Cup Size Calculator
Measure your underbust and fullest bust, then get an instant cup-size estimate with confidence score, sister sizes, nearby comparisons, and a clean fit breakdown that keeps users engaged on the page.
Cup size is estimated from the difference between your fullest bust and your calculated band size. Every 1-inch increase usually equals one cup step.
Enter your measurements
Use a soft tape measure. Keep it level all the way around. Measure under the bust snugly, then measure the fullest part of the bust without compressing tissue.
This tool gives a strong starting estimate, not a guaranteed final fit. Bra size can vary across brands, cup constructions, underwires, and fabric stretch.
Recommended size
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Your estimated bra size will appear here after calculation.
Fit confidence
0%Add your measurements and click calculate to see your estimated bra size, sister sizes, and fit guidance.
Sister sizes
Compare nearby sizes
Fit breakdown
Band feel
Your band explanation will appear here.
Cup depth
Your cup explanation will appear here.
Best next step
Your next-step guidance will appear here.
Cup size reference chart
Cup Size Calculator – Find Your Perfect Bra Size Instantly
A cup size is determined by the difference between your fullest bust measurement and your band size. In most sizing systems, every 1-inch increase equals one cup step. This cup size calculator uses your measurements to instantly estimate your bra size, show sister sizes, and guide you toward a better fit.
Getting the right bra size should not be confusing. This smart calculator helps you measure your underbust and full bust, then converts that data into a clear size recommendation with a fit confidence score, cup depth explanation, and nearby size comparisons. Whether you are between sizes, unsure about your current fit, or starting from scratch, this tool gives you a strong, reliable starting point.
Remember, bra sizing is not just about the cup letter. A 34C and a 38C are not the same size because cup volume changes with the band. That’s why this calculator also includes sister size logic and practical fit guidance, so you can move beyond guesswork and find a size that actually feels right on your body.
What Is Cup Size?
Cup size is the difference between your fullest bust measurement and your band size. In most bra sizing systems, every 1 inch of difference usually equals one cup step. A 1-inch difference commonly maps to an A cup, a 2-inch difference to a B cup, a 3-inch difference to a C cup, and so on.
Your cup letter never exists on its own. It always works together with the band size. That is why a 34C and a 38C do not hold the same cup volume, even though they share the same letter.
How This Cup Size Calculator Works
This calculator estimates your bra size by combining your underbust measurement, fullest bust measurement, fit preference, and breast shape. The goal is to give you a strong starting size, then help you refine fit with sister sizes and practical fit checks.
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Measure your underbust
Measure snugly around your ribcage directly under the bust. This number is used to estimate your band size.
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Measure your fullest bust
Measure around the fullest part of your bust while keeping the tape level and relaxed.
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Calculate the difference
The calculator subtracts your estimated band size from your fullest bust measurement. That difference becomes the base for the cup estimate.
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Adjust for fit preference
A comfort-first fit may lean slightly easier in the band, while a snug preference may point toward a firmer feel.
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Refine with breast shape
Breast shape matters because the same measured size can fit differently depending on whether tissue is shallow, projected, fuller on top, or fuller on bottom.
Bra Cup Size Chart
This basic cup chart shows how cup letters are commonly estimated from the difference between bust and band measurements.
| Difference | Cup Size | Typical Meaning |
|---|---|---|
| 1 inch | A | A small step above the band measurement |
| 2 inches | B | One cup size above A in the same band range |
| 3 inches | C | A common mid-range cup progression |
| 4 inches | D | One step above C, still relative to the band size |
| 5 inches | DD | More volume than D on the same band |
| 6 inches | DDD | Another step up in cup depth and volume |
| 7 inches | G | Larger cup progression depending on brand system |
| 8 inches | H | A deeper cup range with more overall volume |
This chart is useful as a starting reference, but it does not tell the whole story. Cup size is relative to band size, which means cup volume grows as the band number increases. That is why the same cup letter can look and feel very different across different band sizes.
What Your Cup Size Result Actually Means
When the calculator gives you a result like 34C or 36B, it is describing two things at once: the band size and the cup size. Understanding both parts is the key to getting a bra that actually fits.
What the band size means
The band size reflects the circumference around your ribcage under the bust. It is the foundation of bra support and should sit level around your body.
What the cup size means
The cup size reflects the difference between your bust and band measurement. It does not describe breast size on its own.
What 34C means
A 34C usually means the bra is built for a ribcage around 34 inches, while the cup is designed for a bust measurement about 3 inches larger than that band size.
If you only focus on the cup letter and ignore the band, you can easily end up in the wrong size. The full bra size matters, not just the letter.
