estimator vs reckoner

estimator and reckoner both are nouns.

Word	Noun	Adjective	Verb	Adverb
estimator	Yes	No	No	No
reckoner	Yes	No	No	No

As nouns, estimator and reckoner are synonyms defined as:

estimator and reckoner: an expert at calculation (or at operating calculating machines)

Other synonyms of estimator include calculator, computer, figurer.

estimator (noun)	reckoner (noun)
an expert at calculation (or at operating calculating machines)	a handbook of tables used to facilitate computation
	an expert at calculation (or at operating calculating machines)