How to get your Bra Size

1. To find your band size: Wearing an unlined, non-padded bra (not over your clothes!), 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.

2. 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,” says Iserlis. “The tape should just skim the bra in front.” Then (bra math alert!) subtract your band size from your bust measurement. “The difference in inches corresponds to your cup size.” For example, if your bust measures 37 inches, and your band size (not the exact measurement, but the band size you found above) is 34, the difference will be 3. On the chart below, you'll see a 3 is equal to a C cup, meaning your bra size is 34C.

Credits to: https://www.goodhousekeeping.com/clothing/bra-reviews/a29114502/how-to-measure-bra-size/