THE HISTORY OF BRA SIZING
The modern bra sizing method was established in the 1930s. The alphabetical cup sizing system was first conceived in 1932 and is attributed to S. H Camp and Company in the USA. The four sizes of A, B, C and D were designed to be used by the majority of women. The first publication featuring these sizes appeared in 1933 in the Corset and Underwear Review.
By the end of the 1930s, many companies adopted the lettering system introduced by S. H Camp and Company and quickly became the standard way of sizing bra cups.
Possibly the biggest influencing manufacturer at the time was Warner's. They adopted the ABC lettering system in the mid 1930s and including it in their advertising campaigns.
Although band sizing developed alongside this as a way of determining underbust measurement, it's still not clear how the "add 4 inches" method came about. Some say that this was due to 'vanity size' marketing by the lingerie companies early on, asking for a size bigger than you really were. Others say that it's due the lack of elasticity of the early fabrics, making a larger length a more comfortable fit. Either way, for many manufacturers today, this is still generally how they size their products. Until an easier to understand and universal sizing method comes along, we are unfortunately left with this 'traditional' method of bra sizing.
As always, use any bra calculator as a guide only and then actually try some products on to obtain the best fit.