1. Measure for your band. Wear an unlined or lightly lined bra that fits well so your breasts are as close to natural as possible. Make sure the band is fitting parallel to the ground just under your breasts. Wrap you tape measure around the band taking care that it's level all the way around.
2. Now to measure the bust. Wrap the tape measure around the fullest part of your bust. Make sure the tape is parallel again.
3. To figure out the cup size, subtract your band measurement from your bust measurement. Every inch difference equals one size up. See chart below.