Why the Same Cup Letter Does Not Mean the Same Cup Volume
One of the most common bra fitting mistakes is assuming that the same cup letter always means the same breast volume. It does not. Cup letters are relative, not absolute.
34B is not the same as 38B
Both are B cups because each has about a 2-inch difference between bust and band, but the overall frame is larger on the 38 band. That creates a physically larger cup.
The same logic applies to every cup letter
A 32C, 36C, and 40C all use the same cup letter, but they do not hold the same amount of volume because the band changes the scale of the cup.
This is why comparing full bra sizes is far more accurate than comparing cup letters alone.
What Are Sister Sizes in Bras?
Sister sizes are bra sizes that keep a similar cup volume while changing the band size. When you go up one band size, you usually go down one cup letter to keep the cup volume close. When you go down one band size, you usually go up one cup letter.
34C sister sizes
34C has sister sizes 32D and 36B.
34B sister sizes
34B has sister sizes 32C and 36A.
36D sister sizes
36D has sister sizes 34DD and 38C.
When sister sizes help
- The cups fit but the band feels too tight
- The band feels right but the cups feel slightly off
- A specific bra brand runs tight or loose in the band
Sister sizing can improve comfort, but it should not be used to force a size that does not match your body. The best sister size still needs proper wire placement, smooth cups, and a supportive band.
Signs Your Cup Size May Be Wrong
Spillage over the top or sides
If breast tissue spills over the cups, the cup size is likely too small.
Gaping or empty space
If the cups wrinkle or gap, the cup may be too large, too tall, or simply the wrong shape for your bust.
Underwire sitting on tissue
The underwire should rest on the ribcage, not on breast tissue. If it sits on tissue, the cups may be too small or too narrow.
Center gore not lying flat
If the center part of the bra does not sit against the sternum, the cup size may be too small or the shape may be mismatched.
Band riding up
A band that rides up in the back is often too loose, even if the cups seem acceptable.
Straps doing all the work
If the straps dig in heavily, the band may not be giving enough support.
5-Step Bra Fit Check After Using the Calculator
A calculated size is your starting point. Real fit is confirmed by how the bra behaves on your body.
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Check the band
The band should sit level around your ribcage and feel snug without pain.
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Scoop and swoop
Lean forward slightly and pull all breast tissue fully into the cups.
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Check the center gore
The center gore should lie flat against your sternum in most wired bras.
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Check the underwire
The wire should surround the breast root and sit on the ribs, not on tissue.
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Move around
Raise your arms, sit down, and walk around. A good bra should stay in place without shifting or digging.
Why Breast Shape Can Change Bra Fit
Even when the calculator gives the correct size range, bra fit can still vary because breast shape affects how a cup fills and where support is needed.
Shallow shape
Shallow breasts tend to spread wider across the chest with less forward projection. Open-top or wider cups may fit better.
Projected shape
Projected breasts need more forward depth. Shallow cups may feel tight even if the size looks correct on paper.
Full on top
This shape may need more upper-cup flexibility to avoid cutting in at the neckline.
Full on bottom
This shape often benefits from strong lower-cup support and a more open upper edge.
That is why the best-fitting bra is about both size and shape, not size alone.
What to Do If You’re Between Bra Sizes
If you fall between two sizes, start by deciding what feels most important: support, comfort, or a specific bra style.
- Choose the firmer band if support is the main goal.
- Choose the more forgiving band if comfort is the priority.
- If the cup feels right but the band feels wrong, try a sister size.
- If one style gaps and another feels perfect in the same size, shape is probably the issue, not the measurement.
- Being between sizes is normal. The right choice depends on how the bra feels in motion, not only on the label.
Compare Nearby Bra Sizes
After finding your estimated size, the next best step is comparing it with nearby sizes. This helps you understand whether you may need a larger cup, a smaller cup, or a sister size with a different band feel.
Helpful Bra Size Tools
If you want a more complete sizing picture, these tools can help you refine your fit further.
Cup Size Calculator FAQs
How do I calculate my cup size?
Is cup size the same as breast size?
Is a D cup always large?
Why do 34B and 38B look different if both are B cups?
What if my bra cup gaps?
What if my bra is spilling over?
Are bra cup sizes the same across brands?
What is a sister size?
Should I size up in the band for comfort?
Can breast shape affect fit even if the size is correct?
Still Unsure About Your Bra Size?
Use your calculator result as a starting point, then compare nearby sizes, check your sister sizes, and confirm the fit with a real bra fit test. If you want a fuller sizing result, explore the broader tools and detailed comparison guides across the site.
