Bra Size Calculator

Research consistently shows that 70–85% of bra wearers are in the wrong size, usually wearing a band too large and cups too small. This happens for several reasons: many people use outdated measurement methods that add inches to the band size; bra manufacturers inconsistently size their products; shopping in stores that carry a limited size range (typically 32–40 bands, A–DD cups) forces customers into the nearest available size; and many people are never professionally fitted. The correct fit dramatically improves comfort, posture, and breast health. Getting professionally fitted at a specialty lingerie store (even if you subsequently shop elsewhere) is one of the most impactful actions for bra fit improvement.

Sister sizes are bra sizes that contain the same cup volume despite having different band sizes and cup letters. When you go up one band size, you go down one cup letter to maintain the same cup volume (36B → 38A); when you go down one band size, you go up one cup letter (36B → 34C). This is because cup volume is proportional to the band size. Sister sizing is useful when you need an exact band size that is not available in your cup letter, or when you want to try a smaller band for more support while keeping the same cup volume. A well-fitted bra should ideally be in your true size rather than a sister size, but sister sizes can be a practical solution.

Bra size changes with significant weight fluctuations (loss or gain), pregnancy, breastfeeding, hormonal changes, and simply with age. The breast tissue and supporting ligaments change throughout life. Getting remeasured annually or after any significant body change (weight change of 10+ lbs, finishing breastfeeding, starting or changing hormonal birth control, menopause) is good practice. Many people discover they have been wearing the wrong size for years after their first professional fitting. The discomfort and posture problems associated with incorrect sizing — shoulder pain from tight straps, back pain from inadequate support, skin irritation from band rubbing — often resolve with a correctly fitted bra.

Bra sizing is not standardized across manufacturers. Each brand designs their own patterns, and a 34C from Victoria's Secret may fit very differently from a 34C from Wacoal or ThirdLove. This is due to differences in cup shape (balconette vs plunge vs full cup), wire placement, band stretch, and overall construction philosophy. Additionally, some brands (particularly UK and European specialty brands) size more accurately toward larger cups, while US department store brands tend to make larger cups under smaller letters. Reading brand-specific fit reviews and checking return policies is essential when trying a new brand online.

Yes, significantly. A balconette or plunge-style bra with lower-cut cups may require going up a cup size from a full-coverage bra because there is less fabric to contain the full breast volume. Sports bras and bralettes typically run differently from underwire bras and are often sized by S/M/L rather than numerical sizes. Convertible and strapless bras require a tighter band for security, so some people go down a band size for these styles. Padded bras add volume, which can affect how the cup fits. Always factor in the bra style when interpreting size recommendations.

Signs of an incorrect fit include: the band riding up at the back (too large or stretched-out band); shoulder straps digging in and leaving marks (cups too small, band not providing enough support); the center gore (front panel between cups) not lying flat against the sternum (cups too small); breast tissue spilling over the top or sides of cups (cups too small); cups with excess fabric or wrinkling (cups too large); underwires poking into breast tissue rather than lying on the rib cage (cups too small); and visible bra lines under clothing due to the bra sitting at an angle. Most of these problems stem from wearing a band too large and cup too small.

Bra sizing is fundamentally based on body measurements — underbust circumference and bust circumference — regardless of gender identity. Anyone seeking a bra for breast support or shaping can use the same measurement and calculation method. Transgender women on hormone therapy may find that breast development follows typical cup-to-band proportions over time. Chest binders are sized differently (usually by chest circumference in a simple S/M/L/XL scale). Specialty brands are increasingly designing inclusive sizing and marketing that addresses diverse gender identities. The fitting principles remain the same: measure underbust for band, measure fullest point for cup.

Cup volume refers to the three-dimensional space inside each cup of the bra, determined by both the band size and the cup letter. A 34D and a 38B have different cup letters but hold different volumes of breast tissue — 34D cups are significantly smaller in volume than 38D cups even though both are labeled 'D.' This is why the cup letter alone is meaningless without the band size: a 'D cup' is not a universal volume. Cup volume increases by approximately one fluid ounce per size increment. Understanding cup volume helps explain why the same cup letter fits differently at different band sizes, and why larger-banded bras need larger letters to hold the same volume as smaller-banded bras.