For example, a 30B has less cup volume than a 40B, even though both bras use the same B cup letter.
On the same band size, B sits between A and C. For example, a 34B has more cup volume than a 34A and less cup volume than a 34C. Once the band size changes, comparing cup letters alone becomes less useful.
So, while B is one of the earlier cup letters in the size sequence, there is no single breast size or appearance that can accurately be described as “a B cup”. The complete band-and-cup size, body proportions and breast shape all affect how the size looks when worn.
To compare nearby cup sizes, see our A Cup Bras Guide and C Cup Bras Guide.
There is no single appearance for a B cup because the cup volume changes with the band size. The visual diagrams above show this by comparing B cups across several different band sizes.
For example, a 32B has less cup volume than a 38B, even though both use the same B cup letter. Breast shape, body proportions and bra style can also affect how the same bra size looks when worn.
To compare B with nearby cup sizes, see our A Cup Bras Guide and C Cup Bras Guide, or view our B vs C vs D Cup Size Comparison.
B cup volume changes with band size, so we have individual guides for B cup sizes from 28B to 56B. Choose your size below for bra recommendations, sizing information and fit guidance.
You can also visit our B Cup Bras Guide for brand availability and an overview of the complete B cup range.
Yes. On the same band size, a B cup is one cup size larger than an A cup. For example, a 34B has more cup volume than a 34A.
There is no single fixed breast size represented by a B cup. In traditional bra sizing, a B cup represents approximately a 2-inch difference between the band size and fullest bust measurement, but the overall cup volume changes with the band size.
B is one of the earlier cup letters in the bra size sequence, but the cup letter alone does not determine overall breast size. A 30B has less cup volume than a 40B, for example, so the complete band-and-cup size needs to be considered.
A B cup can look different depending on the band size, breast shape, body proportions and bra style. For example, a 30B has less cup volume than a 42B. The diagrams on this page provide simplified educational examples of how B cup volume changes across band sizes.
Generally, yes. B is used as the same cup letter in both common UK and US bra sizing, and the band numbers are also generally the same. For example, a UK 34B is generally equivalent to a US 34B. Differences between UK and US cup lettering become more noticeable further along the cup sequence.
For general background on bra sizing systems and cup-size terminology, see the bra size reference on Wikipedia .
For a wider comparison of cup sizes, see our How Big Are Bra Cup Sizes? guide.
If you are unsure which size to try, measure your underband and fullest bust and enter the measurements into our Bra Size Calculator.
You can then check how the bra fits using our Bra Fitting Advice and compare sizing across different brands with our Bra Size Converter.
