Bra Size Calculator
Find a useful starting size without guessing
A bra size calculator is most helpful when shopping has turned into trial and error. Many people know a bra feels off but cannot immediately tell whether the problem starts with a loose band, a cup that is too small, or a size label that never matched their measurements in the first place. This tool gives you a practical first estimate using the two measurements that drive most standard bra sizing systems: the underbust and the full bust. It is not a promise that every bra in that size will feel perfect, but it is a much better place to start than randomly trying adjacent sizes.
The reason this matters is simple. The band does most of the support work, while the cup letter describes how much fuller the bust measurement is than the band measurement used for sizing. If the band is too large, the bra can ride up your back and shift during the day. If the cup is too small, tissue can spill over the top or sides. If the cup is too large, the fabric may wrinkle or gape. A quick estimate helps you sort out which issue is most likely before you spend time comparing styles.
What to measure before you use the form
For the underbust, wrap a soft tape measure around your ribcage directly under the bust. Pull it snugly so it reflects the support area of the bra band, but do not pull so hard that the tape digs in or distorts the number. For the bust measurement, stand naturally and measure around the fullest part of the bust with the tape parallel to the floor. The bust measurement should be firm enough to stay level, yet loose enough that it does not compress the breast tissue. Using the same tape and the same body position for both measurements usually gives the most consistent result.
If possible, measure while wearing a thin, non-padded bra or while braless. Heavy padding can inflate the bust number and produce a cup estimate that looks larger than what you would actually buy. It also helps to breathe normally and check the tape in a mirror. Small tilts matter. A tape that drops in the back or angles upward in front can change the numbers enough to move you into a neighboring size.
The three form inputs are straightforward, but they are worth interpreting carefully. The underbust field is the snug ribcage measurement. The bust field is the circumference at the fullest part of the bust. The unit selector tells the calculator whether the numbers are already in inches or need to be converted from centimeters. That last step is important because the cup mapping in this calculator follows the common inch-based method. If you enter centimeters, the calculator converts them first and then applies the same rules.
How the calculator turns measurements into a size
The logic is compact. First, the calculator converts the measurements to inches if you entered centimeters. Second, it rounds the underbust to the nearest even band size. Third, it compares the bust measurement with that rounded band size to get a difference in inches. That difference determines the cup letter. A difference of about 1 inch maps to A, 2 inches to B, 3 inches to C, 4 inches to D, and so on through the table shown later on the page.
Like most practical calculators, the broader structure can still be described mathematically as a function of measured inputs:
In this calculator, those inputs are not abstract business variables or engineering loads. They are your body measurements. The idea is the same, though: collect the necessary inputs, normalize units, and transform them into a label that is easier to use while shopping. The page originally included a generic weighted-sum formula, and it is still worth keeping here because it highlights a broader calculator pattern:
Bra sizing is simpler than that generic model. You are not summing many weighted components. You are rounding the underbust to a band size, then using the bust-to-band difference to choose the cup. That is why this calculator is easy to use once the measurements are sound. Most confusion comes from measuring loosely, mixing centimeters and inches, or forgetting that the cup letter depends on the final band size used in the calculation, not just on the raw underbust measurement alone.
A realistic worked example
Suppose your snug underbust measurement is 31 inches and your bust measurement is 35 inches. The calculator rounds 31 to a 32 band because the tool uses the nearest even number. Next, it subtracts the band size from the bust measurement: 35 minus 32 equals 3. In the sizing table used here, a 3-inch difference corresponds to a C cup. The estimate is therefore 32C.
That result is a starting point, not the end of the conversation. If a 32 band feels firmer than you like in a particular brand, you might also test a sister size such as 34B. The cup letter changes because the band changed, but the cup volume is roughly comparable. On the other hand, if a 32C feels loose in the band and roomy in the cup, the fit issue is not always solved by moving straight to a smaller cup. Sometimes the more useful adjustment is a tighter band with a coordinated cup shift.
How to read the result like a shopper, not a spreadsheet
When the result appears, treat it as the best next size to try on or search for online. It is not a diagnosis of every fit problem you have ever had. Different bra constructions can feel very different at the same labeled size. A full-coverage bra may contain tissue well in one size, while a plunge bra in the exact same size might cut in at the top because the cup shape is lower and more open. Likewise, a stretchy bralette-style bra may tolerate a broader range of body measurements than a structured underwire bra with firm fabric.
Use the number and letter together. A band without the right cup can be misleading, and a cup letter without the band is incomplete because cup volume is relative to band size. A D cup on a 30 band is not the same volume as a D cup on a 38 band. That is one reason bra sizes can sound more dramatic than they really are. The letter is only part of the label.
After you calculate a size, it helps to ask a few practical questions. Does the band seem plausible given the ribcage measurement you took? Does the cup letter look consistent with how your current bras fail? If your favorite bra rides up in the back, the band estimate may explain why. If cups cut in but the band already feels firm, a larger cup within the same band family may be the likely direction. This is the kind of real-world interpretation that turns a number into a useful shopping decision.
Assumptions, edge cases, and why trying on still matters
This calculator follows a common U.S.-style rule set, which makes it broadly useful but not universal. Brands can grade cups differently, some regions use different letter progressions after D or DD, and certain specialty bras fit more by shape than by pure measurement math. People with very projected breasts, very shallow breasts, significant asymmetry, or changing size across a cycle may find that the estimate is close but not complete. That is normal.
