Similar in meaning:
* Tranquil: Peaceful and serene, often describing a place or atmosphere.
* Peaceful: Free from disturbance or conflict, often used to describe a state of mind.
* Serene: Calm and peaceful, often used to describe a person's demeanor.
* Still: Not moving or making any sound, often used to describe water or air.
* Quiet: Free from noise or commotion, often used to describe a place or situation.
* Composed: Calm and in control, often used to describe a person's behavior.
* Placid: Calm and peaceful, often used to describe a person's temperament.
* Relaxed: Free from tension or anxiety, often used to describe a person's physical state.
* Unruffled: Not upset or disturbed, often used to describe a person's reaction to a situation.
More specific meanings:
* Equanimous: Calm and even-tempered, especially under pressure.
* Unflappable: Not easily upset or flustered.
* Poised: Calm and self-assured.
* Collected: Calm and in control, often used to describe a person's thoughts.
* Cool-headed: Calm and rational, often used to describe a person's thinking.
The best word to use will depend on the specific context of your sentence.