Formal:
* Je suis calme. (This is the most straightforward translation.)
Informal:
* Je suis tranquille. (This implies a sense of peacefulness and relaxation.)
* Je suis détendu(e). (This emphasizes the feeling of being relaxed and at ease.)
More nuanced:
* Je suis serein(e). (This conveys a sense of inner peace and composure.)
* Je suis apaisé(e). (This suggests that you have calmed down after a stressful situation.)
The best choice will depend on the specific situation and the level of formality you wish to convey.