How to Measure Your Bra Size at Home Accurately

Bra sizing comes down to two numbers. The band measurement is taken snugly around your ribcage, directly under the bust, and it decides the number part of your size such as 34 or 36. The bust measurement is taken around the fullest part of your chest, and the difference between it and the band determines the cup letter.

Use a soft cloth or tailor tape measure, not a rigid metal one, and take both measurements while wearing an unpadded, non-push-up bra or nothing on top at all. Padding adds volume that throws off the bust number and pushes you toward a larger cup than you actually wear.

The cup is based on the gap between your bust and band measurements, not on the bust number alone. As a rough guide, each inch of difference is one cup size. A one inch difference is roughly an A cup, two inches a B, three inches a C, four inches a D, and so on up the scale.

This is why two people with very different bust measurements can wear the same cup letter, and why the same person can look like a completely different size on paper depending on the band. The letter is meaningless without the number attached to it, which is why a 32D and a 36D are not the same shape at all.

The calculator does the arithmetic so you do not have to memorize any tables.

Sister sizes are different size labels that share the same cup volume. If you go down one band size, you go up one cup letter to keep the same volume, and if you go up a band you go down a cup. A 34C, a 32D, and a 36B all hold a similar cup volume but fit different ribcages.

Sister sizes are practical, not just trivia. If a bra fits your cup well but the band feels loose or tight, or your usual size is out of stock, a sister size lets you keep the volume while adjusting the band. Keeping your two or three closest sister sizes in mind makes shopping far less frustrating.

A calculated size is a strong starting point, not a guarantee. Cut, brand, style, fabric stretch, and even the day of the month all change how a given size feels, so treat the result as the first size to try rather than a fixed answer. Brands vary enough that the same label can fit very differently between two makers.

Signs of a good fit are a band that sits level around your body and stays put when you raise your arms, a center panel that lies flat against your chest, and cups that fully contain the bust without gaps or spillover. If something is off, use your sister sizes to zero in on the right fit. These results are general estimates, and a professional fitting can help if you are consistently between sizes.