First put on the best fitting, unpadded bra you own. If necessary pin the straps up to bring the breasts to the correct level. Get a tape measure and either measure yourself or have a friend measure you. The general rule of thumb for all measuring is: less than ½”, round DOWN, more than a ½”, round UP. So if your measurement is 32 ¼”, call it 32. Diagrams here.
2. Next measure the circumference above the bust under the arms (upper bust).
3. Measure the full bust circumference (nipple level). Make sure the tape measure is straight. Do not let the tape droop in the back.
4. Subtract measurement 2 (upper bust) from measurement 3 (full bust). Each inch difference is a bra cup size. For example, 1" corresponds to an A cup, 2" to a B cup, 3" to a C cup.
Second, you must try on bras. As we women know, there are variance between different styles and manufactures, so now you take your measurements and go try on different styles. Find the bra style that feels and looks best on you. You want the area between the cups to touch or almost touch the chest wall (no big air gap). The bra is not a sling for the breasts. They should be supported by the whole bra, not hanging from the straps.
Women with augmented breasts (implants) may find it more difficult to find a good fit as the implant shape may alter the way the bra fits. Because of this two plastic surgeons developed a bra designed for the augmented breast. "Le Mystère No.9 is a collection of innovative, designer bras incorporating patented technology so as to compliment the unique changes in shape and size that occur after a breast augmentation. Le Mystère has created the only bra on the market designed specifically for augmented women that fits true to size. You will no longer have to go up one cup size to accommodate the increased depth of your breast, since the design of the No.9 garments has taken into account all of the anatomic changes that occur after your breast enhancement. Now it’s easy for you to find a superior fitting bra for your post augmentation breasts. Look no further, Le Mystère No.9 is here!" Their fit guide used different guidelines.
Fitting a Bra--the Breast Site
Teen Health--Finding the Right Bra
Playtex--How to Measure a Bra
Sandra Saffle, Nordstrom's top bra fitter for Oprah's Bra Fitting Tips
Sew Sassy Fabrics (if you want to make your own)