According to Playtex, a 38C bra cup has a measurement of 41 inches around the fullest part of the bust and is 3 inches greater than the band size. The band size of 38 inches is measured above the bust and under the underarm.
The difference between the measurement around the fullest part of the breast and the band size determines the cup size. If there is no difference, the cup size is AA. With a 1-inch difference, the cup size is A. Each inch of difference increases the cup size letter by one until D. At a 5-inch difference, the cup size is DD. A 6-inch difference is DDD and a 7-inch difference is G.