What is the real difference between 34C and 34D?
34C and 34D share the same band measurement. The only difference is one cup of volume — about a one-inch difference between under-bust and bust.
34C and 34D are one cup apart on the same band. The difference is roughly one inch of bust circumference, or about 30 ml of cup volume. If you have noticeable gapping or overflow at one of these sizes, the other is the natural next try.
How to know which size fits you
The simplest way is to measure. Take your under-bust snug and your bust loose. Round the under-bust to the nearest even inch — that is your band. Subtract the under-bust from the bust — every inch of difference equals one cup letter (1 = A, 2 = B, 3 = C and so on).
If your measurement is right between 34C and 34D, try 34C first on the loosest hook and walk around. If the band rides up, try 34D.