Bra sizes vary from country to country and if you're an American, you'll need to know how to convert your bra size to a metric bra size if you're going to be living or staying in another country for an extended period of time. The biggest difference in sizing is moving from the American system to the metric system, which is used in most European countries. While it is handy to know how to make the conversions yourself, there are also many websites that have conversion calculators or charts, though many give different information and some are not accurate.

The most important conversion you will have to make when looking at a bra from a country that uses the metric system is inches to centimeters. While we use inches in the U.S., most European countries use centimeters. There are 2.54 centimeters in an inch. This doesn't make for an easy time converting sizes. If you're a 34 band size in the U.S., you would multiply 34 by 2.54, giving you a band size of roughly 86. If you're using a conversion calculator, you could end up with a different result, as all are calculated slightly differently, depending on the creator.

The cup sizes between American bras and bras in other countries stay the same when moving from AA to DD but then vary slightly thereafter. In the U.S., bra cup sizes continue to go up a letter at a time after DDD. So the next size would be E, followed by F, followed by G, going all the way to an O cup. European sizes differ in the fact that they continue to double letters when moving up a cup size. If you begin with the F cup, the next size is FF, followed by G, followed by GG, going all the way to a K cup. Therefore, if you're an M cup in the U.S., you'd be a J cup in the UK.

Bra cup letters sizing was introduced in 1935 by The Warners Company. European companies accepted this bra sizing method not long after U.S. lingerie manufacturers accepted it. The band size, in reality, measures a woman's underbust. When they were originally introduced, the sizing methods needed to sound more appealing to women, so manufacturer's added 4 to 5 inches to the underbust measurement to come up with the band size that was ultimately used. This difference is why it is so difficult to convert a bra size. Some European manufacturers add the extra inches (or centimeters) to keep up with the sizing method and some do not.

The best advice when it comes to purchasing a bra from another country is to ask someone in the know. Many times, establishments that sell lingerie will have professionals on hand who can point you in the right direction when it comes to a size. The people who work at the stores that sell bras in other countries can likely tell you how they size their bras and what the best size will be for you given your American size. When all else fails, always ask.