An Important Note About Bra-Sizing
This will affect the fit of a corset, especially if you have chosen a style which covers the breasts. Most women do not wear a bra that is the correct size. Many are still wearing a bra that is the same size as the one they wore in high school. Pregnancy, weight fluctuations, fitness and ageing all affect the size and fit of a bra. Bras are sized by the circumference of the chest (band size) and the volume of the breasts (cup size).
To find your bra size:
Measure (in inches) around the ribcage just below the breasts. Add 5" to this measurement and this will determine your bra size (i.e. 29" ribs + 5" = Size 34).
To figure your cup size:
Measure firmly around the upper chest (Above the breasts, at armpit level). Then measure around the fullest part of the bust. The difference between this measurement and the chest size is the cup size:
1" = A, 2" = B, 3" = C, 4" = D, 5" = DD etc.
These measurements should be taken over an unpadded bra (nude measurements can be inaccurate, especially with larger figures).
A properly fitting bra will not have wrinkling cups, a band that rides up in the back, excessive strain across the back, bulges across the top of the breast or across the back or breasts bulging into the armpit. It will also be more comfortable and flattering than one that is the wrong size. Bra sizes are probably the most standardized of all apparel however there are variabilities from company to company. There are even sometimes differences in fit within one company's range, depending on the cut of the bra.
For men who wear bras
The same rules apply for general sizing. Many crossdressers are wearing a bra that is multiple sizes too small. A correctly fitting bra will not be an ordeal to put on, and shouldn't leave marks on the body that last for hours. As men are often much larger around the chest than women, finding a nice bra in larger sizes is a challenge (as it is for larger women). As a rule 'A' is a small cup, 'B' is an average cup, 'C' is a large cup. A good pair of weighted breast forms are the best investment for a natural looking bust line. A bust that looks natural is in proportion with the body (i.e. a a pair of 36B forms will look very small on a body that properly fits into a size 44 bra). A cup size does not remain the same size throughout the size range A 34C bra cup will be much smaller than a 40C, although they are both C-cup size
Merry widows and bustiers are often awkward for men to wear, unless they are very petite or short-waisted, as the waistline of the garment is usually much too high. A regular bra plus an underbust corset is a better solution, and will give a much smoother line under clothes.