I like boobs so yes :D I've learned by experience and guesstimating
Your only option is to guess the girl's bra size. If you are not buying her a bra, you don't need to know her bra size. Try to find out something that's important to her instead.
bra sizes are organized by the number (ex. 34A), which is the length around your rib cage. And the letter (ex. 34A) is your cup size.
It ranges from cup sizes AA-G and rib sizes 30-56
The different numbers before "A" in a bra size are the size of the band. A bra size 32A will fit a woman who is about 32 inches around the rib cage.
You can know a woman's bra size by asking her. If you do not think it is appropriate to ask her for her bra size, then her bra size is probably none of your business.
he wants in your bra
Well if your talking bra sizes then... A A34 bra size is what most girls 12 to 13 wear. A B36 bra size is the bra size right after the A cup.
36b
36/80
Not that I know of, but there might be......
Decrease each by 75% .
Its called La Mar- The Beautiful Girls. great song (:
YES, though most stupidheads say it is bigger. WRONG. I am a girl and I know and DOUBLE SIZES ARE INDEED SMALLER.Actually, you are wrong there. You may be a girl, but you do not know bra sizes very well, Double A bras are SMALLER THAN AN A cup. Double D bra's are BIGGER THAN A D CUP.
The average bra size for teenage girls can vary widely, but it is generally around A to C cup sizes. It's important to remember that bra size can change frequently during the teenage years due to growth and development.
They're embarrassed because if they know other girls have bigger boobs than them, they feel like the girls that have bigger boobs are better and more attractive than them. If that's not the case, then maybe they think their boobs are too big. I don't know... You shouldn't be embarrassed about how big your boobs are. 
bra sizes
Bra sizes for 12 and 13 year olds vary greatly based on individual factors such as genetics and growth rate. Typically, girls may start wearing bras around this age with sizes ranging from AA to B cups. It's recommended to get professionally measured for a well-fitting bra.