quieten vs calm down

quieten and calm down both are verbs.

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

As verbs, calm down is a hyponym of quieten; that is, calm down is a word with a more specific, narrower meaning than quieten:

Other hyponyms of quieten include shush, lull, shout down, gag, muzzle.

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

Other synonyms of quieten include calm, lull, quiet, still, tranquilize, tranquillise, tranquillize.

quieten (verb)	calm down (verb)
cause to be quiet or not talk	become quiet or less intensive
make calm or still	make calm or still
become quiet or quieter	become quiet or calm, especially after a state of agitation