calm down vs simmer down

calm down and simmer down both are verbs.

Word	Noun	Adjective	Verb	Adverb
calm down	No	No	Yes	No
simmer down	No	No	Yes	No

As verbs, calm down and simmer down are synonyms defined as:

calm down and simmer down: become quiet or calm, especially after a state of agitation

Other synonyms of calm down include calm, chill out, cool it, cool off, settle down.

calm down (verb)	simmer down (verb)
become quiet or less intensive	become quiet or calm, especially after a state of agitation
make calm or still
become quiet or calm, especially after a state of agitation