A bra size calculator is a tool that takes your body measurements and figures out your bra size for you. You give it two numbers: your band size (the measurement around your ribcage) and your bust size (the measurement around the fullest part of your chest). It does the math and spits out a size like 34C or 38B.
Modify the values and click the calculate button to use
Bust size: The loose circumference measured around the chest over the fullest part of the breasts, while standing straight with arms to the side, and wearing a properly fitted bra.
Band or frame size: The firm circumference, fitted not tightly, measured directly underneath the breasts.
| Band Size | FR/BE/ES | EU (EN 13402) | US and UK | AU, NZ |
|---|---|---|---|---|
| XXS | 75 | 60 | 28 | 6 |
| XS | 80 | 65 | 30 | 8 |
| S | 85 | 70 | 32 | 10 |
| M | 90 | 75 | 34 | 12 |
| L | 95 | 80 | 36 | 14 |
| XL | 100 | 85 | 38 | 16 |
| XXL | 105 | 90 | 40 | 18 |
| 3XL | 110 | 95 | 42 | 20 |
| 4XL | 115 | 100 | 44 | 22 |
| 5XL | 120 | 105 | 46 | 24 |
Band size equivalents across different international sizing systems
| Difference (in) | US Cup | UK Cup |
|---|---|---|
| < 1 | AA | AA |
| 1 | A | A |
| 2 | B | B |
| 3 | C | C |
| 4 | D | D |
| 5 | E or DD | DD |
| 6 | F or DDD | E |
| 7 | G or DDDD | F |
| 8 | H | FF |
| 9 | I | G |
| 10 | J | GG |
| 11 | K | H |
| 12 | L | HH |
| 13 | M | J |
| 14 | N | JJ |
| Difference (cm) | EU Cup |
|---|---|
| 10-11 | AA |
| 12-13 | A |
| 14-15 | B |
| 16-17 | C |
| 18-19 | D |
| 20-21 | E |
| 22-23 | F |
| 24-25 | G |
| 26-27 | H |
| 28-29 | I |
| 30-31 | J |
| 32-33 | K |
A bra size calculator is a tool that takes your body measurements and figures out your bra size for you. You give it two numbers: your band size (the measurement around your ribcage) and your bust size (the measurement around the fullest part of your chest). It does the math and spits out a size like 34C or 38B.
Sounds simple, right? But here's the thing. A lot of calculators are wrong. Or they give you a size that doesn't account for how different brands fit. Our calculator is different. It's built on the most common sizing system used by major brands, and it gives you a solid starting point. Think of it as your first step, not your final answer.
Before you measure, you need to understand what you're measuring. These two numbers are the foundation of your bra size:
The measurement around your ribcage, right under your bust. This is where the bra band sits. It should be snug but not tight — like a firm hug. The band does about 80% of the support work, so getting this number right is crucial. If it's an odd number, round up to the next even number (31 → 32).
The measurement around the fullest part of your chest, usually across your nipples. Keep the tape loose — just touching your skin, not digging in. This number minus your band size gives you your cup size. The difference in inches = your cup letter.
🧮 The Simple Math:
Bust Size − Band Size = Cup Size
Example: Bust 37" − Band 34" = 3" difference = C cup → Size: 34C
Okay, so you're ready to find your size. Awesome. But before you grab a measuring tape, there are a few things you need to do. If you skip these steps, your result will be off. And we don't want that.
Stand up straight and breathe out. Wrap the measuring tape around your ribcage, right under your bust. The tape should be snug, but not tight — like a bra band that fits well. Make sure the tape is level all the way around. If it's an odd number, round up to the next even number. 31 → 32, 33 → 34, etc.
Now measure around the fullest part of your bust. Keep the tape loose this time — just touching your skin, not digging in. Make sure the tape goes across your nipples and is level all the way around. Write down that number.
Put both numbers into our bra size calculator. That's it. The calculator will do the rest and give you your size plus sister sizes.
So how does the calculator actually work? It's not magic. It's just subtraction. The calculator takes your bust measurement and subtracts your band measurement. The difference tells you your cup size.
📊 Cup Size Chart (Difference = Bust − Band):
0"
AA
1"
A
2"
B
3"
C
4"
D
5"
DD
6"
DDD
So if your band is 34 inches and your bust is 37 inches, the difference is 3 inches = C cup = 34C.
Sounds straightforward, right? But here's where it gets tricky. That formula is just a starting point. Your actual fit depends on the shape of your breasts, the brand, and even the style. That's why we need to talk about sister sizes.