The result is also sensitive to rounding. Because the band is rounded first, a small shift in underbust measurement can change the band and therefore change the cup letter attached to the final size. That does not mean your body changed dramatically; it means the size label depends on the rounded band used in the method. In practice, that is why neighboring sister sizes are often worth keeping in mind whenever you are between measurements.
If you want the best outcome, use this calculator as the measurement step in a broader fitting process. Start with the estimated size. Try that size in the bra style you care about. Then judge the band tension, wire placement, cup containment, strap comfort, and how the bra feels after a few minutes of wear. The calculator gives you a reliable launch point. Your body, the bra design, and your comfort preferences determine the final choice.
How Bra Sizing Works
Choosing a bra that fits comfortably starts with understanding what the size label is trying to describe. The number is the band size, which relates to the ribcage measurement under the bust. The letter is the cup size, which reflects how much larger the full-bust measurement is than the band used in the final size. This calculator uses the familiar underbust-plus-difference approach because it is easy to measure at home and it provides a practical baseline for most U.S. bra shopping.
The core cup formula used on this page is preserved below in MathML. It expresses the cup-driving difference as the bust measurement minus the band measurement:
Here, Bu is the bust measurement and Ba is the band measurement used in sizing. Each inch of difference corresponds to a larger cup step. When the form is set to centimeters, the calculator divides both measurements by 2.54 first so that the same inch-based cup mapping can be applied consistently.
The band result is intentionally rounded to the nearest even number because most mainstream band sizes are sold that way. That rounding step is easy to overlook, but it explains why two very similar underbust measurements can produce slightly different labels. A 31-inch underbust often rounds to a 32 band. A 33-inch underbust often rounds to a 34 band. Once the band changes, the cup letter linked to the final size can shift even if the bust measurement hardly moves.
Common Band Sizes
| Underbust (inches) | Band Size |
|---|---|
| 28-29 | 30 |
| 30-31 | 32 |
| 32-33 | 34 |
| 34-35 | 36 |
| 36-37 | 38 |
| 38-39 | 40 |
Cup Size Differences
| Difference (inches) | Cup Size |
|---|---|
| 1 | A |
| 2 | B |
| 3 | C |
| 4 | D |
| 5 | DD/E |
| 6 | DDD/F |
Those tables are intentionally simple. They are good for learning the logic and generating a useful first size, but real products vary. A bra with firm fabric and shallow cups may feel small even when the numbers suggest it should fit. A softer, stretchier style may feel forgiving in the same labeled size. That is why the calculator gives an estimate rather than an absolute verdict.
One of the most helpful concepts for interpreting the result is the idea of sister sizes. Sizes such as 34C, 32D, and 36B do not have identical fit, because the band changes, but the cup volume stays in a similar neighborhood. If you know the calculator is close yet the band feel is not right, sister sizing helps you adjust intelligently rather than wandering randomly through the size rack.
Fit clues also matter. If the center gore does not tack, the cup may be too small or the bra shape may be wrong. If the band rides up in the back, it is often too loose. If straps dig deeply into the shoulders, that can be a sign the band is not carrying enough of the load. If cups gape at the top while the underwire feels wide, the issue may be cup shape rather than simply needing a smaller size. These observations work best when paired with a measurement-based estimate like the one this calculator provides.
Accurate measuring technique makes a bigger difference than many shoppers expect. Re-measuring once or twice is worth the minute it takes. A tape that slips downward at the back can make the underbust seem larger. A padded bra can make the bust seem fuller. Holding the tape too loosely at the underbust or too tightly at the bust can send the final estimate in the wrong direction. If your first result surprises you, the smartest next step is usually to measure again before assuming the math is wrong.
Regional sizing is another source of confusion. This calculator follows a U.S.-style cup naming sequence, especially in the higher letters. UK and EU systems often use different lettering beyond D or DD, and some brands print multiple regional sizes on the same tag. That does not make the calculator unhelpful; it simply means the resulting size may need a label conversion when you shop internationally.
The formula below is repeated here because it captures the heart of the method in the clearest possible way:
To summarize, start with accurate measurements, apply the formula , and then interpret the result alongside real fit feedback. Armed with that combination, you can shop more efficiently, compare neighboring sizes with more confidence, and understand why the band number and cup letter always have to be read together.
Body size can also change over time. Weight shifts, hormonal changes, pregnancy, breastfeeding, or athletic training can all move the numbers enough to change the fit of bras that used to feel reliable. Rechecking your measurements every so often is a low-effort way to keep your bra drawer working for you rather than against you. If a once-comfortable bra suddenly feels off, a fresh measurement is often more useful than forcing the old size to keep working.
In short, this calculator is strongest when you use it as a starting map. It narrows the search, gives you a logical first size, and helps you understand why a sister size might solve a fit issue better than simply going up or down one cup. Pair the estimate with careful try-ons, and you will make faster, more confident decisions than guessing from habit alone.
Your estimate is a practical starting size for trying on bras or narrowing online options. Final comfort still depends on brand, cup shape, and style.
Mini-Game: Fit Match Rush
This optional canvas game turns the sizing rule into a fast boutique challenge. A shopper card shows an underbust and bust measurement. Your job is to tap or click the fitting-room door with the correct estimated size before the shopper loses patience. Early rounds stay in inches, then the game mixes centimeters and trickier sister-size decoys so you feel why conversions and band rounding matter.
Best score: 0. Build a streak for bonus points and survive the mixed-unit rush.
Takeaway: the band is rounded first, so a small change in underbust measurement can change the final size label even when the bust measurement stays the same.