To find out your bra size, all you need is a measuring tape. You should do this at least yearly.
To find your band size:
Wearing an unlined, non-padded bra, run the measuring tape all the way around your back where your band sits, being careful to keep the tape level. It should be as snug as possible without digging.
If you don't get a whole number (33 and 1/2 inches), round to the nearest whole number (34 inches), and then use the chart below to find your band size.
Note: Inches do not equal size, meaning 34 inches does not equal a size 34 band. (You must apply Your band size+4)
To find your cup size:
Run the measuring tape around your back, just under your shoulder blades, and up around the fullest part of your bust ,The number in inches corresponds to your cup size according this table: