They are sister sizes, meaning interchangeable. It depends on how you want the bra to fit. Say your band measures at 36.5 inches and your bust measures at 41.5 Inches there is a 5 inch difference between the two so an inch for every cup size (a,b,c,d,dd). If you try in the 36dd and it is a little tight around the band you could go up to a 38d to make it more comfortabl.
The next bra size after 36DD is typically 36DDD (or 36E, depending on the brand). Bra sizing can vary between manufacturers, so it's always a good idea to try on different styles and sizes for the best fit. Additionally, you could also consider increasing the band size to 38 while decreasing the cup size to D, resulting in a 38D, which can also provide a different fit.
My guess is 36D or 36DD. Been into some 36C and those look bigger!
36DD
38D
A 38D bra is equivalent to a 40C or a 36DD (or 36E, depending on the sizing system used). The number (38) represents the band size, while the letter (D) indicates the cup size. When adjusting the band size, you typically shift the cup size to maintain a similar volume.
She wears 36DD.
38d
16D
38D
get out of here pervert!
36DD or 36E
~im 36B ~i'm 38D