This is the thing most bra size calculators don't tell you. And it's the reason why you might get a 34C from the calculator, but a 36B fits you better. A sister size is a different size that holds the same cup volume. The cup volume stays the same, but the band and cup letter change.
⬆️ Go up one band → go down one cup | ⬇️ Go down one band → go up one cup
👯 Sister sizes of 34C:
Why does this matter? Because not all bras are made the same. A 34C from Brand A might feel tight in the band. So you try the sister size 36B. The band is looser, but the cup volume is the same. It might fit perfectly. Our calculator gives you your primary size AND your sister sizes. Try them all.
Mistake #1: Measuring Over Your Clothes
Your shirt, sweater, or padded bra all add inches. Measure directly on your skin or over a very thin, non-padded bra. Otherwise, your numbers will be too big.
Mistake #2: Using a Stiff Measuring Tape
A metal tape measure from your toolbox won't work. It can't curve around your body. You need a soft, flexible one from any craft store.
Mistake #3: Pulling the Tape Too Tight
Your band measurement should be snug, not painful. If you pull the tape as tight as it will go, your band size will be too small and your bra will feel like a torture device.
Mistake #4: Ignoring the Sister Size
Your calculator size is a starting point. The sister size might be your actual perfect fit. Don't ignore it — try them all.
Mistake #5: Thinking Your Size Never Changes
Weight changes, pregnancy, breastfeeding, even your menstrual cycle can affect your size. Re-measure every six months or so.
You just want a bra that doesn't drive you crazy. You want to put it on in the morning and forget about it. Getting the right size is the first step.
Sports bras are a whole different beast. But the same principles apply. A good sports bra should be snug but not restrictive. Our calculator helps you find your starting size.
Your body has been through a lot. And your bra size has probably changed dramatically. Nursing bras need to be comfortable and easy to use. Our calculator helps you find a size that works for your new shape.
Growing up is hard enough without dealing with a bra that doesn't fit. Our calculator is private, easy to use, and gives them a place to start.
Here's a secret that professional bra fitters use. The band is the most important part of the bra. It does 80% of the work. The straps are just there to help. If your band is too loose, your straps will dig into your shoulders. If your band is too tight, you'll be uncomfortable all day.
The perfect band should be snug on the loosest hook. As the bra stretches out over time, you move to the tighter hooks. That's how you get the most life out of your bra.
Also, did you know that most people wear a band size that's too big? They think a 36 feels comfortable, but a 34 would actually give them better support. The calculator might tell you something unexpected. Trust it. Try it.
Double-check your measurements. Did you measure in the right spots? Was the tape level? Did you breathe out?
Try the sister size. If the calculator says 32DD, try a 34D or 36C. One might feel more familiar.
Different brands fit differently. A 32DD from Victoria's Secret might fit differently than Aerie. Focus on how the bra feels, not the number.
Get a professional fitting. It's free at most stores. They can help you figure out what's going on.
It's very possible. Most people wear a band that's too big and a cup that's too small. A 32DD has the same cup volume as a 36B (they're sister sizes), but the band is tighter and provides better support. Try a 32DD and see how it feels. You might be shocked at how much better it is.
Your band is too big. The band should be level all the way around your body. If it's riding up, it's not snug enough. Try going down a band size and up a cup size (your sister size).
This is usually a band size problem, not a strap problem. If the band is too loose, the straps have to do all the work, and they'll slip off your shoulders. Try a smaller band size. You can also look for bras with racerback or convertible straps.
This is a shape issue, not a size issue. You might have breasts that are fuller on the bottom than the top. Look for bras designed for this shape, like balconette or demi-cup styles. You might also need to go down a cup size.
Again, this is a band problem. Your straps shouldn't be carrying the weight of your breasts. The band should do that. If your straps are digging in, your band is too loose. Try a smaller band size.
No, you're not weird. You just have a less common size. Many brands don't make a lot of bras in 38A. But they do exist. Look for brands that specialize in fuller busts or plus sizes. And remember, you can try sister sizes like 36B or 40AA.
This is frustrating, but it's normal. Different brands use different sizing systems. And different fitters have different methods. Use our calculator to get a starting point, then try on bras in that size and its sister sizes. The right size is the one that feels best on your body.
Light, temporary red marks (like from socks or jeans) are normal. But deep, painful marks that last for hours are not. If your bra leaves angry red grooves, your band is too tight or the cups are too small. Try a bigger band size or a larger cup. Your bra should be snug, not painful.