What is the real difference between 40D and 38DD?
40D and 38DD are sister sizes — they share the exact same cup volume but use a different band. If your 40D feels too tight in the band but the cups fit, 38DD (or vice versa) is the first size to try.
Because 40D and 38DD are sister sizes, you can swap between them when one is out of stock or when a brand fits slightly differently. The cup volume is identical — only the rib-cage band changes.
Most fitters recommend trying both sizes if your usual size feels off. The right one depends on what feels comfortable on your back rather than on your bust.
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 40D and 38DD, try 40D first on the loosest hook and walk around. If the band rides up, try 38DD.