Bra Size Calculator

About the Bra Size Calculator

A bra size calculator helps you find your correct bra size in US, UK, and EU sizing from two simple measurements — bust and underbust — eliminating the guesswork that leads to poor fit. Research suggests that a large majority of women wear the wrong bra size, typically with a band that is too large and cups that are too small. A properly fitting bra provides support through the band (80%) rather than the straps (20%), reduces back pain, and improves posture and clothing fit. The calculation is straightforward: band size comes from the underbust measurement rounded to the nearest even number; cup size comes from the difference between full bust and band size. Our calculator provides your US and UK size (which share the same numbering), EU metric size, and sister sizes — equivalent volumes in adjacent band sizes for when your exact size is unavailable. This tool is relevant for shoppers in the USA, UK, Canada, and Australia where sizing systems overlap, and for international shoppers converting between systems.

How It Works

Band size: measure snugly around the ribcage just under the bust in inches. Round to the nearest even number (minimum 28). If underbust = 31", band = 32". Cup size: measure loosely around the fullest part of the bust. Subtract band from bust measurement: cup difference = bust − band. Each inch = one cup size: 1" = A, 2" = B, 3" = C, 4" = D, 5" = DD/E, 6" = DDD/F. Example: bust = 37", underbust = 31". Band = 32 (round 31 up). Cup diff = 37 − 32 = 5 → DD. Size = 32DD. EU size uses underbust in centimetres rounded to nearest 5: 31" × 2.54 = 78.7 cm → EU 80. Sister sizes: moving up one band and down one cup (or vice versa) gives the same cup volume: 32DD = 34D = 30DDD.

Tips & Best Practices

• ✓Measure without wearing a padded bra — a padded or push-up bra adds volume that distorts your true bust measurement. A lightly padded or unlined bra gives the most accurate reading.
• ✓The band should be snug but you should be able to slide two fingers underneath. If you can pull the back of the band more than one inch away from your body, go down a band size (and up a cup to maintain volume).
• ✓Straps should not bear the majority of the support. If your straps are digging in and you are constantly adjusting them, it usually means the band is too loose — try a smaller band size first before tightening straps.
• ✓Sister sizes are useful when your exact size sells out. A 34C and a 32D have the same cup volume; a 36C and 38B do too. Going up a band size while down a cup (or vice versa) keeps the same cup capacity.
• ✓Breast tissue changes with weight fluctuation, pregnancy, nursing, and age. Remeasure your bra size any time you have a significant weight change, after pregnancy and nursing, or if your current bras feel uncomfortable even on the loosest hook.

Who Uses This Calculator

Women of all ages determining their correct bra size for the first time or after a body change. Online shoppers comparing sizes between US, UK, and EU brand sizing before purchasing. Maternity and nursing wear shoppers who need to account for expected changes during and after pregnancy. Parents helping teenagers understand proper fit for their first bras.