What is the real difference between C and D?
Cup C and cup D are commonly compared, but the actual size depends on your band. A C cup on a 28 band is much smaller than a C cup on a 38 band.
"C cup vs D cup" only makes sense when you specify a band. A C cup on a 28 band is much smaller than a C cup on a 38 band. Always pair the cup letter with the band number when you compare sizes.
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 C and D, try C first on the loosest hook and walk around. If the band rides up, try D.