What fitting method should you use?

There are fitting guides all over the internet. Most guides and calculators will lead you to believe you need to add 4 inches to your underbust measurement and get the difference between that number and your bust measurement. I certainly understand why retailers follow this method- it crams nearly everyone into 32-38 bands and A-DD cups, which cuts down production costs. What I don't understand is why unaffiliated calculators still use this method!

My personal favorite calculator is the one from SophisticatedPair. This guide on reddit will walk you through how to get the most accurate measurements. If your underbust measures 30 inches or fewer, I recommend setting the calculator to spit out the "moderately snug" size; otherwise, use the "very snug". (There are exceptions to this, but for the sake of simplicity, we'll stick with this rule of thumb for now.) You will get two sizes: your US and  your UK. Going with your UK size will nearly always be better, as the US market does not produce small bands with large cups, and only a couple brands even make anything larger than a DDD size in any band size.

Maybe you're surprised at what this calculator is telling you. Thinking that there's no way that size is accurate is referred to in the bra-fitting community as "letterphobia". The size you get from the calculator may not be the exact size you end up in, for one reason or another, but it is a great starting point.

Sometimes it can be appropriate to add inches to the underbust- not 4, though. Bras I Hate recommends adding 2 inches; you can read more about her reasoning here. I don't think that's the best solution for everyone, but I personally follow it because I am pretty bony, and I have problems with the wires being too wide in GG cups vs G.

It's worth noting that the reddit guide I linked tends to fail women who are larger than a 36 band or so. If you measure in that range, you have more variables to consider when figuring out your size. FullerFigureFullerBust,  XL Hourglass, and Fussy Busty are better resources for full-figured women than my blog will be.

Hopefully this was helpful and not an information overload. Feel free to leave any questions you may have in the comments.