Enter your two measurements to get recommendations across all sizing systems.
Sports bra fit is an art, not a science. Your results are a starting point, not a final answer. If your bust is 4 or more inches larger than your ribcage, consider sizing up or trying a band/cup sports bra for a better fit.
✦D+ fit — your bust is 4+ inches larger than your ribcage. Band/cup styles often provide better support.
!You’re not alone
Lots of athletes share your size, but many sports bra brands don’t make it yet. Many start their band sizes at 32, and when the band fits, the cup often doesn’t. That’s a gap in what’s available, not a problem with your body.
Things to try:
A sister size with a bigger band and a smaller cup letter
Highly adjustable styles (like SheFit) with adjustable straps, bands, and closures
An in-person fitting at a specialty bra shop
Alpha Size
—
Band / Cup
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T-Shirt / Womens
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SheFit
—
Numerical
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Youth Alpha
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1
Ribcage measurement
Wrap a soft measuring tape snugly around your torso directly under your bust. Exhale normally, then note the number. Round to the nearest whole inch. This is your ribcage measurement.
2
Bust measurement
Wrap the tape around the fullest part of your chest — usually across the nipple line. Keep it parallel to the floor and snug but not tight. Round to the nearest whole inch.
3
Enter and calculate
Type both numbers into the calculator above and press Calculate my size. Your sizes across all systems appear instantly.
💡 Best results: Measure in a non-padded, well-fitting bra (or no bra). An underwire or heavily padded bra can distort both measurements.
Tap a chart to open it. After you calculate, your row is highlighted in coral.
Ribcage → alpha size
Ribcage
Alpha size
D+ alpha size
Bust minus ribcage → cup letter. Letters beyond D vary by brand; this shows the most common US conventions.
Difference
Cup size
What it means
Ribcage → numerical size, with alpha cross-reference
Ribcage
Numerical
D+ numerical
Alpha size
What is a sister size? Sister sizes share the same cup volume with a different band. Go up one band and down one cup letter — or down one band and up one cup — for the same fit in a different band tension. Useful when a size is out of stock or a band feels too tight or loose.
Enter your measurements above and tap Calculate to see your personal sister sizes.
Each row is one set of sister sizes. Calculate above and your row lights up.
Band sizes run across the top.
How cup size works
Cup size is the difference in inches between your bust and your ribcage. That’s it. A is 1 inch. B is 2 inches. C is 3 inches.
Bust
36"
−
Ribcage
34"
=
2 inches = B cup
B
Simple enough through D. Then brands get creative with the alphabet — see below.
Let’s clear something up
D and DD are not big sizes. They’re just… sizes.
Somewhere along the way, a myth took hold: that D or DD means an exceptionally large chest. It doesn’t. A D cup simply means your bust measures 4 inches more than your ribcage. That describes an enormous range of bodies — from a petite 28D to a 44D — which have almost nothing in common in terms of actual volume, shape, or how a bra fits.
This myth has done real harm. It led brands to avoid letters like E, F, and G — doubling up on D instead — as if those letters were something to be ashamed of. It has contributed to generations of women and girls being told they’re wearing the “wrong” size when the industry simply stopped making the right one.
Bodies and breasts come in every possible shape and size. A cup letter doesn’t capture breast shape, tissue distribution, root width, or projection.
Same cup, three names
US
DDD
UK
E
EU
F
These are often the same cup. The letter depends on where the bra was made and how the brand labels it.
A tag says Medium or 34C. Your body doesn’t read tags.
There’s no universal sizing standard behind those letters and numbers. A Small in one brand and style can fit completely differently from a Small in another brand and style — even a different style within the same brand. That’s not you sizing wrong — it’s the label doing an incomplete job.
Different fit models. Every brand designs around its own idea of a “typical” body — a fit model that may look nothing like yours.
Different styles. A racerback tank and a molded compression crop are built on different patterns, even when they come from the same brand.
Different fabrics. A firm, compressive fabric behaves differently on the body than a soft, stretchy one, even at the exact same size label.
Different cup and band construction. How the cup is shaped and where the band sits changes where support actually lands on your body.
