Calculate your UK bra size from underbust and overbust measurements with sister sizes and international equivalents.
Bra sizing varies between brands and styles. This calculator provides a starting point based on UK sizing standards. Always try on for the best fit.
Finding the right bra size is essential for comfort, support, and confidence. Research consistently shows that a large proportion of women wear the wrong bra size, often because they have not been measured recently or because sizing conventions are confusing. The UK uses its own bra sizing system that differs from both EU and US standards, particularly in the cup sizes above D. UK bra sizing is based on two measurements: the underbust (band) measurement and the overbust (bust) measurement. The difference between these two measurements determines the cup size. The band size is the underbust measurement rounded to the nearest even number, and the cups follow the sequence AA, A, B, C, D, DD, E, F, FF, G, GG, H, HH, J, JJ, K. This double-letter pattern (DD, FF, GG, HH, JJ) is unique to UK sizing and can cause confusion when comparing to other systems. This calculator takes your measurements and determines your UK bra size, along with sister sizes (alternative sizes with equivalent cup volume) and international equivalents for EU and US sizing. Whether you are shopping online, buying from an international brand, or simply checking your current size, having accurate measurements is the foundation of a good fit.
To calculate your bra size: 1. Choose your measurement unit: inches or centimetres. If you are measuring in centimetres, the calculator converts automatically. 2. Measure your underbust. Wrap a soft measuring tape snugly around your ribcage, directly beneath your bust. The tape should be firm but not tight. Exhale normally and read the measurement. Enter this value in the "Underbust Measurement" field. 3. Measure your overbust. Wrap the tape around the fullest part of your bust. Do not pull the tape tight or compress. Stand upright and breathe normally. Enter this value in the "Overbust Measurement" field. 4. Review your results. Your UK bra size is shown as the primary result (e.g. 34D). Sister sizes show alternative fits with the same cup volume. EU and US equivalents help when shopping from international brands. 5. Use the sister sizes if your band feels too tight (try the sister size up, which has a larger band and one cup size smaller) or too loose (try the sister size down, which has a smaller band and one cup size larger).
UK bra sizing follows a straightforward formula: Band Size: Round the underbust measurement to the nearest even number within the range 28 to 44. For example, an underbust of 33 inches rounds to 34. Cup Size: Calculate the difference between the overbust measurement and the band size. Each inch of difference corresponds to one cup size: 0 = AA, 1 = A, 2 = B, 3 = C, 4 = D, 5 = DD, 6 = E, 7 = F, 8 = FF, 9 = G, 10 = GG, 11 = H, 12 = HH, 13 = J, 14 = JJ, 15 = K. Sister Sizes: The sister size up increases the band by 2 and decreases the cup by 1 letter. The sister size down decreases the band by 2 and increases the cup by 1 letter. This maintains roughly the same cup volume. EU Equivalent: UK band sizes map to EU centimetre bands: 28=60, 30=65, 32=70, 34=75, 36=80, 38=85, 40=90, 42=95, 44=100. US Equivalent: Band sizes are the same, but cup sizes diverge after D. UK DD = US DD/E, UK E = US DDD/F, and so on. If you provide measurements in centimetres, they are converted to inches by dividing by 2.54 before the calculation.
Keep in mind that sizing varies between brands and even between styles from the same brand. Moulded and padded bras may fit differently from unlined styles. When trying on a new bra, the band should be snug on the loosest hook (so it can be tightened as the elastic stretches over time). The centre panel should sit flat against your chest, and the underwire should follow the natural crease of your bust. If you are between sizes, try both the calculated size and the nearest sister size. Professional bra fitters at department stores and specialist retailers can provide personalised advice beyond what any calculator can offer.