Before we go through how to figure out your bra size, you need to realize that this is not something you do one time. Your body changes continually. Your breasts are no different. You should resize your breast once a year. You'll probably be shocked the first few times when you realize how much change is occurring. Okay, let's get on with the measuring process.
Our first step is to get a tape measure. Now wrap it around your chest and back so that is perpendicular to the ground. Do not run the tape over your breast. Instead, run it just under them at their base. Now measure one full loop. This is your band size. The "band" is the material that runs around the bottom edge of your bra and should provide the majority of the support to your breasts.
Now we need to get our second measurement. Do the same thing with the tape measure with one exception. Instead of placing it at the bottom of your breast, place it over the top where they are fullest. This is your bust line, often called your "bust size". By definition, it will be larger than your band size.
Now to figure out your cup size, we need to do a bit of math. Subtract your band size from your bust size to the closest inch. You should have a single digit number. Now you must match it the cup size as follows: 2 inches is a B cup; 3 inches is a C cup; 4 inches is a D cup; 5 inches is a DD cup; and 6 inches is a DDD cup.
So, what is our final measurement for our bra size? It is simple. You just take the band size and add the cup size after it. Let's assume I have a band size of 36 inches and bust size of 39 inches. The difference is 3 inches, which means I have a "C" cup. My bra size is a 36C.
Does this mean I should hop online and just start ordering 36C bras? Unfortunately, it does not. Bra brands have their own ideas of how bras should fit regardless of the sizes. As a result, you should try them on first to find a version that fits well and then pop online to buy. Online sites are usually cheaper because they do not carry the overhead that a brick and mortar store does. Most also offer free shipping, which cuts your cost further and we can all stand to save a buck these days.