Sizing shifts over time and by market. A label today doesn’t always mean what it meant a decade ago, or what it means from a brand overseas.
Picture three 13-year-old girls. Each measures a 30" ribcage and a 33" bust, so the calculator gives all three the same size. The same bra could feel great on one, slide around on another, and dig in on the third. Nothing is wrong with any of them. Two numbers simply can’t describe a whole body.
Athlete 1
30" / 33"
Broad shoulders, short torso
Athlete 2
30" / 33"
Long-waisted, narrow sloped shoulders
Athlete 3
30" / 33"
Wide back, breasts sit lower
Shoulder shape. The distance from the top of your shoulder to your collarbone, and how sloped your shoulders are, affects whether straps stay put, slip off, or dig in.
Torso length. A shorter or longer torso changes where the band lands and how much strap length you need.
Breast shape and placement. Where breast tissue starts on the chest, how wide it spreads, and how far it projects forward all affect how a cup or compression panel fits.
Back width and ribcage shape. A rounder or flatter ribcage changes how a band hugs your body, even at the same measurement.
What you like to feel. Some athletes love snug compression and size down to get it. Others notice every seam, tag, or tight band. Your comfort preferences are part of fit too.
Band sits snug and parallel to the floor, without riding up
Straps stay put without digging in or slipping off
No gaping, spilling, or pinching at the cup
Full range of motion — jump, reach, twist — without shifting
Not quite right? Try a sister size, a different style, a different brand, or size up or down.
A reminder
Your body is not the problem.
The sports bra you see in an advertisement, or even one that’s “supposed” to be your size, won’t always be the right fit for you. Models in ads are chosen to fit the bra, not the other way around. If something doesn’t feel right, that’s information, not failure. Keep searching until you find the one that lets you move, play, and feel like yourself.
Keep searching. Your fit is out there.
Additional Bra Fit Resources
Watch our Bra Fit Video
FAQs
"Alpha size" refers to Sports bras that are sized by Letters like, “S” for Small, “XS” for Extra Small, “M” for Medium, etc. Alpha size bras are typically determined using the ribcage measurement.
“D+” is meant to accommodate individuals with a 4 inch difference (or greater) between the ribcage and bust measurement. An individual may have a ribcage measurement that indicates they might fit in a certain size (XS, S, M, L, etc). However, having a fuller bust means that you might consider sizing up to accommodate your chest. So if the ribcage measurement suggests a size M, you might size up to a size L. As always, wear what makes you feel most comfortable and confident.
“Band/cup“ refers to Sports bras that are sized using both the Rib Cage measurement and the Bust measurement. The different between these measurements in inches tells you the Cup size (A Cup=1 inch difference, B Cup= 2 Inch difference, etc). We combine the Ribcage measure (Plus 2-4 inches) with the cup size for the band/cup size for example: 34D, 38C, etc.
A band/cup size bra can offer more support for bodies with fuller chests especially when the ribcage measurement and bust measurements are significantly different. The band along the ribcage provides the most support during activity (not the straps at the top).
Each brand has different methods for sizing. Some brands use sizes between 0-22 in even increments for sizing.
This will vary by brand. For some brands, 1X will come after size XL. These iterations typically go XS, S, M, L, XL, 1X, 2X, 3X, 4X, etc... However other brands will add and X to indicate an increase in size like, "S, M, L, XL, XXL, XXXL, XXXXL, etc) If you have access to the brand's sizing guide it should help you determine what measurements correspond to your recommended size.
A training sports bra is a great option for younger athletes who are just starting to develop breast buds. The material is usually very light without too much compression so that the wearer can start to get used to wearing a sports bra under clothing. These typically come in youth sizes. Sizing is determined by the rib cage measurement.
The calculator is just a recommendation. If you have been comfortably wearing a certain size in a certain brand then you can certainly continue to wear what is most comfortable. Our bodies are always changing. Even from day to day your chest size might change based on a number of factors. You might consider the calculator recommendations as a sister size option. Ultimately it is up to you to determine what is most comfortable for you and your activities.
