There is no such thing as an average bra size for anyone. There is a range of "average sizes" typically going from 30AA - 38H which is considered normal depending on your height, weight, genetics and proportions.



If you measure:



25, 26, 27 inches under your bust you are a 28 band size. Now measure over your breasts.



1 inch of less = AA

2 inches = A

3 inches = B

4 inches = C

5 = D and so on.



So if you measure a 26 under your boobs and a 30 over your boobs (neither tight nor loose, just sitting on them comfortably) you will be a 28 C



I am 21 and a 32B cup which is one of the most common and average sizes. My genetics seem larger but when my mum lost weight her boobs dropped so that's how I know my genes are not ever going to be more than a 32B-D cup. Yes, you can still grow, I have started growing again at my age but I won't go more than 1-2 cup sizes any longer. You cannot determine how big you will grow and don't count anyone in the family who has had children, their boobs have been altered.



Don't go to Ann Summers etc, they will fit you a bra at the smallest they sell when this is likely not your size, they did to to me and my friend who is actually a 28B and I was a 30B and they wanted me in a 32A at the time (quite a few years ago, did it fit? no way)



Your bra should fit you like a glove, if it is tight and leaving marks, it is too small. If the cups are loose when you bend over or flat or roll or have a gap, they are too big, if your band is loose and needs to be on the tightest hook it is too big.