reckoner vs estimator

reckoner and estimator both are nouns.

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

As nouns, reckoner and estimator are synonyms defined as:

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

Other synonyms of reckoner include calculator, computer, figurer.

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