estimate vs guess

estimate and guess both are nouns.

Word	Noun	Adjective	Verb	Adverb
estimate	Yes	No	Yes	No
guess	Yes	No	Yes	No

As nouns, guess is a hyponym of estimate; that is, guess is a word with a more specific, narrower meaning than estimate:

As verbs, estimate and guess are synonyms defined as:

estimate and guess: judge tentatively or form an estimate of (quantities or time)

Other synonyms of estimate include approximate, gauge, judge.

estimate (noun)	guess (noun)
a judgment of the qualities of something or somebody	an estimate based on little or no information
an approximate calculation of quantity or degree or worth	a message expressing an opinion based on incomplete evidence
the respect with which a person is held
a document appraising the value of something (as for insurance or taxation)
a statement indicating the likely cost of some job

estimate (verb)	guess (verb)
judge tentatively or form an estimate of (quantities or time)	expect, believe, or suppose
judge to be probable	judge tentatively or form an estimate of (quantities or time)
	guess correctly; solve by guessing
	put forward, of a guess, in spite of possible refutation