For the one piece, we recommend that you take the size of your bottom:

34, take XS
36, take S
38, take M
40, take L
42, take XL.
44, take XXL.

The one-piece 1.0 is more indented than that of the other one-pieces.

If you make a mistake, you have 14 days to change size. If you have any hesitation, contact us at hello@posidonie.io. We will be happy to help you and avoid round trips that could alter the planet.