Fit Guide

How Bra Sizing Works | The Cup Size Myth Dispelled

Cup size means NOTHING, NADA, NICHTS…without the proper band size to define it!

In fact, bras sharing the same cup letter, but different bands DO NOT share the same cups. Fundamentally, cup size is measured by the difference between your under bust (i.e. band size) and your bust – measured around the fullest part of the breasts (generally across the nipples).

Most of us assume, for example, if one wears a 34D and finds that the band of a particular bra is too tight, that she should go up to a 36D. This is incorrect! The woman who wears a 34D has less breast tissue than the woman who wears a 36D. Instead she should choose a 36C in order to achieve a more comfortable band with the same size cup because cup volume increases as band size increases. This is due to the inverse relationship between band and cup. We will elaborate further on this below.

But first, let’s look at a few examples of D cups across varying band sizes! The D cup is representative of a 4-inch difference between one’s band size and bust measurement. 

As evidenced by the chart above D cups are not equivalent across the board and in fact are completely dependent on band size. So, the D cups on a 36D are bigger than the D cups on a 34D which are both bigger than the D cups on a 32D.

Basically, a D is not a D is not a D!

This is partially why so many of us are wearing bras with bands that are too big and cups that are too small – resulting in the dreaded muffin boob. We’ve been led to believe that D’s are big, DD’s are enormous and DDD’s are just plain porn-starish. Well, nothing could be further from the truth.

That’s the dirty little secret of the lingerie industry.

Cup size represents the volume of your bosom and is completely relative to your band size. For example, a 36D holds 710cc of breast tissue - the same as a 34DD, 32F and 30G. A 34B, on the other hand, holds 390cc of breast tissue – the same as a 32C, 30D, and 28DD. This band-to-bust ratio, also known as Sister Sizing, represents bras that have the same cup volume. Assuming you have the right cup size, an increase or decrease in band size to achieve a slightly looser or snugger fit requires an opposite move in cup size. It's counterintuitive, I know, but it's the way bra sizing works!

It's no wonder so many of us have been wearing the wrong size! 
Discover your Sister Size in the chart below.


We do not recommend choosing a bra more than one Sister Size away from your measured bra size. While, for example, 38B, 36C, 34D, 32DD, and 30F all hold the same volume, the bras are designed for very different body shapes. As you go up in band size, for example, the entire bra scales upward with everything from the distance between the cups to the placement of the straps to the length of the wings increasing in proportion.

Now with that said, please remember that No Two Bras Fit Alike.

For some reason, as women, we are told a bra size early on and get it stuck in our heads that it is our size for a lifetime. Bras, however, are no different than the other articles of clothing we wear. That is to say that style, brand, manufacturer can all cause variations in our requisite size. Not to mention breast shape, fluctuations in weight, and babies; all of which can affect the size you need.

Keep in mind that with frequent wear bras lose their elasticity thus becoming less supportive. Furthermore, we only recommend going up an extra band size if you have less body fat around your ribcage since 80% of the support for your breasts comes from having a snug band. Finally, with strapless bras we recommend a snug fit to aid in avoiding the dreaded slippage.

So ladies, isn’t it time to get past our aversion to larger cup sizes. Instead of the 34C you're currently you might actually need a 30DD. And with the right size you could eliminate the dreaded muffin boob, side spillage and discomfort of straps digging into your shoulders.