Bra Size Calculator
How Bra Sizing Works
Choosing a bra that fits comfortably and supports properly begins with accurate measurements. This calculator uses the widely adopted underbust plus difference method. Measure snugly around your ribcage directly under the bust to obtain the band measurement and loosely around the fullest part of your bust to obtain the bust measurement. The difference between these two numbers determines the cup size, while the band measurement is converted into an even-numbered band size.
The core formula for calculating the cup size can be expressed in MathML as follows:
where Bu is the bust measurement and Ba is the band measurement. Each inch of difference corresponds to an incremental cup letter (1 inch = A, 2 inches = B, etc.). For centimeter inputs we convert the values to inches by dividing by 2.54 before applying the same formula. The resulting band size is the nearest even number to the adjusted underbust measurement.
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 |
The tables above give a quick reference, but bra sizing can vary between brands and regions. European and UK sizing use similar band numbers but may differ in cup labeling beyond DD. The calculator provides a starting point, yet trying on bras remains the best way to ensure comfort and support.
Accurate measurements should be taken while standing straight with a non-padded bra or without any bra. The tape measure must be parallel to the floor for consistency. If your measurements fall between sizes, consider the fit you prefer. Some individuals favor a tighter band for additional support, while others opt for a looser band for comfort during extended wear.
Beyond the basic calculation, understanding the biomechanics of support can deepen appreciation for proper bra fit. The band carries the majority of the breast weight, while the straps offer stabilization. A band that rides up the back or straps that dig into shoulders typically indicate an incorrect size. When the cup is too small, overflow occurs; when too large, there is gaping. Addressing these issues starts with correct sizing, which this calculator facilitates.
In addition to comfort, properly fitted bras can have health benefits. Poor fit may contribute to back or shoulder pain. By ensuring the weight is distributed across the band rather than the straps, you reduce strain on shoulder muscles. Athletes often use sports bras engineered with compression or encapsulation designs based on the same sizing principles described here.
Different styles—balconette, plunge, full coverage—will fit differently even at the same measured size. The shape of the underwire, the stretch of the fabric, and the manufacturer’s pattern grading can all affect fit. Therefore, consider the calculator a guide that yields a theoretical size. The experiential aspect of trying on bras remains crucial to find the ideal combination of support, comfort, and aesthetic.
To summarize, start with accurate measurements, apply the formula , and consult the tables. Armed with this information, you can more confidently navigate the vast array of bras available. Whether shopping online or in-store, having precise band and cup numbers reduces frustration and increases the likelihood of finding the perfect fit.
Many shoppers also encounter the concept of sister sizes. These are bra sizes where the cup volume remains roughly the same while the band size changes. For example, a 34C shares a similar cup volume with a 32D or a 36B. Sister sizes can provide alternative fits when your usual size is unavailable or when slight adjustments are needed for comfort. Understanding this relationship empowers you to experiment confidently within a narrow range of sizes.
International conversions add another layer of complexity. While this calculator follows the U.S. convention, you might encounter European, U.K., or Japanese sizing systems. Each uses distinct labeling for cups beyond the standard A-D range. Referencing a conversion chart can translate your calculated size into the appropriate local designation. Awareness of these regional differences is essential for online shopping or travel.
Finally, remember that body shapes change over time due to weight fluctuations, hormonal shifts, or athletic training. Regular reassessment ensures your bras continue to fit optimally. By pairing periodic measurements with this calculator, you maintain a wardrobe that supports both comfort and confidence.