Because there is no sizing standard for bras, bra pattern sizing will differ from company to company. To further complicate matters, there is no one measurement method to determine what size will work for every body (including the one on my pattern envelope!).
The only reliable way to determine bra size is to try on finished bras made from that pattern! Given the bra sizing challenge, I recommend starting with your ready to wear size – if it fits you well!
I always recommend making a muslin first to test the fit of the bra. This applies not only to new patterns but also to any new fabric you decide to use. I have two posts on how to make muslins (a.k.a. toiles), a fast way and a more thorough way.
If you do not have a ready-to-wear size or at a loss for what size to start with, I offer the below sizing guideline for my bra sewing patterns.
Sister sizes are bra sizes that have the same bra cup volume and underwire size. The difference between sister sizes is the size of the band and frame. For example, the sizes 30D, 32C, 34B and 36A all have the same cup volume and use the same underwire. The difference between those sizes is the band. This blog post has instructions for how to modify the band to reach the sister sizes on this table