To find out how bra sizes work, there are two main measurements to consider.
1. The band size.
The band size of your bra is determined by the circumference of your underbust, which is found in the ribcage region near your breasts.. In the United Kingdom, most band sizes are measured in inches and have numerical names. e.g., 32, 34, 36, etc. They are typically equivalent to the women’s underbust circumference and are rounded off to the nearest even number.
2. The cup size.
The cup size of your bra is based on your breast or bust size. This is the difference between your underbust circumference and your chest or bust circumference.
The chest circumference is measured over the top of your breasts’ highest point, and it is generally recorded in inches per U.K. sizing standards like underbust circumferences.
Once the difference between your bust and underbust is determined, the answer is converted to its corresponding alphabetical letter e.g., A, B, C, D in the U.K. cup size chart.
For example, if your bust measurement is 36 inches and your underbust is 34 inches, then the difference between the two is 2 inches. The alphabetical equivalent of this in a generic U.K. cup size chart is B. Therefore, your recommended bra size is 34B.
So bra sizes work by adding the band size + cup size.