Bra Fitting for Trans Women
Grab your tape measure! Here is a somewhat helpful guide for trans women to find the right bra fit: https://bustyresources.fandom.com/wiki/How-to_determine_bra_size_for_transgender_women
But what this doesn't say is that because trans women tend to have an inverted-triangle shape, the overbust (upper chest underneath your arms and across your back) can sometimes measure the same or greater than your fullest bust. It is particularly likely for very thin or athletic trans women. If this is the case for you, and you are not using breast forms, it makes more sense to take your overbust as your band size. You will get a more comfortable fit in the upper chest this way.
The only problem may be that the bra gaps in the cup. But you can solve this problem by choosing bras in a smaller cup size, and/or padding the cups with inserts. I'm a fan of the IBB Flatter Me plus-size bras for trans women because they are unlikely to gap and are available in large band/small cup sizes: https://www.brasoutsidethebox.com/single-post/flatter-me-sizes-40aa-and-up
As I've said, for trans women the sizing of these will be different. Follow the guide as indicated, but take your overbust measurement as your band size. If you get an odd number, round up. Next, measure your fullest bust (across your breasts and around your back) and compare this to the rounded-up overbust measurement. If they are the same, or your overbust measurement is greater, your cup size is AA or less.
Some brands in addition to IBB, such as The Little Bra Company, Lula Lu, Dobreva, and Marks & Spencer, do sell AA cups --and Lula Lu also sells AAA. If your fullest bust measures one inch larger, you are an A cup. Two inches larger, B cup; and so forth.