Go to a store that sells lingerie, there the saleswomen are very kind and would explain everything to you. They will let you try and you will find out which size is right for you. Usually, the size of the cup increases with each subsequent letter - A is the smallest, then B, etc.
I think you are Cup B, but there are still tables on the Internet, you measure yourself and look and when you buy a bra you try several sizes and the one that is most comfortable for you and does not look big, nor does it seem to burst is yours.
You're a little confused, at least that's how it seems to me. You have two options - to do it yourself or go to a lingerie store and ask a saleswoman to help you.
To do it yourself you only need a sewing meter and find a size chart on the Internet. You measure the chest and chest circumference, not just the chest. You find the difference and this difference determines the cup (A, B, C). The chest circumference determines the number in front of the cup (for example 75B). Because you are thin but tall, you will probably be cup A or B, more likely B.
In a lingerie store they will choose something nice for you, or you can try it.
Otherwise, the chest circumference (just below the chest you measure) corresponds to the tightness and size of the bra, and the letter to the size of the cup. If you live in a city with a mall, just go there to an N&M store and try different models and sizes, which is convenient for you and you like the size.
Chest circumference is not a sufficient indicator. You can be broad-shouldered.
The bras have 2 size indicators. One is for the cup, the other for the length of the belt. The exact size is very important because the bra is worn for a long time and can affect the posture.
1 nohaa_mian answered