Bust size mean?

The size of a bra is commonly described by two values: the band size (underband) (a number based on the circumference of the chest under the bust, excluding the breasts), and the cup size (given by a letter, and relating to the volume of the breasts themselves) (e.g. a 30D bra is for a 30-size band and a D-size cup). Band size is usually determined by measuring body circumference either under or above the breasts and then adding a specified amount to account for the fact that the ribcage is generally wider at the height of the breasts than at the point at which one measures. The cup size is worked out from the difference between the band size and a measurement of the chest circumference over the fullest part of the breasts (overbust). A common mistake is to take the measurements with a bra on, instead of braless. The mean underband circumference in the UK is 85.5 cm and in the US 88.7 cm. Overbust measurements are 100.8 cm and 104.7 cm respectively, for women 18–64 years.