How to Use a Bra Size Calculator. One of the most common mistakes women make when buying bras is not buying the correct the size. In order to get a correct fitting bra to give you the most lift and support it's important to take a few minutes to get two correct measurements. These measurements can be put into a bra size calculator to determine your correct bra size.
Take a tape measure and wrap it around your rib cage just below your bust. The tape measure should sit where an underwire would go on a bra and be close fitting around your body.
Add five inches to the measurement. This number will become your band size for a bra. If the number is odd, go up to the next even number for a correct size.
Measure around your bust at the fullest point. This is generally directly over a woman's nipples. Double check to make sure the tape measure goes straight around you and isn't too tight.
Subtract the measurements in Step 2 from Step 3 above. The difference will be your cup size. Every inch difference signifies a cup size. For example, a three inch difference would mean you're a C cup.