What you'll need: a measuring tape and an unlined or lightly padded bra.

Step 1: Circumference of Your Bust

Wrap the measuring tape around the fullest part of your bust without pulling it too tight. Make sure that the measuring tape is straight across your back. Note down the measurement in inches and round up if you're between sizes.

Step 2: Circumference of the Band Under Your Bust

Wrap the measuring tape around your ribcage, under the bust, without pulling it too tight. The measuring tape should remain parallel to the ground and hug your body. If the measurement is an odd number, round up (e.g., 35 inches = 36 inches). This measurement is your band size.

Step 3: Final Calculation

Subtract the result of step 2 from that of step 1. The difference between the circumference of the bust (measurement 1) and the circumference of the band (measurement 2) determines your cup size.

To find out what that is, refer to the following table.