Band size is expressed as a number, whereas cup size is represented by a letter (A,B,C,etc.). The conventional method of determining cup size does not rely on the direct measurement of the breasts but instead relates the circumference of the chest immediately below the breasts to the circumference of the chest around the fullest part of the breasts. In the most common method of bra sizing, band measurement is determined by wrapping a tape snugly around the chest immediately below the breasts. Five inches is then added to that number. If the number is odd, it’s rounded off to the next highest even number (because bras are only offered in even numbered sizes).
For example, if the under-bust chest circumference is 29 inches, the band size is 34 (29 + 5). If the underbust chest circumference is 32 inches, the band size is 38 (32+5=37, rounded up to the nearest even number=38).
The bust circumference is determined by measuring the distance around the chest loosely with a tape across the fullest part of the breasts, usually at the level of the nipples, with the woman wearing a non-padded bra. Cup size is then calculated by the difference between bust circumference to underbust plus five measurements.
A difference of 3 inches equals an A cup, 4 inches a B cup, 5 inches a C cup, 6 inches a D cup, and so forth. For example, a woman with a bust circumference of 38 inches and a band size of 34 (underbust chest circumference of 29 inches + 5 inches) would be a B cup (38-34=4-inch difference =B cup).
Dr. Pechter’s study measured bra cup size by the traditional means and compared with measurements directly across each breast. This measure was horizontal distance across the skin of each unclothed breast with a tape from the lateral (outer) breast crease to the medial (inner) breast crease.
For women with a band size of 36, a measurement of 7 inches corresponds to an A cup, 8 inches a B cup, 9 inches a C cup, etc., with each one-inch increment determining a cup size up or down. For every change in band size up two or down two, the cup size measurement in inches increases or decreases one-half inch respectively.
Thus, a woman with 34 band size will be a C cup if the measurement across her breast is 8.5 inches. In small or firm breasts, the circumference can simply be measured in the standing position. Women with large or sagging breasts are more accurately measured in the supine position (laying on back).
The point where the breast mound begins laterally can sometimes be more easily discerned with the woman’s arms elevated. Gently pushing the breast laterally to the outside or medially toward the inside may help define the breast creases for accurate measurement.
The video below demonstrates a clear demarcation of medial (inner) breast margin (crease) when she pushes upward and slightly inward on breasts. The same pressure on the breast defines outer (lateral) breast crease. It is from the inner to the outer crease that measurement in inches most accurately determines the correct cup size, as described above.
In his study of 100 women, Pechter’s method was accurate for over 80% of women, and never off by more than one cup size. Traditional cup sizing methods provided results that were smaller than those stated by women subjects, often by two or more cup sizes.