What is one size up from a 36DD a 38D?

The number and the letter are actually two different measurement. The number is the measurement around your chest, just under your breast. They are counted in even numbers, so the next size up if it is tight around your rib cage would be a 38. The letters are actually the cup size. You would measure around your back and breasts. If that number is one inch more than your number size, than you are an A, 2 inches a B, 3 inches a C, 4 inches a D, and so on. The double D comes in when your breasts are extend 4 inches past your chest, but are wider than what will fit into a D cup. A D and a DD still have the same depth, but the cup will be slightly wider.

So in short, you will have to measure to see what the next size up is. If you need more room in the rib cage area, you will likely be moving up to a 38DD. If the cups are too tight depth wise, but the ribs are fine, you will be likely going to a 36E. If is deep enough and okay around the ribs, but you find the cups are not wide enough, you will likely be going into a 36DDD. Most lingerie shops such as Victoria's Secret or Frederick's will measure you as a free service. They do it over your clothes, so it is nothing too embarrassing. If you are unsure, that is an option that you want to try.