unbelievable vs improbable

unbelievable and improbable both are adjectives.

Word	Noun	Adjective	Verb	Adverb
unbelievable	No	Yes	No	No
improbable	No	Yes	No	No

As adjectives, unbelievable and improbable are synonyms defined as:

unbelievable and improbable: having a probability too low to inspire belief

Other synonyms of unbelievable include unconvincing, unlikely.

unbelievable (adjective)	improbable (adjective)
beyond belief or understanding	not likely to be true or to occur or to have occurred
having a probability too low to inspire belief	having a probability too low to inspire belief
	too improbable to admit of belief