Breast size is determined by a number of factors, including body type, overall weight and body fat percentage. The average breast measurement in the U.S., U.K. and Ireland by bra size is 34DD, also called 34E, according to both lingerie company Intimacy -- as reported by "Glamour" magazine online -- and British retailer Debenhams, as reported in Mirror Online. The average bra size has increased over the years, which could be due in part to weight gain or breast augmentation, but also could be attributed to the fact that more women are getting properly sized for their bras.

Breast measurement by bra size is a combination of two numbers: ribcage circumference and bust line circumference. For a proper bra sizing, the ribcage measurement is taken directly below the breasts with the tape parallel to the floor, while the bust measurement is taken where the breasts are fullest, usually at nipple level. The ribcage measurement is related to the number, or band size, of a bra, while the difference between ribcage and bust line indicates the letter, or cup size.

At a 34DD bra size, the average women has a ribcage measurement of 29 to 30 inches and a bust line measurement of 38 to 39 inches. Since cup size is relative to band size, the letter measurement of a bra alone isn't an accurate measure of breast size. For example, 34DD breasts would be significantly smaller than 38DD breasts, because the ribcage and overall bust circumference would be smaller.