Getting fitted for a bra at a store might seem like the most "professional" way to know your size. But that comes with the commitment that you'll at least buy a bra from them. So the best way to really know your size is by doing it at home.
First, put on a bra that fits you well and grab a measuring tape. Wrap the tape around your rib cage, right where the bra band is. Make sure the tape remains parallel to the ground all throughout. Take the number of inches and round it up to the nearest even number to get your band size.
Next, move the measuring tape up to the fullest part of your breasts while making sure the tape stays parallel to the floor. Get the measurement and round it up to the nearest even number to get your cup size.
Lastly, it's time to do a little math. Subtract measurement A from B. The difference determines your cup size:
A = 1
B = 2
C = 3
D = 4
DD = 5
And so forth. So if your band size is 36 and your cup measurement is 38, your cup size is B. That means your perfect bra size is 36B!