Bra Size Calculator

How Does the Formula Work?

The bra size calculator determines your correct bra size from two simple measurements: the underbust (band) circumference and the bust (fullest point) circumference. It converts between US, UK, and EU sizing systems and provides sister sizes — alternative sizes with the same cup volume but different band lengths. Studies consistently show that approximately 80 percent of women wear the wrong bra size, most commonly a band too large and cup too small. A properly fitted bra improves posture, reduces back pain, and provides better support and comfort throughout the day.

How to Measure Correctly

Accurate measurements are essential for a correct fit. Use a soft measuring tape and wear an unpadded bra or no bra. For the underbust measurement, wrap the tape snugly around your ribcage directly below your breasts. The tape should be level all the way around, firm but not tight enough to dig into skin. For the bust measurement, wrap the tape around the fullest part of your bust (usually at nipple level), keeping the tape level and not pulling it tight — it should just touch the skin. Stand straight with arms at your sides. Measure in front of a mirror to ensure the tape is level. Take both measurements in the same unit (either centimeters or inches) and enter them into this calculator.

Understanding Sister Sizes

Sister sizes are a powerful fitting concept that most women do not know about. Sister sizes share the same cup volume but have different band circumferences. If your calculated size is 34C, the sister size up (smaller band, larger cup) is 32D, and the sister size down (larger band, smaller cup) is 36B. All three hold the same cup volume — the difference is only in how tight the band fits. This is incredibly useful when a store does not carry your exact size, when a particular brand runs differently, or when you want a tighter or looser band feel. Moving up one sister size gives a snugger band with a labeled larger cup; moving down gives a looser band with a labeled smaller cup.

Common Fit Problems

Band riding up in the back means the band is too large — try a smaller band (and go up a cup size to maintain volume). Cups overflowing (creating a double-breast effect) means the cup is too small — go up a cup size. Cups wrinkling or gapping means the cup is too large — go down a cup size. Straps digging into shoulders means the band is not providing enough support (the band carries 80 percent of the support, not the straps) — try a smaller band. The underwire poking or sitting on breast tissue means the cup is too small or the wrong shape. Center gore (the piece between the cups) not lying flat against the sternum indicates a cup size too small.

Size Systems Explained

The US system uses even numbers for bands (30, 32, 34, 36, 38) and letters for cups (A, B, C, D, DD/E, DDD/F, G, H). The UK system uses the same bands but different cup labeling beyond D (DD, E, F, FF, G, GG, H). EU sizing uses centimeter-based bands in multiples of 5 (65, 70, 75, 80, 85) with international cup letters. French and Spanish sizing adds 15 to the EU band (EU 75 = FR/ES 90). Italian sizing uses numbered cups (1, 2, 3, 4, 5). Australian sizing is close to UK. Japanese sizing uses centimeter bands similar to EU. This calculator handles the three major systems — US, UK, and EU — which cover the vast majority of global brands.

When to Get Remeasured

Bra size can change throughout life due to weight fluctuations, hormonal changes, pregnancy, breastfeeding, menopause, and aging. Experts recommend measuring every 6 to 12 months or whenever you notice fit changes. Weight gain or loss of 5 kilograms or more typically affects bra size. During pregnancy, breast size can increase by 1 to 3 cup sizes. Hormonal birth control can also affect breast size. Even within a single menstrual cycle, breast size can vary by half a cup size. This calculator is designed for quick remeasurement — keep a measuring tape handy and check your size whenever your current bras start feeling uncomfortable.

Sports Bra Sizing

Sports bras require special attention because inadequate support during exercise can cause breast pain and long-term damage to Cooper's ligaments (the internal support structure). For low-impact activities like yoga, a compression-style bra in your regular size usually works. For medium impact (cycling, hiking), an encapsulation bra with cups provides better support. For high impact (running, HIIT, jumping), you need maximum support — often a band size down and a cup size up from your regular bra for a snugger fit. Sports bra sizes often use S/M/L rather than band-cup — use your calculated size to match the brand's conversion chart. This calculator gives you the foundation to select the right sports bra for your activity level.