Our tanks are true to size. If you have bigger boobs, like a DDD or more, I would size up. It won't fit that different, and it just gives you an extra bit of coverage on the boobs. The model in the photo shoot is wearing a small, and the model wearing the bra and bloomer set is wearing an XS. Here's a chart:
US size | UK size | Buttercup size |
0 - 2 | 2 - 4 | XS |
4 - 6 | 6 - 8 | S |
8 - 10 | 10 - 12 | M |
12 - 14 | 14 - 16 | L |
16 - 18 | 18 - 20 | XL |