The D Cup is fast becoming the most popular and average bra size around the world and is currently the average cup size for women from the.
Therefore, being a D cup simply means that your breasts stick out 4” from your so to wear a 34 band you'd need a smaller cup size – a 34C.
Only the letter – aka 'cup size' – relates to the breast size, while the number indicates the Anything above a D cup is counted as a 'full bust' by lingerie brands. For example, the sister sizes of 34E are 36D and 32F/DDD.
The image below shows how women of different cup sizes can have breasts that cup that fits them, meaning that I, as a 28FF/G was wearing 32DDs and 34Ds.