How to Read Bra Size Labels and What They Actually Mean

Quick Summary

Why does the size matter, and what happens if you don’t wear the right size
Bra size - band, cup, and sister sizes
Why sizes vary across brands
Bra myths demystified
Underneat’s Fit-osophy

Finding your bra size is not rocket science. But the bra size labels - full of letters and numbers might make it feel like you’re looking at some hieroglyphics, and make bra shopping a frustrating experience. The bra size label is often misunderstood, leading to an ill-fitted and uncomfortable fit.

This guide will demystify bra size labels, explaining what the numbers and letters truly represent and why understanding them is critical for both comfort and health.

The Health Connection: Why a Proper Fit Matters

Posture and Back Pain: An ill-fitting bra, especially one with a loose band or inadequate cups, forces your shoulders and back to take on the weight, leading to poor posture, back pain, and even headaches.

Skin Irritation and Chafing: A tight band can dig into the skin, causing red marks, chafing, and general discomfort. A small cup size bra can cause boob spillage and irritation.

Circulation and Breathing: When the band is tight, it can restrict blood flow and breathing. Similarly, overly tight straps can cause painful grooves in the shoulders and impede circulation.

Breast Health: A well-fitting bra provides the necessary support to prevent the stretching of breast ligaments, which can cause discomfort and sagging over time.

What Does a Bra Size Really Mean?

You don’t have to break your head over this; let us break it down for you with a classic example:

If you have a 34C bra, here’s what the sizing means:

34 = Your band size (in inches)

C = Your cup size

The letter isn't a universal size. A 34C and 36C don’t hold the same volume. That’s where sister sizing comes in (we’ll get there).

Band Size

This is the number that you see on bra size labels and is measured snugly just below your bust. If the measurement comes out to be 33 inches, then round it up to the nearest even number, which means that your band size is 34. This fitting should feel firm but not like a corset from 1800s England.

Pro Tip: If the band rides up your back, it's too loose. If you can’t breathe, it’s too tight. In either case, you need to change to the right bra size.

Cup Size

The difference between your bust and band measurements calculates cup size. To get your bust measurement, loosely measure your breast around the fullest part. Once you have the number, subtract it from the band size. The number represents the cup size of the bra.

Difference (in inches)	Cup Size
1	A
2	B
3	C
4	D
5	DD/E
6	DDD/F

So if your band measures 34” and your bust is 37”, the 3” difference = C cup = 34C.

Sister Sizing

Ever tried a bra that wasn’t your usual size, but it fit like a dream? It could be because of sister sizing. These are sizes with the same cup volume, but different band sizes.

How they work: For every band size you go down, you go up one cup size to maintain the same cup volume (e.g., a 34B has the same cup volume as a 32C). Conversely, for every band size you go up, you go down one cup size (e.g., a 34B has the same cup volume as a 36A).

Example:

Band Size	Cup Size	Sister Size
32	D	34C
34	C	36B

So a 34C, 32D, and 36B all hold the same breast volume. This knowledge is a powerful tool for finding a better fit. If the band feels right but the cups are too small, you can try the sister size with a larger cup size bra.

Why Sizes Vary Across Brands

Just like how your “M” in one brand feels like an “XS” in another, bra size also fluctuates. Different brands use different measurement systems, materials, and moulds. That’s why you can be a 32D in one and a 34C in another. It can be a little confusing, but if you follow the measurement guidelines, then you should be good to go.

Tip: Always try before you buy. Or buy from brands with flexible return/exchange policies.

Common Misconceptions About Bra Sizes

Your cup size is always the same. Even your mood “swings,” so no, cup size is not always the same. They change with band size (remember sister sizes).

D cup = big bust. A 28D and 38D are very different.

Only busty girls need support. Wrong! Everyone needs a good fit and structure because it’s about comfort, not just size.

Bras stretch out, so you have to size down or go tighter. They’re your breasts, not guinea pigs. Start with the loosest hook, tightening it as the band becomes looser to have the same fit and comfort.

Underneat's Fit-osophy

At Underneat, we’re not about fitting your body into a bra; we believe in finding a bra that works with your body. All our designs come with inclusive sizing, soft but supportive cups, and buttery-soft fabrics that move with you and support you like how you support your besties.

TL;DR

If Cinderella didn’t have glass slippers that fit her like a dream, she wouldn’t have made it to the ball, and you deserve the same kind of magic when it comes to your bra. And that starts with understanding your bra size and fit.

If your bra straps keep sliding, your cups are throwing tantrums, and your band’s halfway to your shoulder blades, then the label of your bra size is lying to you. Or at least… It’s trying to be helpful, but needs some context. The truth is, knowing your size means nothing if you settle for a bra that doesn't fit properly.

Need help decoding? Check out our size guide, because you deserve lingerie that fits like it knows you.