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How Do Bra Sizes Work: A Full Guide

Written by Uplifted Lingerie on 09 March 2021

How Do Bra Sizes Work


How do bra sizes work? Bra sizes consist of a band size (the number) and a cup size (the letter). The band size is the measurement around your rib cage in inches, whilst the cup size reflects the difference between the size of your rib cage and the size of your bust. Wearing the right bra size is the key to getting the right support, as well as a comfortable fit. 

There’s no better feeling than putting on a bra that fits perfectly. It can boost your confidence and self-esteem, leaving you feeling like you’re ready to take on the world. But not many people fully understand how bra sizes work. In short, there are two measurements you will need to know in order to understand how a bra size works - your band size and cup size. In this article, we’ll explain more about how bra sizes are calculated, as well as how you can ensure that you’re wearing the right size bra. 


The Legend Of The Perfectly-Fitted Bra

Do you remember seeing bra sizes for the first time as a teenager? They look like a jumble of numbers and letters and you may be left feeling confused and overwhelmed. 

You may be tempted to head to a high street store to get measured for a bra. After all, that would take some of the confusion away, right? 

The problem is, many high street stores use outdated methods of measuring bra size, adding on several inches to your band size and leaving you with a bra that rides up your back and doesn’t give you the support that you need. 

A properly-fitted bra leaves you oozing with confidence, and we believe that every woman deserves this. But how do you ensure that you’re wearing the right size bra if even the high street retailers aren’t always getting it right? We’ve got you covered. 

View our full range of bras here

How Do You Calculate Bra Size  

How Do You Calculate Bra Size?

Two measurements are needed to calculate your bra size. These are your band size and your cup size. Your band size makes up the number while your cup size is represented by the letter. 

The only equipment that you need to measure your bra size is a flexible tape measure. You’ll find your bra size by measuring around your rib cage and your bust. Read on to discover how to calculate your bra size correctly. 


Band Size

Your band size is the number that you see within your bra size. For example, 30, 36 or 40. This refers to the size of your rib cage. 

To measure your band size, wrap your tape measure around your rib cage, directly under your bust. The tape measure should lie exactly where the band of your bra would naturally sit. It should be level and snug. The number in inches is your band size. 

This is where many high street stores go wrong as they are renowned for adding several inches to this measurement. This is because back in the 1950s, bras were made from silk and satin and did not stretch. However, fast-forward to the 21st century and our bras now allow for stretch, so we don’t need to add anything onto this measurement. 

So, if you measure around your rib cage and find that it is 32 inches, this is your band size. If the number is an odd number, you might need to try the band size below and above to see which fits you best. 


Cup Size

Your cup size is letters than you see in your bra size, for example B, DD or G. This refers to the size of your breasts in relation to the size of your rib cage. 

To measure your cup size, wrap a measuring tape around the fullest part of your bust, level with your nipple, and then round the number in inches to the nearest whole number. You’ll then need to subtract your band measurement from this number. 

So, if your bust measurement is 41” and your band measurement is 34”, you’ll do the calculation 41-34 and be left with 6”. You can then use the table below to discover your cup size. In this example, your bra size would be 34E. 

Difference (inches) Cup Size
Less than 1" AA
1" A
2" B
3" C
4" D
5" DD
6" E
7" F
8" FF
9" G
10" GG
11" H
12" HH
13" J
14" JJ
15" K
16" KK
17" L

Find our D+ bras here


Fit Is More Important Than Measurements

Whatever the calculations say that your bra size is, the fit of your bra is far more significant. Use this calculation as a starting point to try on different bra sizes and find the right bra for your body type. 

It might be that you need to increase or decrease your band or cup size slightly to find the right fit. You may also notice that your size varies slightly between different brands and styles of bra. 

Get the perfect fit with our fitting guide


What Is A Sister Size

What Is A Sister Size? 

You might have heard people talk about sister sizes – and no, it’s not the size bra that your sister wears. A sister size refers to a bra which has the same cup volume as another. 

As we’ve already discussed, your cup size directly related to your band size as it is the difference between the size of your breasts and your rib cage. If you’re a D cup, that means that your bust is 4 inches bigger than your rib cage. If you decided to opt for a bigger band, you’d need a C cup as your bust would then only be 3 inches bigger than your band size. 

If your rib cage became wider but your breasts stayed the same size, you’d need to increase your band size and decrease your cup size. If you wore a 28F bra and gained weight around your ribs but not on your breasts, you may need to buy a 30E. 

A sister size refers to bra sizes which have the same size cups. For example, 28E, 32C and 36A all have the same size cups and are therefore sister sizes. 

Being aware of your sister sizes can help you to adjust your bra size to find the most comfortable fit. If your cups fit comfortably but your band is too loose, you’ll need to decrease your band size and increase your cup size to keep the same cup volume. 

If you were wearing a 34E and wanted a smaller band, you’d need to try a 32F. Although it seems as if the cup volume has increased, it’s actually stayed the same as it’s relative to your band size. If you’d tried a 32E, you’d have noticed that the cup size would have got smaller. 

For on-trend DD+ support, discover our Curvy Kate range of bras 


How Should A Bra Fit?

Getting the right fit is critical to ensuring your comfort. You’ll probably find that your bra size will vary slightly depending on the brand and style of bra, so it’s important to know how your bra should fit so that you can tell whether it’s the right size. 

When you put your bra on, it’s important to begin with a ‘swoop and scoop’. Reach into your bra cup and scoop your breast tissue from underneath your armpit so that it sits inside the cup. Check that your breast tissue isn’t spilling out of the top of the cup and make sure that any wiring isn’t pressing into your breast tissue. 

Then, check how your band is sitting on your back. It should be level all the way around and shouldn’t be riding up at the back. You should only be able to fit two fingers under the band when it is fastened on the loosest hook. 

Next, make sure that the centre of the bra sits flat against your chest. It shouldn’t move when you lift your arms up in front of you. If the centre doesn’t sit flat on your chest, the band is too big. 

Finally, tighten your straps just enough that they don’t fall off your shoulders, but not so much that your shoulders begin to support the weight of your breasts. 80% of your support should be coming from your band. This will help to prevent back and shoulder pain. 

Read our full fitting guide here


Related Questions

What Is ABCD In Bra Size? 

The letter in your bra size refers to your cup size. This is relative to your band size. It’s calculated by subtracting your ribcage measurement from your bust measurement. The number that you are left with will tell you your cup size. For example, if the difference between your rib cage measurement and your bust measurement is 6 inches, your cup size would be an E. 


Which Cup Size Is Bigger B Or C? 

If the band size remains the same a C cup would be bigger than a B cup. However, your cup size is directly related to your band size as it reflects the difference between the size of your rib cage and the size of your bust. If you reduce your band size, you’ll need to increase your cup size to keep the volume the same. This means that the cup volume would be bigger in a 36B bra than a 32C.  


Is D Cup Considered Big?

Your cup size is relative to your band size. If your breasts are a D cup, this means that your bust measurement is four inches larger than your band measurement. Whether or not a D cup is considered big depends on your own perception. However, it’s important to remember that a 28D is a smaller cup volume than a 36D, as the cup volume depends on your band size.  


Find The Perfect Bra For Your Body Shape

How do bra sizes work? If you’re ready to discover the perfect bra for your body shape, we are here to help. If you have any questions about how to find the right bra size, feel free to send us an email or give us a call. We’re so confident that we can help you to find the perfect bra that we are happy to offer an exchange or refund if your new bra isn’t quite right for you. 

Discover our full range of bras here


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