罩杯是表示胸罩的大小的表示方式。

罩杯的尺寸标准，根据制造商的不同，版型可能有所不同因而穿着效果上有所差异。罩杯尺寸大致是下列公式换算可得知：

部分特定内衣罩杯尺寸会自行升级或因加垫升级，其他或因版型关系罩杯有些微深浅差异，各地也会有所不同。

罩杯由小到大依序是：

Determining the correct bra size (also known as brassiere measurement or bust size) is the process manufacturers engage in to design and manufacture bras that correctly fit a majority of women, and for individual women, the process of identifying a correctly fitting bra. Bra sizes usually consist of one or more letters indicating the breast cup size and a number, indicating a band size around the individual's torso. Bra cup sizes were invented in 1932 and band sizes became popular in the 1940s.