Bra Size Calculator
Find your bra size in US, UK, or EU sizing from your underbust and bust measurements, with sister size alternatives.
About the Bra Size Calculator
Finding the right bra size makes a significant difference in comfort and support. Most people are wearing the wrong size — either a band that's too loose, a cup that's too small, or both. Our bra size calculator takes two simple measurements (underbust and bust) and calculates your size in US, UK, or EU sizing systems, plus shows sister sizes for when your exact size isn't available.
Use a soft tape measure for best results. Measure twice: once snugly just below your bust for the underbust/band measurement, and once loosely across the fullest part of your chest for the bust measurement. The difference between these two measurements determines your cup size.
How It Works
The band size is your underbust measurement rounded up to the nearest even number. The cup size is determined by the difference between your full bust and band size: 1 inch = A, 2 = B, 3 = C, 4 = D, etc. Sister sizes share the same cup volume but use a different band (go up one band, down one cup — or vice versa).
How to Measure Correctly
Taking accurate measurements is the most important step in finding your correct bra size. For the underbust measurement, wrap the tape measure around your ribcage directly under your bust. The tape should be snug — not tight enough to restrict breathing, but firm enough that you can only fit one finger underneath. This measurement determines your band size. Take the measurement on an exhale for the most consistent result. If your measurement falls on an odd number, the calculation rounds up to the next even number for your band size.
For the bust measurement, lean forward slightly at about 45 degrees so your breast tissue falls naturally forward into the cup, then wrap the tape measure around the fullest part of your chest at nipple level. Keep the tape parallel to the ground and avoid pulling it tight — the tape should rest against your skin with no tension. This is your bust measurement. The difference in inches between your bust measurement and your band size determines your cup size.
Measurements alone won't give you a perfect fit — different brands, styles, and materials fit differently even in the same nominal size. Use this calculator as a starting point, then try on several sizes around your calculated size. Signs of a good fit: the band sits level around your body (not riding up in the back), the centre front gore sits flat against your sternum, underwires lie flat against your ribs, and cups contain all breast tissue without spillage or empty space. When in doubt, a professional bra fitting at a lingerie retailer is the most accurate method.
US, UK, and EU Sizing Differences
Bra sizing systems differ significantly between countries, which causes confusion when shopping across borders. The US and UK share the same band sizes (both in inches), but the cup labelling diverges starting at the E cup: what the US calls DD, the UK calls DD, but what the US calls DDD, the UK calls E. European sizing (France/Belgium, Spain, and Italy) uses band sizes in centimetres — typically 65, 70, 75, 80, 85, 90, 95 — and the cup progression uses the alphabet without the DD convention. A US 36C is roughly equivalent to a UK 36C and an EU 80C.
Australian sizing largely follows the UK system. Japanese sizing uses centimetre measurements for both band and cup. When purchasing from international retailers, always check the brand's specific size conversion chart — no universal standard exists, and even within the same country, brands may fit differently. This calculator provides US, UK, and EU sizing, which covers the most common international shopping scenarios.
Sister sizes are an important concept for international shopping and when your exact size isn't available. Sister sizes share the same cup volume as your true size but pair it with a different band. Going up one band size and down one cup gives a looser band with the same cup volume. Going down one band size and up one cup gives a tighter band with the same cup volume. For example, 34C, 36B, and 38A are sister sizes — the cup holds the same volume, but the band fit changes. If you fall between sizes, sister sizing helps you find the best available option.
Common Fitting Issues and Solutions
Band riding up: The most common fitting problem. If the back of your bra rides up toward your shoulder blades, the band is too large. Try a smaller band size (and one cup size up to maintain volume). The band should provide 80% of the bra's support — if the band doesn't stay down, the bra can't do its job regardless of cup size. As bras age and the elastic stretches, the band becomes looser — this is why bras have multiple hook positions: start on the loosest hook and tighten as the bra ages.
Cup spillage or quadboob: If breast tissue spills over the top or sides of the cup, the cup is too small. Go up one or more cup sizes. Many women wear cups that are too small because they resist the letter progression — there's no reason a DD or larger cup should feel different than a B or C. Cup size is a ratio relative to band size, not an absolute measurement. A 32G and a 40G look very different on the body despite sharing a cup letter.
Underwire digging: If the underwire pokes your underarm or sits on breast tissue rather than against your ribs, the cup is too small or the underwire shape doesn't match your breast root width. Underwires should encircle the entire breast base and sit flat against the chest wall all the way around. If underwire comfort is a concern, try plunge styles (which have a shorter centre gore) or soft-cup bras that provide support without underwire. Some people find that going up a band size and down a cup size (sister sizing) helps the underwire sit more comfortably.