Source : Webster's Revised Unabridged Dictionary (1913)

Estimator \Es"ti*ma`tor\, n. [L. aestimator.]
   One who estimates or values; a valuer. --Jer. Taylor.

Source : WordNet®

estimator
     n : an expert at calculation (or at operating calculating
         machines) [syn: {calculator}, {reckoner}, {figurer}, {computer}]