Your band size represents your diaphragm or your body. The number 30, 32, 34 and so on is known as the underbust measurement because it is taken from underneath your bust and around your rib cage.

Your cup size is the letter which represents the depth of your bust. The letters A,B,C and so on, known as the overbust measurement is taken around the fullest part of the bust.

With your bra on use a standard dressmaker's tape measure and measure snugly around your rib cage, under your bust and shoulder blades. Add 5  to that number.  Once you add the 5, if the number is even then that is your Band Size. If the number is odd round up to one.  If, let's say, it's 31 then add 1 to make it even and it becomes 32. The best idea is to measure in inches, since that's the industry standard.

Still using the same tape measure with a bra on and your clothes off place the tape measure under your arms and over the fullest part of your bust.

Subtract the final number obtained from Step 1 (rib cage measurement) from number you just obstained. E.X. Step 1 = 38, Step 2 = 42. 42 - 38 = 4. 4 Converts to D cup. Use the table below to see the rest of the values.

Note that in the table below there IS some minor ambiguity. Usually when that's the case, the sizes are interchangeable. The closer one is to the next cup up, the more interchangeable the cup sizes will be.