How Big Is a 38B Bra Cup Size?

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Finding your bra size can be a frustrating process. It becomes even more confusing due to the general misconception that the bigger the bra size number, the bigger the boobs. However, that is not the case.

You would be surprised to know that 38B boob size is actually not very large. This leads me to the question, “how big is 38B, really?” 

How Big Is 38B?

Your bra size will be 38B bra if your underbust measures around 34 to 35 inches and your bust measures around 40 inches. Breasts with a B cup size extend around 2 inches from your ribcage and this size is considered average.

Graphic image of a woman wearing an orange tank top and black shorts with a text explaining that a 38B bra size has a bust size range of 39-40 inches and 37-38 inches band size

The number (38) is your band size and the alphabet (B) is your cup size. The cup size is determined by the difference between the bust size and band size.

Despite the comparatively smaller size, you can achieve amazing cleavage with the right kind of bra, such as push up bras and padded bras

If you have a larger frame with small breasts, like a 38B, take a look at my #1 recommended push up bra: the Maidenform Demi Push Up Bra. Super comfortable, this specific bra features Wonderbra technology to give you a natural looking lift!

You can find your perfect bra size by measuring your band size and cup size. When you try out the new try, make sure that:

38B Bra Size Details

38B SizeDetails
Band Size37 to 38 inches
Bust Size39 to 40 inches
WeightAround 4.3 pounds
Sister Size Up40A
Sister Size Down36C

Is 38B an Unusual Size?

A 38B is not an uncommon bra size. What makes the bra size unusual is the combination of a wider back and smaller breasts.

If you have a 38B bra size, chances are that you have a bigger bone structure with less body fat. Compared to C cup sizes, consequently, your cup size is small because of the lack of tissues on your body.

However, there are many factors that affect your breast size.

A white full-coverage bra with a lace design placed on a white mannequin

Regardless, it can be a challenge to find a bra this size in every retail store. Even though I recommend wearing a bra made for your dimensions, I understand that it may not always be possible. In that case you can explore the sister sizes of your bra size. 

What Are the Sister Sizes of 38B?

Sister sizes refers to sizes that have a similar cup capacity. The rule of thumb with sister sizes is that you go up a band size and down a cup size, or the other way around. In this way, the 38B bra size’s sister sizes are 36C and 40A.  

Graphic image with drawings of three white body mannequins representing 36C, 38B and 40A as the sister sizes for a 38B cup size

Here is how you can decide which sister size is more appropriate for you:

  • If you fasten your bra using the last hook or the loosest setting, go for a sister size up i.e. 40A.
  • If you fasten your bra using the first hook or the tightest setting, go for a sister size down i.e. 36C.

Related Questions

How Will I Know if I Have a 38B Bra Size?

You’ll know you have a 38B bra size if your bust measures around 39 to 40 inches while your underbust ranges from 37 to 38 inches. You should wear bras with B cups if there’s a two-inch difference between the bust and band measurements.

What Are the Best Bra Styles for a 36DD Size?

The best bra styles for a 38B are plunge bras, bandeau bras, and halter bralettes. This size has a small bust yet a wide ribcage, so you must prioritize side panels and contouring cups.

How Can I Make My 38B Size Look Bigger?

You can make your 38B size look bigger by wearing padded or push-up bras. You may also consider using bras with less cup coverage, such as demi and balconette bras.

Bra Size Measurement & Cup Size | A, B, C, D, DD Bra Sizes AND Extra Large Breasts


Women with 34-35 inches band size and 40 inches bust size have the bra size 38B. It is amongst the least common sizes since women with broad backs usually have bigger boobs. Nonetheless, if you cannot find a bra in your size, check out the sister sizes (36C and 40A) to see what feels most comfortable.


Allena is the Founder and Editor of The Better Fit, a platform dedicated to empowering women with practical advice on bras and fitness, inspired by her own revelation of the widespread issue of incorrect bra sizing. Her expertise and tips have been recognized by major publications such as Cosmopolitan, Better Homes & Gardens, Insider, and NBC, making her a trusted voice in women's wellness and lifestyle.