Yet 8 out of 10 women wear bras that don't fit.

Finding the right bra doesn't have to be an ordeal if you know the basics of bra sizing. There are two parts to a bra's siz​e: the chest siz​e (also called the band siz​e) and the cup siz​e.

The chest, or band, siz​e is the part of the bra that runs across a girl's chest and around her back. The cups are the parts of the bra that hold the breasts. Bras come in many siz​es, which are different combinations of the chest siz​es (shown as numbers — in the United States, these are 32, 34, 36, etc.) and cup siz​es (shown as letters — AA, A, B, C, etc.).

It's important to get both the chest and cup siz​es right to ensure a proper fit. If a girl wears a bra that's the wrong chest siz​e, the bra's band will either feel tight and uncomfortable around her ribcage or it will be too loose and ride up her breasts. Bras that are too small in the cup will compress the breasts and force them to bulge out the sides. And cups that are too big can wrinkle and pucker, which won't look good under clothes.

So how do you calculate your chest and cup measurements? It's easy, and it's something you can do in the privacy of your own room at home. All you need is a tape measure.

Chest measurement. Bra fitting experts offer two methods of calculating the chest measurement. The most common method involves running a tape measure just under your breasts, all the way around your back and ribcage. The tape measure should rest flat on your skin and lie straight across your back — not so tight that it digs in, but not so loose that it sags down in back. Make a note of your measurement and add 5 inches. That's your chest siz​e.

You can also estimate your chest siz​e by running a tape measure across your chest above your breasts, under your armpits, and around your back (measure from the same back position as in the first chest measurement option). This method is easier because you don't have to do any addition or other calculation, but some people feel it's less accurate than using the first method.

If your chest measurement comes out as an odd number (such as 31 inches or 33 inches), it's usually a good rule to round down to the next number. Most bras have a few sets of adjustable hooks and eyes, so you can adjust the tightness. It's a good idea to round down, rather than round up, since bras tend to stretch over time.

Cup measurement. As with the chest measurement, when you're measuring for cup siz​e, make sure the tape is not too loose or too tight. If you already own an unpadded bra that fits well, wear it when measuring for cup siz​e. Be careful about the type of bra you wear to take your measurements, though. Sports bras can flatten breasts and give a cup reading that's too small, and padded or lined bras have the opposite effect.

This time, when you run the tape measure around your body, you're going to take the measurement across the fullest part of your breasts. Write down this number. Now subtract your chest measurement. If the difference between the two numbers is less than 1 inch, your cup siz​e is AA. If it's 1 inch, your cup siz​e is A; 2 inches, you're a B; 3 inches you're a C, and so on.

So you've taken your measurements and you're in the fitting room trying a bra in what's supposed to be your siz​e — but it doesn't seem to fit. Now what?

Different brands and styles of bras will fit differently. So bring several options with you into the dressing room to start. And before you pile on all your clothes just to go out and look for a different siz​e, experiment by making some adjustments to the bra. If the bra has an adjustable closure, extend or shorten the band slightly by moving the hooks to a different spot or adjusting the velcro if the bra has a velcro closing. The bottom band of a properly fitting bra should ride across the middle of your back and pass under your shoulder blades to provide the right support.

A bra's straps allow you to modify how the cups fit and support your breasts. When the straps are the right length, a bra lifts the breasts comfortably and the back of the bra will run straight across your back (if a bra is pulling upward in the back, it may be a sign that the straps are too tight). You should be able to get one finger under the straps to prevent them from digging into your shoulders. In general, girls who are petite will need to wear their bra straps shorter than girls who are tall. If you've made the straps looser and they are still digging into your shoulders, the cup siz​e might be too small for you.

The cups also might be too small if you notice your breasts bulging out the sides, top, or bottom of the bra. If you notice that the cups are puckering or that there is a gap between your breasts and the bra, the cups are too big.

In addition to fit, there are some other things you might want to take into account when choosing a bra.

Breast growth. A girl's breasts can grow and change rapidly during the teen years, which means that until you stop developing, you might have to change your bra siz​e several times. If your breasts are still growing, consider buying fewer bras and measure yourself frequently to double-check your siz​e.

Because growth can make a girl's breasts sensitive, you might find certain styles or fabrics work better for you. For example, some girls like seamless cups because they don't irritate the nipples; others choose styles that minimize jiggle to ease the occasional aches of breast development.

Menstrual changes. Some girls' breasts change siz​e or shape at different times in their menstrual cycles. Many girls find they like to wear different styles of bras at different times in their cycle.

Different-siz​ed breasts. Most girls' breasts grow at different rates — and many girls find themselves with one boob that's bigger than its partner. This is so common that bra manufacturers design their products so they can be adjusted to accommodate asymmetrical breasts. Start by moving the strap adjusters on a bra to different lengths to see if this helps.

Some girls also use the trick of buying a bra that has removable padding and then taking the extra padding out of the cup for the larger breast. Push-up bras can work well for this because they often come with an easily removable padded section (called a "cookie") that sits under the breast.

Hooks, wires, and other hazards. Take a look at the bra you're about to buy and imagine how it might survive a full day of wear. If the front closure pops open too easily, could this bra let you down in front of your entire drama class? The bra's straps may feel fine in the fitting room, but if the adjusters are sitting right on your shoulder or collarbone, how will that feel under your backpack?

Test to see if the bra band is too loose by lifting your arms in the air and putting them back down by your sides a couple of times. If the bra rides up across your breasts, the band is too loose. If you're looking for a sports bra, jump up and down in the fitting room a couple of times to evaluate the bra's bounce control.

You may also want to try on a T-shirt or sweater over a bra you're thinking of buying to get the finished effect. That pink rose on the front may look cute on the bra itself, but does it make you look as if you've sprouted a third breast once your T-shirt's on? Also pay attention to the color of the bra - ones that are closest to your skin color will be the least noticeable under light colored clothing.

No matter what the siz​e or shape of a girl's breasts, the most important thing to remember in choosing a bra is comfort. Try on a variety of bras to find the one that's right for you. Some girls prefer the comfort of cotton, others like the support of synthetics or underwires. There are minimizer bras for girls who'd like their breasts to look smaller, or padded bras for girls who'd like a little extra shape. No one bra is better than another — it all depends on which style works best for you.

Reviewed by: Larissa Hirsch, MD

Date reviewed: November 2009